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Active Isolated Stretching:
An Investigation of the Mechanical Mechanisms
Alison Longo,
BSc (Human Kinetics)
Submitted in partial fulfillment of the requirements for the degree
Master of Science in Applied Health Sciences
(Kinesiology)
Supervisor: Dr. Gail Frost
Faculty of Applied Health Sciences
Brock University
St. Catharines, ON
Alison Longo © June, 2009
ii
ABSTRACT
The Active Isolated Stretching (AIS) technique proposes that by
contracting a muscle (agonist) the opposite muscle (antagonist) will relax through
reciprocal inhibition and lengthen without increasing muscle tension (Mattes,
2000). The clinical effectiveness of AIS has been reported but its mechanism of
action has not been investigated at the tissue level. Proposed mechanisms for
increased range of motion (ROM) include mechanical or neural changes, or an
increased stretch tolerance. The purpose of the study was to investigate changes
in mechanical properties, i.e. stiffness, of skeletal muscle in response to acute
and long-term AIS stretching for the hamstring muscle group.
Recreationally active university-aged students (female n=8, male n=2)
classified as having tight hamstrings, by a knee extension test, volunteered for
the study. All stretch procedures were performed on the right leg, with the left leg
serving as a control. Each subject was assessed twice: at an initial session and
after completing a 6-week AIS hamstring stretch training program. For both test
sessions active knee extension (ROM) to a position of “light irritation”, passive
resisted torque and stiffness were determined before and after completion of the
AIS technique (2x10 reps). Data were collected using a Biodex System 3 Pro
(Biodex Medical Systems, NY, USA) isokinetic dynamometer. Surface
electromyography (EMG) was used to monitor vastus lateralis (VL) and
hamstring muscle activity during the stretching movements. Between test
sessions, 2x10 reps of the AIS bent knee hamstring stretch were performed daily
for 6-weeks.
iii
Subjects extended the knee significantly further (session 1: 158.4°±12.6;
session 2: 173.3°±11.5) after completing the long-term stretching program
(p≤0.05). After a single bout of AIS there was a trend toward increased ROM
within the first session, however it was impossible to determine conclusively
whether this change was statistically significant, due to changes in the control leg
ROM. No significant change was found in stiffness values. In both test sessions
hamstring activity was significantly less than VL activity during AIS, when
expressed as %MVC. Long-term AIS appears to be effective at increasing ROM.
A trend for the immediate benefits is also evident. AIS does not appear to affect
mechanical mechanisms because there was no change in stiffness values. The
contribution of neural mechanisms is also apparent and requires further
investigation.
iv
ACKNOWLEDGEMENTS
I have thoroughly enjoyed my time at Brock and am deeply grateful to
several people, without whom this accomplishment would not have been
possible. Dr. Gail Frost, thank you for your guidance, support and the endless
editing of my “final” thesis versions. Dr. David Gabriel, thank you for your
contributions to the project design and statistical analysis, and for the
encouragement at OBC. Dr. Bareket Falk, thank you for being the “devil‟s
advocate”, your comments were very valuable. I would still be trying to figure out
Matlab if it wasn‟t for Cam Mitchell, thank you for the amazing programs and for
patiently answering each and every one my questions. Thank you for the last
minute modeling session Emily. And thank you to the Brock Athletic Therapy
Staff for introducing me to the AIS technique, encouraging me to begin this whole
process, and for allowing me the opportunity to work with BWH for the last two
seasons.
v
TABLE OF CONTENTS
Page
Abstract ................................................................................................................. ii
Acknowledgements .............................................................................................. iv
Table of Contents ................................................................................................. v
List of Figures ...................................................................................................... vii
List of Tables ...................................................................................................... viii
Chapter 1 INTRODUCTION ................................................................................ 1
1.1 Background .......................................................................................... 1
1.2 Active Isolated Stretching (AIS) ........................................................... 2
1.3 Purpose ............................................................................................... 4
1.4 Hypotheses .......................................................................................... 4
Chapter 2 LITERATURE REVIEW ...................................................................... 5
2.1 Tissue Composition ............................................................................. 5
2.1.1 Muscle Components ................................................................. 6
2.1.2 Fascia and Tendons ................................................................. 6
2.1.3 Joint Capsule ............................................................................ 9
2.1.4 Relevance to Stretching .......................................................... 10
2.2 Mechanical Properties ....................................................................... 11
2.2.1 Viscoelasticity ......................................................................... 11
2.2.2 Force-Deformation Relationship ............................................. 14
2.2.3 Passive Stiffness ..................................................................... 15
2.2.4 Biomechanical Effects of Stretching ........................................ 19
2.3 Active Isolated Stretching .................................................................. 22
2.3.1 AIS Research .......................................................................... 23
2.3.2 PNF Techniques ..................................................................... 25
2.4 Neural Pathways ................................................................................ 27
2.4.1 Muscle Spindles ...................................................................... 28
2.4.2 Golgi Tendon Organs .............................................................. 28
2.4.3 Stretch Reflexes ...................................................................... 29
2.4.4 Neural Impact of Stretching..................................................... 30
2.4 5 Fast Limb Movements ............................................................. 37
2.4.6 Mechanical and Neural Factors .............................................. 39
2.5 Pain Tolerance ................................................................................... 40
2.5.1 Pain Tolerance is Irrelevant .................................................... 41
2.5.2 Pain Tolerance is Relevant ..................................................... 42
vi
Chapter 3 METHODOLOGY.............................................................................. 48
3.1 Subjects ............................................................................................. 48
3.2 Testing Procedures Summary ........................................................... 48
3.3 Range of Motion ................................................................................ 53
3.4 Passive Resisted Torque ................................................................... 53
3.5 Active Isolated Stretching .................................................................. 54
3.6 AIS Intervention Program ................................................................... 55
3.7 Gravity Correction .............................................................................. 56
3.8 Data Treatment .................................................................................. 57
Chapter 4 RESULTS .......................................................................................... 59
4.1 Subjects ............................................................................................. 59
4.2 Range of Motion ................................................................................ 59
4.3 Stiffness ............................................................................................. 62
4.4 EMG ................................................................................................... 63
Chapter 5 DISCUSSION .................................................................................... 65
Chapter 6 CONCLUSIONS ................................................................................ 71
6.1 Limitations of the Study ...................................................................... 72
6.2 Future Investigations.......................................................................... 75
REFERENCES ................................................................................................... 76
vii
LIST OF FIGURES
Page
Figure 1 Muscle fiber and fascia layers ............................................................. 7
Figure 2 Visual representation of the elastic (A) and viscous (B) properties of
tissue ................................................................................................. 12
Figure 3 Visual representation of the viscoelastic properties of tissue ............. 12
Figure 4 Stress-strain curve.............................................................................. 14
Figure 5 Summary of the testing procedure for each test session ................... 49
Figure 6 Biodex chair set-up ............................................................................. 52
Figure 7 AIS bent knee technique .................................................................... 56
Figure 8 Maximal knee extension position for the right leg ............................... 60
Figure 9 Left leg and right leg ANCOVA results .............................................. 60
Figure 10 Right leg ANCOVA results, within and between sessions .................. 61
Figure 11 Torque-angle curve for a single subject .............................................. 62
Figure 12 Stiffness over the final 10% of right knee extension ROM .................. 63
Figure 13 Muscle activity during the AIS procedure ........................................... 64
viii
LIST OF TABLES
Page
Table 1 Subject Characteristics .......................................................................... 59
Table 2 MVC muscle activity values (RMS) for the VL ....................................... 64
1
CHAPTER 1
INTRODUCTION
1.1 Background
Therapists and athletes regularly include stretching skeletal muscles as
part of their rehabilitation and training programs. Stretching is a general term
used to describe any manoeuvre designed to lengthen shortened soft tissue
structures. Therefore stretching increases the range of motion (ROM), the degree
of movement, at the joint associated with the skeletal muscle. Investigations of
static, dynamic, ballistic and proprioceptive neuromuscular facilitation (PNF)
stretching techniques have identified mechanical and neural mechanisms which
may explain the increased ROM. A change in stretch tolerance is a third
proposed mechanism to account for the change in ROM due to stretching (Etnyre
& Abraham, 1986b; Taylor, Dalton, Seaber & Garrett, 1990; Halbertsma &
Goeken, 1994).
Static stretching is a process of slowly lengthening a muscle and then
holding it at a constant length for 30-60 seconds. This stretch technique utilizes
the mechanical properties of stress relaxation and creep, as well as the neural
mechanism of autogenic inhibition to increase ROM (Holcomb, 2000).
Dynamic stretching mimics sport specific movements by using quick
controlled movements that are within a normal ROM. Bouncing at the end ROM
is avoided. The movement patterns are specific to the sport and are often
included as part of the active warm-up because they help prepare the athlete for
2
athletic competition. The neural mechanism of reciprocal inhibition is utilized to
achieve the increase in ROM achieved from dynamic stretching (Holcomb, 2000).
Ballistic stretching involves fast active contractions of the agonist muscle
to elongate the antagonistic muscle. The end position is not held which creates a
bouncing type of movement that causes the joint to move into an extreme ROM.
Ballistic stretching triggers the myotatic stretch reflex, which limits the ability of
the antagonist muscle to achieve optimal lengthening (Holcomb, 2000).
Proprioceptive Neuromuscular Facilitation (PNF) stretching uses a
combination of active and passive movements with isometric, concentric and
eccentric muscle contractions to facilitate reciprocal and autogenic inhibition.
These neural mechanisms optimize muscular relaxation, which allows the
musculoskeletal structures to achieve a greater ROM (Holcomb, 2000).
These stretch techniques have been thoroughly investigated and have
been shown to be effective at improving joint ROM. Another stretch technique,
active isolated stretching (AIS), has not been investigated as rigorously. The
clinical effectiveness of AIS has been reported (Leimohn, Mazis & Zhang, 1999;
Marino, Ramsey, Otto & Wygand, 2001; Middag & Harmer, 2002) but, to my
knowledge, its mechanism of action has not been investigated at the tissue level.
1.2 Active Isolated Stretching (AIS)
The basic principle of AIS, as explained by its creator Aaron Mattes, is that
by contracting a muscle (agonist) the opposite muscle (antagonist) will relax
through reciprocal inhibition and lengthen without increasing muscle tension. AIS
3
uses multiple repetitions of stretches lasting less than two seconds, allowing the
muscle to optimally lengthen without triggering the protective myotatic reflex,
which inhibits the stretch potential. It is suggested that a greater stretch is
obtained via AIS because of the relaxed state of the antagonist muscle (Mattes,
2000).
The Mattes method is based on the practical application of Wolff‟s and
Sherrington‟s Laws. Wolff‟s Law states, “The form of the bone being given, the
bone elements place or displace themselves in the direction of the functional
pressure and increase or decrease their mass to reflect the amount of functional
pressure” (Mattes, 2000). The sheets of fascia that surround muscle have been
laid down in a very precise way along the lines of stress within the body and
therefore they need to be stretched along the same lines. The positioning of the
muscle and tendon is critical in stretching as proper alignment minimizes the
tension and friction allowing full elongation to occur.
When a muscle on one side of a joint is contracted, the muscle on the
opposite side sends a neurological signal to relax or release. This is
Sherrington‟s Law of reciprocal inhibition and muscle contraction (Mattes, 2000).
Inhibition of the reflexive myotatic stretch is also incorporated in the AIS
technique with slow, controlled, rhythmic stretches of less than two seconds.
Rapid stretching would trigger the myotatic stretch reflex, causing a contraction
of the antagonist muscle which would limit the stretch potential.
4
1.3 Purpose
Current research on the technique of active isolated stretching has
focused on the effect of AIS on ROM and comparison with the other stretching
techniques. The purpose of the study was to investigate mechanical properties of
the hamstring muscle group in response to the AIS technique. The study
assessed the acute and long term effectiveness of AIS and observed the
underlying mechanism of action.
1.4 Hypotheses
1. Active isolated stretching of the hamstrings will increase ROM at the knee.
2. Mechanical mechanisms will be responsible for the increase in ROM,
because the stiffness of the hamstring muscles will be decreased after
performing AIS.
5
CHAPTER 2
LITERATURE REVIEW
In order to investigate the three proposed mechanisms which account for
the change in ROM due to stretching, the underlying tissue components must be
analyzed. The structure, and the effects of stretching on the structure, of muscle
and connective tissue will be addressed. The neural components of muscle and
their effect on elongation will also be discussed. Research on the three proposed
mechanisms will be presented, as well as the research that has been conducted
on AIS and similar stretch techniques.
2.1 Tissue Composition
There is a close association between the contractile elements of muscle,
the muscle fibers, and the non-contractile elements of connective tissue, such as
fascia. Contractile muscle components are mostly protein based and surrounded
by connective tissue at several levels. Connective tissue is made up of various
densities and spatial arrangements of collagen fibers embedded in ground
substance. Collagen is a fibrous protein that has a very high tensile strength.
Collagenous tissue is found in many different structures, including tendons,
ligaments, joint capsules, aponeuroses and fascial sheaths, among others
(Hunter, 2000).
6
2.1.1 Muscle Components
The ability of muscle to contract is due to its very organized structure.
Actin and myosin myofilaments are responsible for the shortening of muscles.
Myofilaments are arranged longitudinally in the sarcomere, which is the smallest
contractile unit of skeletal muscles. Sarcomeres are linked successively to form a
long, thin strand called a myofibril. A muscle fiber is a cluster of hundreds to
thousands of myofibrils that have been grouped together by connective tissue.
The myofibrils run the entire length of the muscle fiber and create a cylindrical
cell that approximates the diameter of a human hair (Hunter, 2000). Up to 150
muscle fibers are bundled together by connective tissue to create a fasciculus.
The muscle belly is composed of numerous fasciculi.
The sarcomere is considered the contractile unit of the muscle. During
muscle contraction the length of the sarcomere decreases because the actin
filaments slide over the myosin filaments. Conversely, when the muscle relaxes
or is passively lengthened, the sarcomere increases in length. The ability of the
actual muscle fiber to shorten is due to the series arrangement of the sarcomeres
(Hunter, 2000).
2.1.2 Fascia and Tendons
Fascia is a three dimensional fibrous matrix that provides interconnections
throughout all cells of the body. There are four levels of fascia that surround
muscle: epimysium, perimysium, endomysium and sarcolemma. Epimysium
encases the entire muscle belly and is continuous with the tendons at the ends of
7
the muscle. Perimysium surrounds the bundles of muscle fibers (fasciculi).
Endomysium surrounds each individual muscle fiber (Hunter, 2000). The close
association of fascia and muscle fibers means the ability of the muscle to
elongate is directly influenced by the fascial layers.
Fig.1 Muscle fiber and fascia layers (Marieb, 2004)
Tendon is a continuation of the endomysium, perimysium and epimysium.
Tendon joins the muscle to the periosteum of bone (Figure 1). It is composed
mostly of collagen, and a relatively low proportion of elastin. Three protein
strands twist around each other in a triple helix to create a procollagen molecule.
The helical structure is stabilized by hydrogen bonds between the three strands.
The procollagen molecules align end to end, but they do not actually connect.
This creates a collagen filament. Filaments align themselves side by side, the
8
procollagen molecules in one filament overlap the gap between the molecules of
the adjacent filament. The parallel arrangement of filaments is called a microfibril.
Microfibrils are grouped together to become a fiber and the fibers are bundled
together to create the tendon. The strong chemical bonding, called covalent
cross-linking, that forms between adjacent collagen molecules throughout the
collagen bundle creates the strength of collagen. A complete collagen fiber is not
enclosed by a cell membrane, it is just a parallel arrangement of protein strands
that have been bunched together in the extracellular space because of a mutual
attraction and then chemically cross linked to form a stable structure (Hunter,
2000). Distributed between the collagenous bundles are parallel rows of
fibroblasts and a fine reticulum of elastic fibers (LaBan, 1962). Collagen fibers
are capable of only a slight degree of extensibility but are very resistant to tensile
stress. The small percentage of elastin fibers provides minimal elasticity to the
tendon.
When a relaxed muscle is physically stretched most, if not all, of the
resistance to stretch is derived from the extensive connective tissue framework
and sheathing within and around the muscle, not from the myofibrillar elements
(Sapega, Quendenfield, Moyer & Butler, 1981). The parallel arrangement of the
fascia layers and muscle fibers allows this to take place. The muscle fiber
maintains its normal resting length while the connective tissue reacts to the
stretch forces. Change in length when the muscle relaxes or is passively
lengthened occurs as the parallel connective tissues slide over the myofibril
elements.
9
2.1.3 Joint Capsule
All synovial joints in the body are surrounded by a joint capsule. The joint
capsule is connective tissue composed of collagen and elastin. The capsule joins
the articulating bones. Ligaments also surround a joint to provide stability. The
structure is similar to that of tendons; they are mostly parallel bundles of collagen
with interwoven elastin fibers. However, the higher percentage of elastin fibers
and the more random arrangement make ligaments more pliant and flexible
(Conroy & Earle, 2000). All joints have a close-packed and an open-packed
position. The close-packed position for the joint is when the articular surfaces of
the bones are most congruent. To create the firm approximation of the boney
structures the joint capsule and ligaments are taut and often twisted. During
passive movements these inert structures may provide resistance to limit the end
ROM for the joint. The close-packed position for the knee, specifically the
tibiofemoral joint, is knee extension with lateral tibial rotation (Hartley, 2000).
Therefore, when the knee is fully extended the ligaments and joint capsule are
stressed, along with the fascia and tendons of the hamstring muscles. There are
no boney limitations to the tibiofemoral motion in the sagittal plane, and as a
result, anterior and posterior stability is controlled by the ligaments and muscles
around the joint (Hartley, 2000). Therefore, the position of the joint during a
stretch maneuver is important as it can determine whether certain structures are
affected or unaffected by the stretch.
10
2.1.4 Relevance to Stretching
Although the amount of joint ROM is determined primarily by the shape
and congruency of the articulating surfaces, under normal circumstances
connective tissue is the major structure limiting joint motion (Sapega et al., 1981).
Relative contributions to the resistance to stretch in the midrange of joint motion
are 47% joint capsule, 41% passive motion of muscle, 10% tendon and 2% skin.
The restraining effect of the tendons becomes more important towards the
extremes of joint motion (Johns & Wright, 1962). The extensibility of tendon and
fascia surrounding the muscle belly controls the ability of the muscle-tendon unit
to elongate and therefore protects the myofibrils from being stretched to a point
of damage (Purslow, 1989). It has been said that tendons act as mechanical
buffers to protect muscle fibers from abrupt length changes (McHugh et al.,
1999).
There are two different methods by which fascia influences the ability of
the musculotendinous unit to elongate. The series elastic component utilizes the
longitudinal arrangement of muscle fibers and tendon, the endomysium in
particular. The function of the endomysium is to transfer force from the contractile
component to the tendon and bone in series (Magnusson et al., 1996c).
Connective tissue also surrounds the muscle fibers and has the ability to slide
parallel to the muscle fibers creating the parallel elastic component of fascia. The
perimysium is suggested to be the principal component in the parallel elastic
component. Its role is to distribute stress evenly and to prevent over-stretching
(Kubo, Kanehisa & Fukunaga, 2002). When a muscle is passively stretched the
11
length-tension relationship is used to define passive muscle stiffness (Gajdosik &
Bohannon, 1987). It is unclear if the passive muscle stiffness represents the
series or the parallel elastic component. A study by McHugh et al. (1999) reports
the passive muscle stiffness measurement is a reflection of the extensibility of
the tendon-aponeurosis complex, the series elastic component. Conversely,
Kubo, Kanehisa and Fukunaga (2001) found that passive stiffness is not related
to the extensibility of the tendon structure, but to that of the connective tissue
elements in parallel with the muscle fibers. Regardless of which component of
the connective tissue it measures, both studies concur that the passive muscle
stiffness represents the extensibility of the fascia, not the muscle fiber.
2.2 Mechanical Properties
A change in the mechanical properties of the muscle-tendon complex is
one mechanism that has been proposed to explain the increase in ROM that
occurs after stretching (Taylor et al., 1990; McHugh, Kremenic, Fox & Gleim,
1998). The mechanical properties characterize how the musculotendinous unit
structures respond to a stretch force.
2.2.1 Viscoelasticity
The process of elongating tissue is stretching, while the actual elongation
or linear deformation is referred to as stretch. There are two types of stretch:
elastic and plastic. Spring-like behavior, where elongation produced by tensile
loading is recovered after the load is removed, represents elastic stretch.
12
Therefore, it is also described as temporary or recoverable (Sapega et al., 1981).
Elastic properties are visually represented by Hooke's model of a perfect spring
(Figure 2A) (Taylor et al., 1990). Plastic stretch exhibits putty-like behavior. Even
after the tensile stress is removed the linear deformation remains. This is
described as nonrecoverable or permanent elongation (Sapega et al., 1981).
Viscous properties of the tissue permit plastic deformation (Sapega et al., 1981)
and are represented by Newton's model of a hydraulic piston known as a
dashpot (Figure 2B) (Taylor et al., 1990). Muscle, as with most biological tissue,
displays both elastic and plastic, or viscous, properties and therefore is said to
behave viscoelastically. Biomechanical models represent viscoelastic
characteristics by combining springs and dashpots in series and parallel (Figure
3) (Taylor et al., 1990).
Fig. 2 Visual representation of the elastic (A) and viscous (B) properties of tissue
(Taylor et al., 1990)
Fig. 3 Visual representation of the viscoelastic properties of tissue (Taylor et al.,
1990)
13
Stretching techniques utilize the characteristic properties of viscoelastic
material to achieve elongation of the musculotendinous tissue. Viscoelastic
properties include stress relaxation, creep, hysteresis and strain rate
dependence. Stress relaxation occurs when the material is stretched and then
held at a constant length. The force at that length gradually declines. Creep is
characterized by continued deformation at a fixed load. Hysteresis is the variation
in the load-deformation relationship that takes place between loading and
unloading. The strain rate dependence of viscoelastic material means it exhibits
higher tensile stresses at faster strain rates (Taylor et al., 1990).
Stretching techniques should primarily focus on promoting plastic
deformation, as the goal is to permanently increase the ROM. The relative
proportion of elastic and plastic deformation varies depending on how, and under
what conditions, the stretching is performed. Factors that influence the proportion
of plastic and elastic stretch are amount of applied force, duration of applied
force and tissue temperature. Low-force, long-duration stretching at elevated
temperatures favors viscous or plastic deformation. Also, low forces and higher
temperatures minimize structural weakening of the tissue (Sapega et al., 1981).
An elevated temperature decreases the stiffness and increases the
extensibility of the tissue. It is thought that this mechanism of action in
collagenous material occurs due to intermolecular bonds becoming partially
destabilized, enhancing the viscous flow properties of the tissue (Sapega et al.,
1981). Therefore, to optimize stretching potential, and to prevent damage, a
warm-up should be performed before any stretching procedure.
14
2.2.2 Force-Deformation Relationship
Force and deformation are two critical mechanical properties that are
commonly measured when testing tendon structures: an externally applied force,
expressed in units of Newtons (N), will cause the structure to elongate (deform),
which is expressed in units of meters (m). Hooke's law describes the linear
relationship between force and deformation. However, during dynamic loading
tendon displays a non-linear viscoelastic response with an initial toe-region
followed by a progressively steeper region (Magnusson, Hansen & Kjaer, 2003).
A stress-strain or torque-angle curve is often used to visually describe this
phenomenon (Figure 4). The stiffness of the material is expressed as the
relationship between stress and strain, the slope of the line in the steeper portion
of the curve (Magnusson, 1998). As previously discussed, during passive
stretching the stiffness refers to the passive extensibility of fascia.
Fig. 4 Stress-strain curve (Kisner & Colby, 1996)
15
The shape of the curve is due to the underlying composition of the
connective tissue. The collagen molecules that comprise connective tissue
possess relatively strong intermolecular van der Waals bonds. However, the
bonds between the helical procollagen molecules are relatively weak. When a
force is applied to these materials, this disproportion in bonding strengths permits
adjacent chains of molecules to slip over one another allowing a response to
stress. Initially the molecules are disoriented; with application of increasing force
the helical composition becomes longitudinally aligned. This change in
orientation strengthens the intermolecular bonds and increases resistance to
further deformation (LaBan, 1962). Therefore, the molecules become aligned in
the toe region and the linear portion represents the actual elongation of the
collagen tissue.
2.2.3 Passive Stiffness
In the past, studies have used goniometric ROM measurements to
represent flexibility, however this is an invalid method (Gajdosik, 1995).
Goniometric results should be reported and interpreted as ROM measurements
only, not as measurements of factors that may affect ROM (Gajdosik &
Bohannon, 1987). Stress and strain account for the dimensions of the structure,
and thereby provide information only about its qualitative properties. Force and
deformation provide information about the quantitative mechanical behavior of a
structure (Magnusson et al., 2003). However, complex passive movements were
found to be difficult to assess reliably. It has been suggested that force
16
dynamometers should be used to standardize the amount of passive force
applied, and thereby decrease the potential for error (Gajdosik & Bohannon,
1987). Nuyens et al. (2000) used repeated passive isokinetic knee movements to
investigate the test-retest reliability of torque measurements in healthy subjects.
Passive flexion and extension movements were repeated 10 times consecutively
with a 5-second rest interval between each movement. The test involved three
sets with the angular velocity set at 60, 180 and 300 °/s respectively. After the
three sets of 10 consecutive movements were performed the subject was
removed from the isokinetic apparatus for 20 minutes, and then repositioned and
re-tested. Results demonstrated that torque can be reliably measured during
consecutive passive movements. They caution that standardization of patient
positioning is essential for the repeatability of measurements on different test
occasions (Nuyens et al., 2000).
Resistance to stretch is defined as the passive torque, in Newton meters
(Nm), offered by a muscle group during passive elongation using an isokinetic
dynamometer (Magnusson, 1998). The isokinetic dynamometer also provides
information on the joint angle positioning. These data can then be used to create
a torque-angle curve. Stiffness is defined as the change in torque (Nm) divided
by the change in position (radians) and is calculated from the slope of the torque-
angle curve (Magnusson, 1998). Passive stiffness can be calculated using
different mathematical models (Nordez, Cornu & McNair, 2006). A second-order
polynomial, a fourth-order polynomial or an exponential model can be used to fit
the torque-angle data to calculate stiffness.
17
The role of gravity during passive movement should be addressed. Gravity
opposes knee extension and therefore, unless corrected for the effect of gravity,
the torque values for passive resisted extension will be an overestimation.
Gravitational moment can be calculated by different methods. The most
frequently used technique is an isokinetic assessment, however, a more accurate
anthropometric model has been identified (Kellis & Baltzopoulos, 1996). In the
isokinetic assessment of gravitational moment the torque resulting from the effect
of gravity on the combined weight of the leg and the dynamometer is the gravity
effect torque (GET) (Fillyaw, Bevins & Fernandez, 1986). To attain the GET for
knee extension the subject is positioned the same as the testing protocol. The
dynamometer is set at a speed of 30°/s. The examiner holds the knee in full
extension. The subject is instructed to remain completely relaxed and allow the
extremity to fall passively against the resistance offered by the dynamometer
from full extension through to 90° of flexion. The peak torque (GETθ1) and the
angle (θ1) of the peak torque during this movement are the important recordings.
With these values the GET can be calculated for the angle of knee flexion at
which peak torque occurs (θ2).
1
21
2cos )(cos
GET
GET
The gravity corrected hamstring peak torque value is calculated by
subtracting the GETθ2 from the uncorrected peak hamstring torque measured by
the dynamometer (Fillyaw et al., 1986). This method assumes that gravitational
moments measured by the dynamometer are not influenced by the elastic forces
developed due to the viscoelastic nature of the connective tissue in the
18
musculotendinous units of the hamstrings. Furthermore, it is assumed that there
is no force exerted by the muscle groups involved because the limb is completely
relaxed during the procedure. This, however, is difficult to achieve in a laboratory
environment. The gravitational moments obtained by an isokinetic dynamometer
have been found to be significantly different from those obtained using
anthropometric data because of these assumptions. The anthropometric model is
not affected by muscle action factors, and therefore is considered to be a more
accurate method for gravitational moment estimation. The anthropometric model
uses the following equation to calculate the moment when the leg is in full
extension:
)06.0()437.0( BWlM
where M = gravitational moment, l = length of the limb (m), 0.437 = COM-
proximal joint distance/segment length, 0.06 = leg-foot weight/body weight and
BW = body weight (N). The gravitational moment at any other angle A (MA) can
be determined by the following cosine function:
)30(cos
)cos(
AM
MA
The gravity corrected hamstring torque values can be calculated by subtracting
the MA from the uncorrected hamstring torque values measured by the
dynamometer (Kellis & Baltzopoulos, 1996).
The validity for the passive stiffness measurement has been investigated
(Kubo et al., 2001; McHugh et al., 1998). Both investigations found a negative
correlation between passive stiffness and maximal ROM. Hence, a decrease in
passive stiffness was related to an increase in maximal ROM. Therefore, it
19
seems reasonable to suggest that the passive stiffness measured in the studies
can be an index of flexibility (Kubo et al., 2001).
2.2.4 Biomechanical Effects of Stretching
The following studies have confirmed that the increase in ROM after
stretching was due to a change in the mechanical properties of the muscle-
tendon unit.
The muscle-tendon unit responds viscoelastically to tensile loads and is
not influenced by reflex activity (Taylor et al., 1990). Clinically relevant
biomechanical stretching properties in an entire muscle-tendon unit under
passive stretch were experimentally evaluated. The study of rabbit extensor
digitorum longus and tibialis anterior muscle-tendon units was divided into three
parts. In the first part characteristics of repeated stretching of muscle-tendon
units to a predetermined length were examined. This is similar to a cyclic
stretching technique to a given length. The second section involved muscle-
tendon units being stretched repeatedly to the same tension and held at a fixed
length. This section simulated the technique of static stretching. The final part of
the investigation examined how varying the stretch rates and denervating the
muscle affects muscle-tendon stretching. Sustained muscle-tendon unit
elongation occurred with the application of simulated static and cyclic stretching
techniques. The viscoelastic properties of stress relaxation, creep and hysteresis
were found to be responsible for the change in tissue length. In other words, a
change in the mechanical properties should allow for a greater ROM and greater
20
flexibility of a joint. The denervated muscles responded similarly to the innervated
muscles, therefore the muscle response to stretch can be explained by
viscoelastic properties alone, exclusive of stretch reflexes. The stretch reflex may
exist during muscle stretching techniques, but in this investigation it had no
significant force contributions (Taylor et al., 1990).
It was also noted that for both stretch techniques the greatest changes in
the muscle-tendon unit occurred in the first four stretches. The ideal number of
repetitions may have been influenced by the magnitude of the stretching force or
the duration of the hold. However, a minimal number of stretches appear to be
most effective in muscle-tendon unit elongation. Any subsequent stretches will
still bring about length increases, but these increases will be small and less
significant (Taylor et al., 1990).
McHugh et al.‟s (1998) data explains musculoskeletal flexibility in
mechanical terms rather than by neural theories. Passive mechanical force and
stretch induced contractile responses to stretch were examined to see if they
limited maximum straight leg raise (SLR) range of motion. Sixteen recreational
athletes performed a SLR stretch with the knee braced in full extension. The end
point was when the subject reported discomfort. Throughout the movement a
load cell was attached to the ankle to measure torque and surface electrodes
recorded the activity of the rectus and biceps femoris muscles. An
electrogoniometer reported the hip flexion ROM. Torque-ROM curves were
plotted for zero ROM to maximum ROM and back to zero ROM. The results of
the study suggest that the SLR test was primarily a measurement of the passive
21
mechanical forces resisting hip flexion motion. The parallel elastic component
accounted for 79% of the variation in hip flexion ROM. A lack of EMG response
during the stretch indicates that the perception of discomfort during slow passive
stretching precedes any reflex contractile response. It was suggested that reflex
activity would contribute to the resistance of the stretch if the ROM had continued
beyond the point of discomfort or during a rapid stretch (McHugh et al., 1998).
McHugh et al. (1999) examined the effect of passive muscle stiffness on
symptoms of muscle damage after eccentric exercise. The SLR stretch was used
to classify subjects as stiff, normal or compliant. The subjects then performed 6
sets of 10 isokinetic submaximal eccentric actions of the hamstring muscle
group. Greater symptoms of muscle damage were reported in subjects with stiff
hamstring muscles, and attributed to the sarcomere strain theory of muscle
damage. They consider the passive muscle stiffness to be a reflection of the
tendon-aponeurosis extensibility. The tendon is supposed to act as a mechanical
buffer by absorbing lengthening strain to protect the muscle fibers. It is proposed
that muscle damage occurs when a rigid tendon-aponeurosis complex transfers
the strain imposed by active lengthening to the muscle fibers.
Reid and McNair (2004) were the first to investigate muscle stiffness in the
new ROM achieved after a stretch program. Forty-three school-aged students
participated in the study, 23 subjects in the intervention group performed a
stance phase static hamstring stretch five days per week, once per day, for 3
repetitions of a 30 second hold. Twenty subjects in the control group did not
stretch over the 6-week intervention period. The experimental group increased in
22
knee extension ROM, passive resistive force and stiffness after the six weeks.
No significant differences were observed in the control group data for the same
variables. The increase in stiffness that accompanied the new ROM, 10° of knee
extension, provides evidence that changes in structural characteristics of the
tissue had occurred as a result of the stretch program. An increase in muscle
fiber length due to the addition of sacromeres in series and an increase in the
amount of connective tissue in the muscle are two suggested mechanisms for
the increase in stiffness, as they have been documented in studies involving
animals (Herbert & Balnave, 1993; Williams & Goldspink, 1973).
2.3 Active Isolated Stretching
The Mattes Method of active isolated stretching has been practiced for the
last 30 years. The creator, Aaron Mattes, developed this therapeutic myofascial
technique to promote the functional and physiological restoration of muscles,
tendons, ligaments and joints to facilitate healthier fascial planes. The technique
ostensibly uses active movement and reciprocal inhibition to achieve optimal
flexibility. The stretch is most effective when it is performed in 1.5 to 2 seconds;
this prevents the reflexive contraction of the antagonist muscle. A specific muscle
or group of muscles is identified and precise localized movements are
implemented to isolate the stretch of the muscle. The muscle is stretched slightly
beyond the point of light irritation by the application of mild pressure, less than
0.5 kg of resistance. The pressure is released and the muscle is returned to the
starting position in which the muscle is at resting length. The procedure is
23
repeated for a prescribed number of repetitions, 5-15, depending on the muscle
(Mattes, 2000).
2.3.1 AIS Research
Research specifically involving AIS is limited to three studies, each
employing a different protocol. The studies have focused solely on the effect of
AIS on ROM and comparison with other stretching techniques. To my knowledge
no investigations have focused on the underlying mechanisms of action or the
effect at the tissue level.
Leimohn et al. (1999) used active straight leg raise performance to
compare the effect of active isolated and static stretch training. Fifteen male and
fifteen female subjects, ranging from 18 to 25 years, were tested. Maximum
iliofemoral flexion ROM was measured goniometrically with the knee braced in
full extension. The AIS and static stretch groups performed nine supervised
training sessions over a 3-week period. The AIS group raised the leg to active
end ROM and then used a rope to pull it to passive end ROM. The stretch was
held for 2 seconds and was repeated 14 times. In the static stretch group the
heel was raised off the ground to a height where end ROM was achieved. The
static stretch was performed once and held for 30 seconds. Both groups showed
significant improvements in ROM. However, the AIS group improved significantly
more than the static stretch group. The results indicated that AIS training
produced greater ROM gains than static stretching, for these procedures
(Leimohn et al., 1999).
24
Marino et al. (2001) also compared the effects of active isolated and static
stretching. Twenty-four female and six male subjects, aged 22.6 ± 1.3 years,
were included in the study. Sit and reach and goniometric hip flexion ROM were
used to evaluate both techniques, as well as a control group. The stretch
intervention groups performed 60 seconds of stretching three times per week, for
13 weeks. Only goniometric measurement of hip flexion for AIS was found to
have significant changes. It was suggested that accuracy of the instrument may
have effected the change in goniometric measurement but not the sit and reach
measurement. It was concluded that a low dose flexibility intervention, 3 sessions
per week of only 60 seconds per session, was insufficient to elicit measurable
changes in ROM (Marino et al., 2001).
Another comparison of active isolated and static stretching techniques
found no difference in their effectiveness (Middag & Harmer, 2002). College-
aged recreational athletes actively warmed up for 5 minutes and then performed
either the AIS or the static stretching procedure once a day, 5 times per week for
3 weeks. Both groups showed statistically significant increases in hamstring
ROM compared to the baseline measurements. However, the groups did not
differ greatly in their improvements as the static stretching group increased 8%
while the AIS group improved 11%. These results show that minimal time and
effort is required to improve hamstring ROM in healthy active individuals.
Traditional static stretching was just as effective as the AIS technique (Middag &
Harmer, 2002).
25
2.3.2 PNF Techniques
Due to the limited amount of research on the actual AIS technique it is
necessary to review other techniques that closely resemble the AIS procedures.
In the late 1940s PNF stretching was developed by Herman Kabat, MD, based
on Sherrington‟s concepts of muscle facilitation and inhibition. The techniques
were originally used to treat neuromuscular disorders by restoring strength. More
recently PNF methods have been developed to increase ROM. The most
frequently used techniques are hold-relax (HR), contract-relax (CR) and agonist
contract-relax (Houglum, 2001). HR uses an isometric contraction of the muscle
to be stretched followed by lengthening. CR uses a concentric contraction of the
muscle to be stretched followed by lengthening. The agonist contract-relax
method, is also referred to as Contract-Relax Agonist-Contract (CRAC). CRAC
uses a concentric contraction of the opposing muscle followed by passive
lengthening of the muscle to be stretched, and is very similar to AIS. The
contraction of an agonist muscle to initiate reciprocal inhibition in the antagonist
muscle is the basis for both techniques. The techniques vary in the timing and
application of external force to promote lengthening. There have been several
studies investigating PNF methodology and effectiveness.
Etnyre and Lee (1987) published a review summarizing the findings of
previous flexibility studies. Based upon practical applications and experimental
procedures among various comparative investigations, PNF methods were found
to be more effective and efficient than static or ballistic stretching techniques in
producing greater ROM. The CRAC technique was found to be the most effective
26
PNF method to increase the ROM. They also conclude that future studies should
continue for an extended period with regular measurements to determine if and
when differences between methods occur (Etnyre & Lee, 1987).
Holt, Travis and Okita (1970) compared the effects of ballistic, static and
CRAC stretching techniques on hamstring flexibility via a sit and reach
instrument. The CRAC technique positioned the subject lying supine with both
knees extended throughout the procedure. The subject flexed the hip to raise the
leg until a stretch was felt in the hamstrings. An examiner stabilized the leg while
the subject performed a 6-second isometric contraction of the hip extensors. This
was immediately followed by a contraction of the hip flexors to increase the
elongation of the hip extensors, the hamstrings. Three contractions were
repeated and followed by a 10-second rest period, after which the other leg was
stretched. This process of alternating between legs was continued for two
minutes. A second CRAC stretching technique was also performed from a
standing position. The same procedures were followed with the subject flexing
the trunk forward to initiate a stretch of the trunk extensors. The trunk extensors
contracted isometrically and then the trunk flexors concentrically contracted to
increase the ROM in the trunk extensors. The CRAC method produced
significantly greater flexibility scores than the ballistic and static stretching
techniques (Holt et al., 1970).
The effect of static, CR and CRAC stretch techniques on hip ROM was
investigated in men, aged 17-26 years (Cornelius & Hinson, 1980), and in
females, aged 16-23 years (Moore & Hutton, 1980). The CRAC technique
27
involved the subjects starting in supine lying position with the leg held straight. In
the study by Moore and Hutton (1980) a brace was applied to passively hold the
knee in extension. The subject actively flexed the hip to raise the leg from the
ground, a 3-6 second isometric contraction of the hip extensors was followed by
a concentric contraction of the hip flexors resulted in further elongation of the
hamstrings. A greater degree of hip flexion was found using the CRAC stretch
technique compared to the static and CR methods (Cornelius & Hinson, 1980;
Moore & Hutton, 1980).
The CRAC stretching technique was also found to be the most effective in
increasing ROM in the soleus muscle. Static, CR, and CRAC techniques were
performed on separate days by 12 subjects. Both PNF techniques were more
effective that static stretching and the CRAC technique was found to be superior
to the CR method of stretching (Etnyre & Abraham, 1986a).
2.4 Neural Pathways
The effect of muscle stretching in humans has been discussed as a
function of the passive mechanical properties of the muscle-tendon unit.
However, it has also been suggested that the increase in joint ROM is due to
neural components that are activated by some stretch techniques (McHugh et al.,
1998).
The central nervous system (CNS), the brain and the spinal cord, collects
information from afferent nerves and initiates the body‟s response via efferent
nerves. Proprioception is the body‟s ability to consciously and unconsciously
28
respond to the afferent stimuli regarding position (Houglum, 2001). Located
within joints, muscles and tendons, are specialized sensory receptors called
proprioceptors. They are sensitive to pressure and tension, and relay information
concerning muscle dynamics to the CNS. Two significant mechanoreceptors are
the muscle spindles and Golgi tendon organs (GTOs) (Harris & Dudley, 2000).
These mechanoreceptors are responsible for the activation of neural reflexes that
influence the ability of the muscle-tendon unit to elongate during a stretch
technique.
2.4.1 Muscle Spindles
Muscle spindles are modified fibers, called intrafusal fibers, which run
parallel to the normal, extrafusal, muscle fibers. They provide information
concerning muscle length and the rate of change in length. When a muscle is
stretched, deformation of the muscle spindle occurs and activates the sensory
neuron. An impulse is sent to the spinal cord where it synapses with a motor
neuron innervating the same muscle, and a sensory neuron innervating the
reciprocal muscle. Thus, muscle spindles control the myotatic stretch reflex and
reciprocal inhibition (Harris & Dudley, 2000).
2.4.2 Golgi Tendon Organs (GTO)
GTOs are located near the myotendinous junction in tendons. They are
attached in series, end to end, with extrafusal muscle fibers. When the
musculotendinous unit is stretched, tension in the muscle and tendon increases
29
causing the discharge from the GTO to increase. The sensory neuron of the GTO
activates an inhibitory interneuron in the spinal cord, which in turn synapses with,
and inhibits, a motor neuron serving the same muscle. The result is a reduction
in tension within the muscle and tendon. It is a protective mechanism against the
development of excessive tension, called autogenic inhibition (Harris & Dudley,
2000).
2.4.3 Stretch Reflexes
Activation of the proprioceptors initiates a neural response which may be
advantageous or disadvantageous to tissue elongation. The myotatic stretch
reflex occurs when rapid stretching of a muscle lengthens both the extrafusal
muscle fibers and the muscle spindles. This deformation activates the sensory
neuron of the spindle, which sends an impulse to the spinal cord. In the spinal
cord, the sensory neuron synapses with motor neurons. This results in an
activation of the motor neurons that innervate the same muscle, and causes the
muscle to contract. The resulting contraction is roughly equal in force and
distance to the original stretch stimulus. This protective reflex prevents injury that
could result from a rapid increase in muscle length. The myotatic stretch reflex
inhibits a muscle‟s ability to lengthen and therefore a stretching technique should
avoid activating this reflex (Holcomb, 2000).
When a muscle contracts the increase in muscular tension is sensed by
the GTO. The sensory neuron of the GTO sends a signal to the spinal cord,
where it synapses with a motor neuron which inhibits the contracting muscle. The
30
GTO causes the muscle to reflectively relax, this is called autogenic inhibition.
Tension built up during an active contraction stimulates the GTO, causing a
reflexive relaxation of the muscle, which optimizes any subsequent passive
stretching (Holcomb, 2000).
Another reflex that helps a stretching technique to optimally lengthen
tissue is reciprocal inhibition. When the sensory neuron from the muscle spindle
synapses in the spinal cord with a motor neuron it also synapses with another
sensory neuron which innervates the reciprocal muscle. Activation of the sensory
neuron causes inhibition in the reciprocal muscle. Therefore, contraction of the
agonist muscle causes a reflex relaxation of the antagonist muscle. The principle
of reciprocal inhibition is also referred to as Sherington‟s Law (Holcomb, 2000).
2.4.4 Neural Impact of Stretching
Reciprocal inhibition is the key concept in both AIS and the CRAC
technique of PNF stretching. Investigations of reciprocal inhibition have found
mixed results, and its role in contributing to musculoskeletal flexibility remains
controversial. The following are studies which support the idea that reciprocal
inhibition promotes muscle relaxation and therefore allows for greater flexibility.
Etnyre and Abraham (1986b) investigated the neuromuscular influences of
different stretching procedures. The neural mechanisms considered in the
rationale of stretching methods involve the activation of muscle spindles and
GTOs. The level of inhibitory or excitatory influence from these proprioceptors on
the motor pool excitability may be observed by evaluating the Hoffman reflex (H-
31
reflex). The H-reflex is an electrical stimulation of a nerve that recreates the
myotatic stretch reflex that occurs when a muscle is stretched. The only
difference is that the H-reflex bypasses the muscle spindle (Palmieri, Ingersoll &
Hoffman, 2004). Each of 12 subjects was randomly assigned to a treatment-
order group. The techniques of static, CR and CRAC stretching were performed
on separate days for each subject. A marked and lasting suppression of the
motor pool excitability, in agreement with the studies of reciprocal inhibition, was
demonstrated by the CRAC-PNF stretching technique. It is believed that optimal
muscle elongation occurs when the muscle is in a relaxed state, thus when
greater motor pool inhibition is evident. The results of this investigation suggest
that PNF methods, particularly those involving reciprocal activation of muscles,
provide the greatest potential for muscle lengthening (Etnyre & Abraham, 1986b).
The rationale that contraction of the muscle being stretched is inhibited by
antagonist contraction is supported by Etnyre and Abraham (1988), and is
consistent with those studies which indicate that the CRAC technique shows
greater ROM gains. They used fine wire and surface electrodes to investigate the
muscle activity in plantar flexor and dorsiflexor muscles during a stretch
technique similar to the CRAC method to stretch the plantar flexors. Examination
of recordings from the wire electrodes showed no activity in the soleus muscle
during tibialis anterior contraction. This indicated that reciprocal inhibition
occurred during the CRAC technique. EMG recorded with surface electrodes
contained inter-muscle cross-talk and therefore appeared to show suppressed
32
reciprocal inhibition phenomena. The researchers suggested that care must be
taken when using surface electrodes (Etnyre & Abraham, 1988).
Surface electrodes are often used because they are less invasive, easy to
apply and more accessible to researchers. A disadvantage is that they are said
to only be effective for superficial muscles and cannot detect signals from small
muscles. However, they are very effective for clinical assessments of time and
magnitude of activation and studies of general gross relaxation to tenseness
(Basmajian & Deluca, 1985).
The motorneuron pool excitability during stretching of muscle by
contracting the antagonistic muscles was evaluated by Guissard, Duchateau and
Hainaut (1988). Static stretching and CR techniques for the triceps surae
muscles were performed by 28 healthy physical education students. The tendon
(T-) and H-reflexes were analyzed during the stretching procedure. The T- reflex
occurs when an external force is used to increase tension in a tendon, thus
activating the neural pathways that are normally stimulated by the GTO during a
stretch procedure. Inhibition of the motor neuron pool excitability was present
during all three stretch techniques. Reciprocal inhibition was the rationale used to
explain the result; contraction of the opposing muscle inhibited the muscle that
was targeted to be stretched. It was noted that the inhibition of the motor neuron
pool stopped as soon as the stretching technique ceased (Guissard et al., 1988).
In 2001 Guissard, Duchateau and Hainaut further investigated the concept
of reciprocal inhibition during stretch techniques. They concluded that both pre-
and postsynaptic mechanisms caused reduced motoneurone excitation during
33
stretching. EMG was used to record activity in the soleus muscle in response to
electrical stimulation of the tibial nerve at the popliteal fossa (the H- reflex), and
at the ankle (the exteroceptive reflex). The results suggested that during small-
amplitude stretching the decreased H-reflex should be related to presynaptic
mechanisms and thus located at pre-motoneural level. It was proposed that the
postsynaptic inhibitory mechanisms contributed to the decreases in both reflexes
and the motor-evoked potential (MEP), for large-amplitude stretching. Inhibitory
afferents from the GTO are thought to play a greater role in decreasing
motoneurone excitability via Ib fibers during large-amplitude stretching. This is
consistent with the fact that the GTOs, which respond mainly to the muscle fiber
contraction force, are hardly sensitive to the mechanical tension of passive
stretching and only seem to be activated during large amplitude stretching
(Guissard et al., 2001).
Shindo, Harayama, Kondo, Yanagisawa and Tanaka (1984) also
investigated the Ia inhibition mechanism by analyzing the suppression of the
soleus H-reflex by stimulation of the peroneal nerve. During dorsiflexion, Ia
inhibition of the soleus motoneurones was demonstrated in four out of the five
subjects. The mechanism for inhibition of the pathway during plantar flexion was
considered to be inhibition of the Ia interneurone of the flexor side by the Ia
interneurone of the antagonist extensors (Shindo et al., 1984).
Investigations have also found results that disagree with the concept of
reciprocal inhibition. In 1980, Moore and Hutton used EMG to investigate the
relative level of muscle relaxation achieved during the application of static and
34
modified PNF stretch procedures. Participants were female gymnasts, 17-23
years of age. It was thought that gymnasts would understand and perform the
stretch training techniques with less apprehension because of their daily
experience with flexibility training. The subjects performed static, CR and CRAC
PNF stretching procedures while one knee was braced in full extension and their
ankle was attached by a cable to a pulley system. Hip ROM was assessed by a
goniometer, electrogoniometer and potentiometer, an indirect measurement via
the pulley system. EMG data were simultaneously collected for the rectus
femoris and semitendinosus muscles. The CRAC method produced the greatest
increase in ROM and was found to elicit the greatest muscle activity, compared
to the other two techniques. Thus, full muscle relaxation was not imperative for
effective stretching. This suggested that the notion of reciprocal inhibition causing
increased stretch potential should be discarded (Moore & Hutton, 1980).
Static stretching, hold-relax (HR), agonist contract (AC) and hold-relax-
agonist contract (HRAC) stretching techniques were investigated in plantar
flexors by Condon and Hutton (1987). The subjects were seated while their foot
was secured to a footplate and torque device for producing dorsiflexion of the
ankle. ROM was measure by an electrogoniometer and EMG activity was
recorded for the soleus and tibialis anterior muscles. The investigation found
similar increases in ROM following the performance of each of the stretching
procedures. The techniques involving agonist contractions, AC and HRAC, had
smaller H-reflex amplitudes, suggesting possible reciprocal inhibition during the
agonist contraction. If reciprocal inhibition had occurred, the effects were masked
35
by other neuronal input, as shown by the existence of high tonic EMG activity
levels. The findings are in agreement with the findings of Moore and Hutton
(1980). An agonist contraction assisting the stretch significantly increased EMG
activity and, therefore, muscle relaxation during stretch appears to have no effect
on ROM achieved (Condon & Hutton, 1987).
Hamstring activity and knee extension ROM was used to evaluate three
modified PNF stretch techniques in the study by Osternig, Robertson, Troxel and
Hansen (1987). All three techniques were performed by the subjects, 6 men and
4 women, aged 23-36, after adequate warm-up was completed. Surface
electrodes recorded EMG activity of the vastus lateralis and biceps femoris while
the stretch-relax (SR), CR and ACR techniques were performed. The subjects
were in the supine position with the left thigh strapped in extension to stabilize
the pelvis and the right thigh strapped in full hip flexion. The ROM of the right
knee was recorded when the knee was extended to the point of restriction. The
CR and ACR techniques did result in sufficient relaxation of the hamstrings, the
muscles opposing knee extension, to overcome facilitation generated by stretch.
During active knee extension the hamstrings exhibited 55% of the EMG activity
produced by a maximal voluntary contraction (MVC). However, knee ROM was
greatest under these conditions. These results suggest that reciprocal inhibition
does not occur with these PNF techniques, and muscle relaxation is not required
to attain maximal ROM (Osternig et al., 1987). It was noted that when a task
requires precision or when subjects are untrained in the task, strong agonist
excitation produces simultaneous activity in the antagonist (Person, 1958).
36
Similarly, when subjects felt more confident and in control of the procedure they
were more willing to extend their knee further in spite of antagonist resistance
and expressed discomfort (Moore & Hutton, 1980).
In 1990 Osternig, Robertson, Troxel and Hansen used the identical testing
procedures to test the effects of the three PNF techniques in different athlete
populations. They found no difference in the effect on hamstring muscle
activation and knee extension ROM in endurance athletes versus high intensity
athletes versus a control group. The ACR procedure produced the most EMG
activity and 9-13% more knee joint ROM than the other two procedures. It was
concluded that decreases in muscle activity are not strongly related to increases
in joint ROM. Also, 64-84% of total ROM increases occurred during the first of
the two phases for all of the PNF techniques. Therefore, ROM gains may be
limited during a given stretch session regardless of repetitions (Osternig et al.,
1990).
During slow (30o/s) isokinetic knee extension involving maximal volitional
quadriceps contraction Åagaard et al. (2000) found considerable antagonist co-
activation of the hamstring muscles. Knee joint moments for 16 sedentary males
were collected using a Kin-Com dynamometer. The subjects were seated to
perform two types of isokinetic knee extension tests. In the first test the
quadriceps actively generated an extension moment and in the second test the
hamstrings eccentrically contracted to generate a flexion moment. EMG signals
for the vastus lateralis, vastus medialis, rectus femoris, biceps femoris and
semitendinosus were collected. The data suggested that during isokinetic knee
37
extension, co-activation of the flexors of the knee occurs due to a neural
pathway. It was suggested that this hamstring co-activation is to assist the
mechanical and neurosensory functions of the anterior cruciate ligament. Near
full knee extension the contractile forces of the quadriceps induce anterior tibial
shear and excessive internal tibial rotation. The antagonist hamstring moments
potentially counteract these movements (Åagaard et al., 2000).
2.4.5 Fast Limb Movements
Results from Marsden, Obeso and Rothwell (1983) support the opinion
that the action of the antagonist muscle is to provide a counter-acting braking
force, to assist in halting a fast movement. Viscoelastic components of a joint are
sufficient to stop movement, below a certain threshold speed. However, at the
extremes of rotation viscoelastic properties offer greater resistance to movement,
therefore, larger movements that approach end ROM require smaller antagonist
activity. Thumb flexion was studied by analyzing the EMG activity of the agonist
flexor pollicis longus and antagonist extensor pollisis longus muscles. Elbow
extension was also assessed by examining the agonist triceps and antagonist
biceps EMG activity. The following variables were measured from each record:
the amplitude and peak velocity of movement, the time of onset and cessation of
the agonist and antagonist EMG bursts, and the integrated EMG activity during
the agonist and antagonist bursts. For quick movement for a short distance the
antagonist burst was large and followed quickly after the agonist activity, and
when the movement was slower, over a longer distance, the antagonist burst
38
was small and late. Antagonist activity does not occur if it is not necessary, as
when the subject knows there is a definite end-stop (Marsden et al., 1983).
Wierzbicka, Wiegner and Shahani (1986) also investigated the role of
agonist and antagonist muscles in fast arm movements. The characteristic
pattern of EMG activity of the agonist and antagonist muscles during fast
movement was originally described as a “triphasic pattern”. The agonist muscle
provides the initial burst of activity which is followed by a silent period during
which the antagonist becomes active. After the antagonist ceases the agonist is
activated again. The first agonist burst provides the force to accelerate the limb.
The function of the antagonist is generally thought to provide the braking force
decelerating the limb. The function of the second agonist burst has not been
studied in detail, but it has been suggested that it secures the movement at the
target position. This study found that by activating both agonist and antagonist
muscles the same peak displacement which was attained by activating only one
muscle can be obtained more quickly. They concluded that a primary function of
the antagonist is to control movement time. In order to produce fast movements,
a large agonist torque followed by an equally large or larger antagonist torque is
required. It has been shown that eccentric contractions produce more force than
similarly-activated concentric contractions. Thus, EMG activity of an agonist and
antagonist could be the same, but because the antagonist is eccentrically
contracting its torque will be greater than the agonist torque. The amount of the
second agonist burst is proportional to the difference between the first agonist
contraction and the antagonist contraction. Therefore, the third burst will increase
39
as the imbalance between the agonist and the antagonist torques increases
(Wierzbicka et al., 1986).
2.4.6 Mechanical and Neural Factors
Studies by Taylor et al. (1990) and McHugh et al. (1998) concluded that
only mechanical factors affected musculotendinous elongation, and that the
neural factors were irrelevant. A few studies have reported the increase in ROM
due to stretching as a combination of both the mechanical and neural
components of the muscle-tendon unit.
In 2003 Magnusson et al. reported on the role of tendons with respect to
the neuromuscular control of limb positioning. Joint position is partially controlled
by the afferent feedback from muscle spindles, which lie in parallel with muscle
fibers. Muscle spindle activation is proportional to the change in length of the
muscle fibers. The myotatic stretch reflex that inhibits muscle elongation occurs
when muscle spindles are triggered. The muscle fibers are in series with
tendons, therefore tendon compliancy will affect the activation of muscle
spindles. In a very stiff or short tendon the length change of the whole muscle-
tendon complex will be sensed by the muscle spindle. Minimal changes in length
and a small change in the ROM of the joint will activate the muscle spindle.
Alternatively, if a muscle has a very long and extensible tendon then the tendon
may lengthen considerably before the muscle fibers are affected. As a result,
more elongation, and a greater ROM, will occur before the muscle spindle is
40
activated (Magnusson et al., 2003). Therefore, both mechanical and neural
mechanisms affect the muscle-tendon unit extensibility due to stretching.
A critical review of the literature by Shrier (2004) investigated the
hypothesis that stretching improves performance. MEDLINE and Sport Discus
search engines were used to find studies related to stretching and performance.
Twenty-three articles investigating the acute effects of stretching were analyzed.
These studies indicate that the viscoelastic behaviour, i.e. stiffness, of the
musculotendinous unit was decreased when assessed within one hour of the
stretching protocol. It was also noted that in most studies EMG was affected,
suggesting that a neurologic mechanism may also be present, however further
investigations are required (Shrier, 2004).
Guissard & Duchateau (2006) also consider the response to stretching to
be attributable to both mechanical and neural factors. In contrast to strength
training, in which neural adaptations are followed by muscle hypertrophy, they
believe that PNF stretch training involves mechanical adaptations followed by
neural adaptations.
2.5 Pain Tolerance
The third proposed mechanism to account for the increase in ROM due to
stretching is a change in pain tolerance. The underlying mechanism of action of
an altered pain tolerance has not been confirmed. It has been hypothesized that
the muscle, and therefore the subject, is able to tolerate a greater exerted
moment, and as a consequence, more elongation before a sensation of pain or
41
stretch is felt (Halbertsma, van Bolhuis and Goeken, 1996). There are conflicting
results regarding the relevance of pain tolerance to stretching.
2.5.1 Pain Tolerance is Irrelevant
The stretch tolerance of muscles could not explain the increase in
hamstring passive ROM that was observed by Krabat, Laskowski, Smith, Stuart
and Wong (2001). They consider the contributing factors to muscle flexibility to
be a combination of mechanical and neural components. The popliteal angle
ROM on the non-injured knee of 15 subjects undergoing arthroscopic knee
surgery was assessed preoperatively, intraoperatively under anesthesia and
postoperatively after recovery from anesthesia. The subjects were in a supine
position with the injured hip and knee in full extension. For the starting position
the non-injured hip and knee were flexed to 90°, and the end point popliteal angle
was measured when the knee was extended until a firm endpoint was obtained.
A hand dynamometer was used to ensure that equal pressure was being applied
at each firm endpoint. The mean popliteal angle in the intraoperative period was
found to be significantly greater than the mean popliteal angle in the
postoperative period. Also, the group of subjects who received spinal anesthesia
showed a greater increase in the mean intraoperative popliteal angle than the
subjects who received epidural or general anesthesia. The results may be
explained by the fact that spinal anesthetics directly interact with nerves at the
spinal cord level providing the densest neural blockage. The afferent signals that
were produced during hamstring stretching were unable to synapse with the
42
efferent nerves. An increase in popliteal ROM was measured because the
deregulation of the efferent signals to the hamstring muscles resulted in
relaxation, allowing more elongation. The data appear to demonstrate a
neurological contribution to hamstring length which is independent of pain
(Krabat et al., 2001).
2.5.2 Pain Tolerance is Relevant
The effect of a 4-week CR stretch training program was tested with an
instrumented SLR set-up, by Halberstma and Goeken (1994). They tested the
extensibility, stiffness and EMG activity of the hamstring muscles before and after
the training program. Extensibility was defined as the ability of a muscle to allow
elongation, more specifically the ROM over which the limb could be passively
moved. Passive stiffness was defined as the ratio of the change in passive
muscle moment to the change in muscle stretch (∆moment/∆angle). Fourteen
subjects, classified via the toe-touch test as having short hamstrings, were
divided into a stretching group and a control group. The stretching group
performed the CR procedure for the hamstrings twice daily for 10 minutes, with
one session at 0900 hours and the other session at 2000 hours. Both pre- and
post-measurements for each individual were made at the same time of day, as
extensibility may be altered through the day (Halberstma & Goeken, 1994). The
data obtained showed a slight but significant increase in the extensibility of the
hamstrings, as well as a significant increase in the stretching moment tolerated
by the passive hamstring muscles. However, the passive stiffness remained the
43
same. It was concluded that the stretching program did not lengthen or decrease
stiffness of short hamstrings. The only change was an increase in the stretch
tolerance of the subjects (Halbertsma & Goeken, 1994).
In 1996 Halbertsma et al. carried out a similar study. The 10 subjects in
the stretching group performed one, 10-minute static stretching intervention. The
hamstrings were stretched statically, from a standing position one leg was raised
on a table with the knee extended while the subjects flexed at the trunk, for 30
seconds. The subject then relaxed, the leg still extended resting on the table with
the body upright, for 30 seconds. This was repeated for the duration of 10
minutes. The results showed no significant change in the passive muscle
stiffness curve with respect to the pre-stretch stiffness curve. Once again they
attributed the increase in extensibility and maximum muscle moment to an
increase in stretch tolerance. In their opinion, the muscle tolerated the greater
moment and as a consequence more elongation was obtained after the
stretching procedure (Halbertsma et al., 1996).
Magnusson et al. (1996a) examined EMG activity, passive torque and
stretch perception during static and CR stretch techniques. Passive torque (Nm),
measured by a dynamometer, was used to quantify the resistance to stretch
offered by the hamstring muscle group during passive knee extension. Ten male
recreational athletes, aged 29.4 ± 4.1 years, were seated with their trunk
perpendicular to the seat and the thigh resting on a specifically constructed pad
so that it was elevated 30° - 45° from horizontal. The lever arm of the
dynamometer was attached slightly proximal to the lateral malleolus. The
44
positioning ensured that none of the subjects were able to reach full knee
extension during the stretch maneuver, so as to apply tension only to the muscle-
tendon unit without involvement of the posterior joint capsule. Gross electrical
activity of the hamstring muscles was recorded by the surface electrodes placed
mid-way between the gluteal fold and the knee joint. The knee was passively
extended to an angle which provoked a sensation of tightness in the posterior
thigh. This was used as the final position for the stretch maneuvers. For the
dynamic testing, two protocols were followed for both the static and CR
stretches. The constant angle protocol passively extended the knee to an angle
10° below the final position where it was held for 10 seconds before continuing to
extend to the final position, where it was held for 80 seconds. In the variable
angle protocol the knee was passively extended and held for 10 seconds at 10°
below the final position. The knee was then further extended until the subject
experienced pain in the posterior thigh and pressed a safety switch that
instantaneously stopped the lever arm. For the static stretch technique the
subjects remained relaxed throughout the protocol. For the CR technique the
subjects performed a 6-second isometric hamstring contraction while the leg was
held 10° below the final position, during both testing protocols. The data showed
that for the constant angle protocol the response of the hamstring muscle was
similar with respect to passive torque and low level EMG activity for both the CR
and static stretching. For the variable angle protocol, the CR stretching
procedure maximal tolerated joint angle and passive torque were found to be
greater than the static stretching values. The low level EMG activity remained
45
unchanged. The data suggested that the viscoelastic and EMG response was
unaffected by the type of stretch maneuver, although it was thought that PNF
stretching altered stretch perception (Magnusson et al, 1996a).
Another study by Magnusson, Simonsen, Åagaard, Sorensen and Kjaer
(1996b) concluded that the increase in ROM achieved from a 3-week static
stretch training program was due to an increased stretch tolerance. There was no
change in the mechanical or viscoelastic properties of muscle. This study
involved seven female subjects, aged 26.0 ± 6.0 years, who only occasionally
participated in recreational sports. The measuring techniques and patient
positioning were identical to the previous study (Magnusson et al., 1996b),
however the stretch protocols varied slightly. For each subject only one leg was
tested, while the other served as a control. The final position for this study was
determined on the first day and was used for the pre- and post-training stretch
protocols. In the first protocol the knee was passively extended to the final
position and held for 90 seconds. Protocol two required the patients to close their
eyes. The dynamometer passively extended the knee until the patient indicated
the onset of pain, at which point they pressed a switch instantaneously stopping
the lever arm. Throughout both testing protocols the subjects were instructed to
remain completely relaxed. The training program involved two sessions per day,
and the stretch was held for 45 seconds with a 15-30 second rest period between
five repetitions. The stretch required the subject to sit with the control knee flexed
and hip slightly abducted to ensure hamstring relaxation. The stretch leg was
extended in front of the subject with the hip in a neutral rotation. The stretch was
46
initiated with the subject leaning forward with a straight back until a „stretch‟
sensation was experienced in the posterior thigh. The results of the study
showed the stiffness remained unchanged. The maximum joint ROM and
corresponding passive torque increased, and it was concluded that this was most
likely due to an increased stretch tolerance (Magnusson et al., 1996b).
Magnusson, Simonsen, Åagaard, Boesen, Johannsen and Kjaer (1997)
used the same protocols as described above to examine the hamstring muscles
in endurance-trained athletes with varying flexibility. Eighteen male elite-level
orienteers were classified as tight, n=10, or normal, n=8, based on a clinical toe-
touch test. Data from protocol one showed that the tight subjects‟ final angle and
stiffness was lower than the normal subjects‟. During the hold phase of protocol
one the tight subjects had a lower peak and final torque than the normal subjects,
but the torque decline was similar. Protocol two results indicated that the tight
subjects reached a lower maximal angle, torque and stiffness than the normal
subjects. In the common range, the stiffness was greater in the tight subjects. It
was concluded that individuals with a restricted joint ROM on a toe-touch test,
had stiffer hamstrings and a lower stretch tolerance. Tight and normal subjects
showed similar viscoelastic stress relaxation at the point of maximal stretch
sensation, indicating that all subjects could benefit from a single static stretch
(Magnusson et al., 1997).
Once again, the role of stretch tolerance in limiting stretch was examined
by Magnusson, Åagaard, Simonsen and Bojsen-Moller (2000). The same
protocols as previously discussed were carried out using seven flexible and six
47
inflexible orienteers as subjects. The results indicated that the flexible subjects
attained a greater angle of stretch with an accompanying greater tensile stress
than the inflexible subjects, due to an apparent greater tolerance of the externally
applied load (Magnusson et al., 2000).
In addition to investigating the acute effects of a stretching bout, the
critical review by Shrier (2004) examined the long term effects of regular
stretching. The nine studies that were analyzed concluded that three to four
weeks of stretching only affected stretch tolerance.
Investigations of static, dynamic, ballistic and PNF stretching techniques
have identified three mechanisms to account for the change in ROM due to
stretching; mechanical, neural and a change in stretch tolerance. The AIS
technique has not been investigated as rigorously, and current research has
focused on the effect of AIS on ROM and comparison with the other stretching
techniques. This study investigates tissue properties of human skeletal muscle in
response to the AIS technique for the hamstring muscle group.
48
CHAPTER 3
METHODOLOGY
3.1 Subjects
Participants (female n=8, male n=2) were recreationally active university
aged volunteers. To participate in the study they had to classify as having tight
hamstrings via the standing toe touch (Kippers & Parker, 1987) and knee
extension with hip flexed (Bandy & Irion, 1994) tests (described below). They
were also free of any lower back, hip or knee pathology, by self-report.
Participants were allowed to continue with their normal sports activities, however
were not allowed to take part in any other form of formal flexibility training, such
as yoga or Pilates. The potential risks and benefits of participation in the study
were explained. They were informed of the procedures to be used as well as the
purpose of the study and that they could withdraw at any time without penalty.
Written informed consent was obtained from the participants before they began
any testing and the study had received clearance from the Brock University
Ethics Board.
3.2 Testing Procedures Summary
The subjects were required to complete two test sessions: an initial testing
session which investigated the acute affects of AIS and a session following the
completion of a 6-week AIS program which provided information on the long term
effects. An outline of testing procedures can be seen in Figure 5.
49
Fig. 5 Summary of the testing procedure for each test session.
Potential participants who contacted the researcher by phone or email
after reading a poster announcing the study, were instructed to complete a self
screening standing toe touch test to determine if they met the requirements of the
study. In the standing toe touch test the participant bent forward from a standing
position while holding their knees in extension. Volunteers whose finger to
ground distance was greater than 0 cm classified as having tight hamstrings
(Halbertsma & Goeken, 1994; Magnusson et at., 1997) and they were invited to
participate in the study.
The first testing session began with an assessment of the hamstring
flexibility of the participant via a repetition of the standing toe touch test and the
completion of a knee extension with hip flexion test. Finger to ground distance of
a standing toe touch was measured and recorded for each subject. The knee
extension with hip flexion test required the participant to be positioned supine
with the right hip and knee supported in 90° of flexion. With the hip held at 90° of
flexion the examiner passively extended the tibia to the terminal position, which is
Active Knee Extension ROM
Passive Resistive Torque (Hamstrings)
AIS Procedure
Active Knee Extension ROM
Passive Resistive Torque (Hamstrings)
Active Knee Extension ROM
Passive Resistive Torque (Hamstrings)
Left Leg
Right Leg
50
the point at which the subject reported a feeling of discomfort or tightness in the
hamstring muscles or when the examiner perceived resistance to stretch. A
goniometric measurement was used to determine the amount of knee extension.
Full hamstring flexibility was considered to be 0° of knee extension. Having
greater than 30° loss of knee extension operationally defined the subject as
having tight hamstrings (Bandy & Irion, 1994). Subjects must have classified as
having tight hamstrings in both tests to continue in the study. Body weight (kg)
was measured on a scale and leg length (m) (lateral femoral epicondyle to
malleolus) was measured using a standard metric tape and recorded.
The Delsys Bagnoli-4 EMG system (Delsys Inc., Boston, MA) was used to
monitor the electrical activity of the vastus lateralis (VL) and hamstring muscle
group. The skin was prepared by shaving, lightly abrading and cleaning with
alcohol. Gel was applied between the skin and the electrode to ensure low
impedance (Osternig et al., 1990). Electrodes were placed mid-way between the
gluteal fold and the knee crease to monitor the hamstrings. An electrode was
placed on the muscle belly of the VL muscle, which is approximately two-thirds of
the way down on a line from the anterior superior iliac spine to the superior
lateral patella (Seniam Project Management Group, n.d.). Muscle activation was
recorded throughout the AIS procedure. Following the AIS sets a maximal
voluntary contraction (MVC) of the quadriceps and hamstrings was collected.
The knee was fixed at 90° of flexion for a 5-second contraction. The muscle
activity between second 1 and second 3 of the 5-second contractions was
51
averaged for three trials and reported as the MVC. All EMG values are reported
as the root mean square (RMS).
After the application of the electrodes the Biodex System 3 Pro (Biodex
Medical Systems, Shirley, NY) isokinetic dynamometer back rest and knee
attachment were set up for the participant. The back rest was set at 90° for all the
subjects, however at that setting the subject‟s hip flexion angle was greater than
90°. Additional support in the form of a pillow placed between the subject‟s back
and the chair back was required to ensure 90° of hip flexion was maintained
throughout the procedure (Figure 6). The height and depth of the back rest were
adjusted to align the axis of the dynamometer with the estimated knee joint
centre of rotation. The knee flexion-extension attachment was positioned
proximal to the malleoli and the lower leg was secured to the dynamometer with
straps. Chest straps stabilized the upper body and the lap strap stabilized the
pelvis. The thigh strap was not fastened, to allow participants who could reach
full knee extension to flex the hip towards the chest to adequately stretch the
hamstrings. The chair and leg attachment locations were recorded to ensure the
same set up was followed for the second test session.
52
Fig. 6 Biodex chair set-up
After the Biodex was prepared, all participants completed a warm up on a
stationary bike for 5 minutes, at a moderate resistance and speed. The initial
baseline values for knee extension ROM and passive resistance to stretch were
determined. The AIS technique was then recreated with the subject seated on
the Biodex. ROM and passive resisted torque were reassessed after the stretch
technique was performed.
After completing the 6 week AIS training the second testing session was
performed following the same procedures as the first session. For each subject
only the right leg underwent the AIS intervention program. The left leg served as
53
a control. ROM and passive resisted torque of the left leg were assessed in both
test sessions, but the AIS program was not performed.
3.3 Range of Motion
For both test sessions knee extension end ROM values were recorded
before and after the stretch procedure, to determine if AIS had an effect on ROM.
While positioned on the Biodex the participant was instructed to actively extend
the knee. The point at which they felt a light irritation in the posterior thigh was
considered the end ROM. An 11-point Likert scale (0-10) was used to classify the
stretch sensation. A 10 was a painful sensation while 0 was completely relaxed.
The subject was instructed to stretch until only light irritation was felt in the
hamstrings, which corresponded to 8 out of 10 on the scale. ROM of the right leg
was measured twice in each test session, before and immediately after
performing the AIS procedure, for a total of four ROM values. The ROM of the
left leg was measured once each session.
3.4 Passive Resisted Torque
Passive resisted torque measurements for the right leg were performed
before and after the AIS procedure in each test session, and once each session
for the left leg. The isokinetic dynamometer was used to assess the passive
resisted torque (Nm) of the hamstring muscles during knee extension. The
passive mode of the Biodex was programmed to extend the knee at a speed of
5°/s from the starting position in approximately 90° of flexion, to the end ROM for
54
each subject, which had been previously recorded. The data collected from this
procedure were gravity corrected (Kellis & Baltzopoulos, 1996) and then used to
create a torque-angle curve from which the stiffness of the hamstring muscles
was determined. Stiffness was calculated for the final 10% of the movement
which represents the new, stretch induced ROM. A change in stiffness in the new
ROM would indicate that changes in the structural characteristics of the tissue
had occurred (Reid & McNair, 2004).
3.5 Active Isolated Stretching
The AIS technique immediately followed the initial ROM and passive
resisted torque measurements. The end ROM was set to the maximum available
ROM of the dynamometer. The starting position was approximately 90° of knee
flexion. The isokinetic mode was programmed to allow the knee to flex and
extend at a speed of 210°/s. This speed removes resistance of the dynamometer
to movement and allows the stretch procedure to be completed in the required
amount of time. A metronome set at 90 beats/min provided the subject with a
rhythmic beat to follow, and ensured that the entire duration of each stretch was
less than 2 seconds. The testing procedure mimicked the bent knee hamstring
AIS stretch technique (Mattes, 2000). The participant actively contracted the
quadriceps to extend the knee to the point of light irritation of the hamstrings, 8
out of 10 on the stretch sensation scale. The examiner provided the overpressure
force by manually assisting the knee extension end ROM, which also ensured
that the knee extended to a point of stretch. If the participant was able to reach
55
full knee extension without irritation they were allowed to bring the hip closer to
the chest, while maintaining the fully extended knee, until the stretch sensation
was felt. The participant then flexed the knee to return to the starting position.
Each participant performed 2 sets of 10 repetitions with a 30 second rest period
between the sets.
3.6 AIS Intervention Program
Each participant performed a 6 week AIS training program using the bent
knee hamstring stretch technique, as outlined in the AIS manual (Figure 7)
(Mattes, 2000). Participants were instructed to warm up before performing the
stretch technique using their normal sports activities or a stationary bike at a
moderate speed and resistance for 5 minutes, if it was available. Otherwise, they
were instructed to jog in place or skip for 5 minutes. For the stretch procedure
participants assumed a supine position with left knee flexed so the foot was on
the ground. The right hip was semi-flexed to a degree which allowed the knee to
fully extend. The starting position of the knee was approximately 90° of flexion.
The quadriceps of the right leg contracted to fully extend the right knee. The hip
remained in the same semi-flexed position unless the knee was completely
extended without the stretch sensation in the hamstrings, at which point the hip
could be flexed closer to the chest. When the knee had been actively extended to
the point at which a light irritation, 8 out of 10 on the stretch scale, was felt in the
hamstrings the participant applied a gentle overpressure to the leg via a rope
wrapped around the ankle and foot. After application of the overpressure the
56
participant flexed the knee to the starting position, the hip remained stationary in
the semi-flexed position. The entire stretch procedure was performed in less than
2 seconds, and the repetitions were performed rhythmically. Daily, each
participant performed 2 sets of 10 repetitions with 30 seconds of rest between
the sets, as recommend by Mattes. They were required to complete a checklist to
keep track of their progress and to provide the researcher with a log indicating
that the stretch training program was successfully completed.
Fig. 7 AIS bent knee technique (Mattes, 2000)
3.7 Gravity Correction
The torque measurements obtained by the isokinetic dynamometer were
gravity corrected before creating the torque-angle curve. An anthropometric
model was used to calculate the gravitational moment for the lower leg. The
following equation was used to calculate the moment when the leg was in full
extension:
)06.0()437.0( BWlM
where M = gravitational moment, l = length of the limb (m), 0.437 = position of the
centre of mass on the long axis of the segment, relative to the knee joint, 0.06 =
57
weight of the leg-foot segment relative to total body weight and BW = body
weight (N). The gravitational moment at any other angle A (MA) was determined
by the following cosine function:
)30(cos
)cos(
AM
MA
The gravity corrected hamstring torque values were calculated by subtracting MA
from the uncorrected hamstring torque values measured by the dynamometer
(Kellis & Baltzopoulos, 1996).
3.8 Data Treatment
Torque and joint ROM was recorded every 10 msec during all procedures
on the Biodex. Every tenth data point was used for data analysis. The passive
resisted torque values were gravity corrected (Kellis & Baltzopoulos, 1996). A
fourth-order polynomial model was used to fit the torque-angle data (Nordez et
al., 2006) and stiffness was determined for the final 10% of the ROM (Reid &
McNair, 2004) using a custom MatLab program (The MathWorks Inc., Natick,
MA).
For the ROM and stiffness values, a 2-way ANOVA (session x time) and
Tukey post-hoc tests were used to determine differences between means. A
paired t-test was used to analyze the differences between the right
(experimental) and left (control) leg values at the beginning of each test session,
and to analyze the left leg between test sessions. ANCOVA was used to assess
the effect of changes in control leg values on right leg differences produced by
the stretching program. An alpha level of 0.05 was considered significant.
58
Statistica for Windows (Statsoft Inc, Tulsa, OK) software was used for data
analysis.
VL and hamstring EMG was collected at 1000 Hz and a bandwidth of 20
to 450 Hz was used. All raw EMG signals were stored on a PC. Muscle activity
collected during each set of the AIS procedure was analyzed, for both sessions
by determining the root mean square (RMS) values. VL and hamstring activity
were expressed as a percentage of the MVC. Differences between the %MVC of
the VL and the hamstring muscles within and between test sessions were
assessed by an ANOVA and Tukey post-hoc tests were used to determine
significant differences. An alpha level of 0.05 was considered significant. Results
are reported as mean ± standard deviation.
59
CHAPTER 4
RESULTS
4.1 Subjects
Ten (female n=8, male n=2) recreationally active volunteers participated in
the study. Analysis using ROM data from a pilot study showed that this number
of subjects would provide adequate statistical power. All subjects were free from
any lower body pathology, and classified as having tight hamstrings according to
a knee extension with hip flexion test. Subject characteristics are summarized in
Table 1. The knee extension test values are number of degrees short of full knee
extension (180°).
Table 1. Subject Characteristics (mean ± standard deviation)
Subjects
Age (yr)
Weight (kg)
Knee Extension Test (°)
n = 10
20.8 ± 1.8
69.4 ± 11.2
40 ± 7
All subjects completed a log book indicating that the stretching intervention
program was performed as required, daily for 2 sets of 10 repetitions with 30
seconds of rest between the sets.
4.2 Range of Motion
Figure 8 displays the maximal knee extension position of the subjects‟
right leg. There was a significant difference between the pre- (158.4° ± 12.6) and
post-AIS (167.9° ± 7.1) position within the first test session. There was also a
significant difference between the pre-AIS position in the first and second (173.3°
± 11.5) sessions. There was no significant difference between the right and left
60
leg in either session (Figure 9). This is attributed to an unexpected increase in
ROM of the subjects‟ left leg (control). Figure 9 displays the significant difference
for the left leg position between the first (161.1° ± 9.0) and second (168.8° ± 8.0)
test session.
Fig. 8 Maximal knee extension position for the right leg (mean ± SD)
Significantly different from Session 1 Pre-AIS (p< 0.05)
Significantly different from Session 1 Post-AIS (p< 0.05)
Fig. 9 Left leg and right leg ANCOVA results (mean ± SD)
Significantly different from Session 1 Right leg (p< 0.05)
Significantly different from Session 1 Left leg (p< 0.05)
61
The left leg ROM was only measured once per test session. Therefore,
the left leg was used as a covariate to assess changes in the right leg ROM
values between the test sessions. An ANCOVA revealed there was still a
significant difference between the starting right leg ROM (158.4° ± 12.6) in the
first session and the starting (173.3° ± 11.5) ROM in the second session (Figure
9).
Consequently, to analyze the right leg ROM within the test sessions with
the left leg as a covariate we assumed that the left leg ROM remained
unchanged within each test session. The adjusted least squares means for the
right leg ROM are shown in Figure 10. The right leg ROM increased, though not
significantly, following the first AIS bout (p=0.063), and also following the 6 week
training protocol (p=0.052), compared to the pre-AIS (160.7° ± 6.5) position in the
first session.
Fig. 10 Right leg ANCOVA results, within and between sessions (mean ±
SD)
Significantly different from Session 1 Pre-AIS (p< 0.05)
62
4.3 Stiffness
Figure 11 shows the torque-angle curve for a single subject. The blue dots
are the raw values from the Biodex. The red line shows the data after gravity
correction and the fourth order polynomial fit. Stiffness was calculated over the
final 10% of the knee extension ROM. This is depicted by the green line. Figure
12 displays the mean stiffness values (Nm/deg) for the right and left leg of all the
subjects. There were no significant differences in stiffness values.
Fig. 11 Torque-angle curve for a single subject.
63
Fig. 12 Stiffness over the final 10% of right knee extension ROM (mean ±
SD)
4.4 EMG
Subjects performed 2 sets of 10 repetitions of the AIS bent knee
hamstring stretch in each test session. The %MVC of the VL and the hamstring
muscles during each set of the AIS procedure is displayed in Figure 13.
Fig. 13 Muscle activity during the AIS procedure (mean ± SD)
Significantly different from VL1 and VL2 Session 1 (p< 0.05)
Significantly different from Hams 1 and Hams 2 Session 1 (p< 0.05)
64
The VL activity in the first set of the second session was significantly
greater than the other VL values. The VL was significantly more active than the
hamstring group during the performance of AIS, within and between sessions.
The hamstring activity in the second session was significantly greater than in the
first. Figure 13 shows the mean values for all subjects, however most individual
values for hamstring %MVC ranged from 5-25%. There were two subjects in
particular whose large %MVC values in the second session, i.e. greater than
80%, influenced the hamstring mean and standard deviation values.
65
CHAPTER 5
DISCUSSION
The results of the current study demonstrate that the bent knee AIS
technique increases knee extension ROM. The right leg ROM increased within
the first session and between the first and second sessions. Our results are
similar to those found in previous AIS investigations. Both Leimohn et al. (1999)
and Middag and Harmer (2002) found significant increases in ROM following
short term AIS training programs of three weeks. A 13-week study by Marino et
al. (2001) found a significant improvement in the goniometric measurements of
ROM after both 7 weeks, and 13 weeks, although the general assessment of
flexibility, a sit-and-reach test, remained unchanged.
In the present study stiffness values remained unchanged in spite of the
AIS intervention. These results are similar to those obtained by Halbertsma and
Goeken (1994) and Halbertsma et al. (1996) who studied CR and static stretch
techniques. These studies suggested that the observed increase in hamstring
extensibility (ROM) could only be achieved by either a change in the elasticity
(stiffness) of the muscle or by an increase in pain tolerance. In 1994 Halbertsma
and Goeken could not establish a significant change in the stiffness of the
hamstrings following a 4-week PNF CR stretch program. In 1996 Halbertsma et
al. found that there was no significant change in the course of the passive muscle
stiffness curve following a 10-minute static stretch, with respect to the pre-stretch
stiffness curve. Therefore, both the investigations concluded that the increase in
66
ROM was caused by an increase in stretch tolerance of the subjects. Neither of
these studies considered the effects of neural components, another identified
mechanism which may explain the increases in ROM (Etnyre & Abraham,
1986b).
An investigation into static stretching by Magnusson et al. (1996a) found
that repeated stretches resulted in decreased stiffness, although the effects were
transient as the stiffness values returned to baseline within one hour. They
concluded that biomechanical variables may be acutely altered by stretch
training, however the long-term effects of stretching remain unclear. Further
investigations by Magnusson et al. (1996b; 1996c) also found that both short-
term and long-term stretching had no effect on stiffness values. They concur with
the studies by Halbertsma et al. (1994; 1996) reporting that the increase in ROM
achieved from stretching is an increase in stretch tolerance rather than a change
in the mechanical or viscoelastic properties of muscle. The mechanism for an
altered stretch perception following stretching is unknown. It has been reported
that pain perception can be affected during dynamic exercise, so it is possible
that a similar response may be stimulated by active stretching. Another possibility
is that nociceptive nerve endings in the joint and muscle play a role via
neurotransmitter modulation or gate control (Magnusson, 1996b).
Because EMG was collected during the performance of AIS in the lab, it is
possible to consider the potential contribution of neural mechanisms to our
findings. Our data show that the hamstring muscles were significantly less active
than the VL muscle during AIS, and that reciprocal inhibition may be occurring. In
67
the bent-knee hamstring stretch the VL muscle actively contracts to extend the
knee which stimulates the muscle spindles of the VL. A synapse in the spinal
cord with a sensory neuron sends an inhibitory signal to the antagonistic
muscles, the hamstrings. Thus, the hamstrings are in a relaxed state, optimizing
their ability to lengthen. Investigations of reciprocal inhibition have found mixed
results. Some research (Moore & Hutton, 1980; Condon & Hutton, 1987;
Osternig et al., 1987; Osternig et al., 1990) has found that an increase in ROM is
independent of muscle activity. They reported that stretch procedures that
produced the greatest increase in ROM, such as CRAC, were accompanied by
relatively high levels of activity in the muscles being stretched. Therefore, they
concluded that full muscle relaxation was not imperative for effective stretching.
The results of our study are in agreement with research that indicates that
reciprocal inhibition may be a contributing factor to the effectiveness of a stretch
technique (Shindo et al., 1984; Etnyre & Abraham, 1986b; Etnyre & Abraham,
1988; Guissard et al., 1988; Guissard et al., 2001). A potential reason for the
disagreement in the literature may be due to the instrumentation and method of
analysis used in the investigations of the neural mechanisms. The studies which
have shown reciprocal inhibition to be a contributing factor have monitored the H-
reflex of the muscles. The H-reflex is an electrical stimulation of a nerve that
recreates the myotatic stretch reflex that occurs when a muscle is stretched
(Palmieri, Ingersoll & Hoffman, 2004). The H-reflex is a specific tool to assess
neurological function. The studies which have disregarded the significance of
reciprocal inhibition have only used surface EMG. Surface electrodes are very
68
effective for studies of general muscle activity, for example the progression from
relaxation to tension in a muscle contraction. However, they are only effective for
studying superficial muscles and cannot detect signals from small muscles
(Basmajian & Deluca, 1985). An investigation by Etnyre and Abraham (1988)
assessed muscle activity of the agonist and antagonist muscles during PNF
stretch procedures using both fine wire and surface electrodes. Examination of
recordings from the wire electrodes showed no activity in the antagonist muscle
during agonist contraction. This indicated that reciprocal inhibition occurred
during the stretch technique. EMG recorded with surface electrodes contained
inter-muscle cross-talk and therefore appeared to show suppressed reciprocal
inhibition phenomena. The researchers suggested that care must be taken when
making conclusions about muscle activity, if surface electrodes have been used
(Etnyre & Abraham, 1988). Our study used surface electrodes to assess muscle
activity but found results which support research which used in-depth, specific
methodology to test neural function. We conclude that neural mechanisms, such
as reciprocal inhibition, possibly played a role in AIS as assessed in this study.
However, further research, specifically assessing the H-reflex, is required to
confirm the neural implications.
The increase in ROM of our control leg was unexpected. However, there
are studies which support this finding and offer an explanation for why it
occurred. Stretch studies are designed to compare values between a control and
a stretch condition. The control can be a separate group of subjects or the
contralateral extremity in the same subject. For both methodological designs
69
there are investigations which report an increase in ROM of the control group or
leg. An investigation of static and ballistic stretch techniques found that the
control group also showed a significant increase in dorsiflexion ROM (Mahieu et
al., 2007). It was concluded that the increase in ROM of the control was due to a
learning effect. They believed that the subjects were able to achieve a greater
ROM in the second session as a result of the practice they had received in the
initial test session. Additionally, familiarity with the testing procedures, i.e. a
learning effect, may have altered the subjects‟ stretch tolerance thus allowing a
greater ROM for the same perceived stretch sensation. The method of examining
the differences between a stretch-trained and a control leg within the same
subject population was used by Handel, Horstmann, Dickhuth and Gulch (1997).
The measurements of the stretched leg were normalized with respect to the
contralateral control leg and then related to the results of the pre-training data.
This methodology was used because it was then possible to filter out influences
that uniformly affected the state of both legs, such as any general improvements
in fitness in the course of the training program or adaptations to the testing
procedures. We performed an ANCOVA on our data in order to eliminate any
bilateral learning effects or physical changes resulting from the recreational
activities that our subjects were allowed to maintain participation in during the
intervention program. The between session ANCOVA showed a significant
increase in ROM. The 2-way ANCOVA indicated an increase in ROM both within
the first session and between the two sessions was close to being statistically
significant (p = 0.063 and p = 0.052 respectively). Our results indicate that long
70
term AIS is effective at increasing ROM, and a trend for the immediate benefits of
AIS is evident.
Alternatively, Grady and Saxena‟s study (1991) of static stretching
observed changes in control leg ROM which were credited to the same
mechanisms of action as PNF exercise. The three experimental groups
performed 0.5, 2.0 and 5.0 minutes, respectively, of static stretching on one
ankle while the opposite ankle served as the control. The increased dorsiflexion
seen in the control leg was explained in terms of muscle facilitation and inhibition.
Before PNF was used as a stretching technique to increase ROM it was a
treatment for restoring strength in patients with neuromuscular disorders.
Patients who were unable to exercise an injured limb performed PNF techniques
on the unaffected side. The injured limb showed benefits from the unilateral
procedures, such as decreased atrophy and a decreased loss of flexibility and
strength, indicating a cross over effect had occurred. We speculate that
performing AIS on one leg, which follows the same concepts of muscle
facilitation and inhibition as PNF, may have a cross over effect. This may be a
reason for the increased ROM of the control leg.
71
Chapter 6
CONCLUSIONS
The purpose of the study was to investigate tissue properties of human
skeletal muscle in response to the AIS technique for the hamstring muscle group.
The study assessed the acute and long term effectiveness of AIS and examined
one possible underlying mechanism of action. The AIS protocol used in the study
produced statistically significant increases in ROM after a 6-week stretching
program. It also produced increases in ROM after a single bout, however it was
impossible to determine conclusively whether these changes were statistically
significant, due to changes in the control leg ROM. Investigations of other stretch
techniques have identified three proposed mechanisms which may explain the
increased ROM: mechanical, neural and a change in stretch tolerance. The
mechanism of action of AIS does not appear to be by way of mechanical
mechanisms because there was no change in the stiffness of the muscle. The
contribution of neural mechanisms is evident, and this requires further
investigation. Also, an altered stretch perception remains a possibility.
72
6.1 Limitations of the Study
We used the Biodex isokinetic dynamometer for testing to ensure that the
subject was stabilized during test sessions, and could be accurately repositioned
for the second session. When assessing the hamstring muscles it is important to
stabilize the pelvis during knee extension as the hamstrings are two joint
muscles, crossing both the hip and the knee. A change in pelvic position can
affect knee extension ROM as it will shorten or lengthen the hamstring muscles
(Nuyens et al., 2000). However, the design of the dynamometer limited our study
to subjects with stiff hamstrings, as flexible persons would reach the maximum
knee extension position before feeling a stretch sensation in the hamstrings. It
has been noted that subjects who classify as having tight hamstrings on a toe-
touch test have both stiffer hamstrings and a lower stretch tolerance than those
who classify as having normal hamstrings (Magnusson et al., 1997). Therefore,
the results of our study may be limited to persons with tight hamstrings. In
addition to limiting the subjects whom we could test, the design of the
dynamometer impacted the effectiveness of the AIS procedure during the second
test session. We found that subjects‟ knee extension ROM improved over the 6-
week training program. As a result, at the second test session during the AIS
procedure some subjects were able to extend their knee to the maximum
extension position without feeling the required stretch sensation of 8 out of 10.
They reported that they could feel a slight stretch but it was not as strong as what
they felt in the first test session. This confirmed that the 6-week AIS program had
affected the subjects‟ knee extension abilities but also may have limited the acute
73
effect of the technique in the second session. The decreased stretch stimulation
may have contributed to the lack of change we found in the stiffness values
during this test session. Subjects attained a greater knee extension position while
performing the stretch procedure compared to when they simply extended their
knee, therefore the passive resisted torque and ROM values were unaffected by
this problem.
A substance in which the past history of movement characterizes the
stiffness or viscosity is said to be thixotropic. This term is commonly used to
describe gels which become fluids when shaken or stirred but which regain their
original high viscosity after they are left to settle (Hagbarth, Hagglund, Nordin, &
Wallin, 1985). Lakie, Walsh and Wright (1984) reported that human
musculotendinous structures possess thixotropic properties. Thus, a warm up
was performed prior to stretching to maximize the ability of the musculotendinous
until to lengthen. We ensured that all subjects completed a 5-minute warm up, at
a moderate resistance and speed, before testing. However, extensibility can also
be affected by the time of day (Halbertsma & Goeken, 1994). We did not test the
subject at the same time of day for both sessions.
Because we had anticipated that the control leg ROM would be
unchanged throughout our study it was assessed only once each session, after
the AIS bout. The unexpected increase in left leg ROM after the 6 week training
protocol suggests that control leg ROM should have been measured both before
and after the AIS bout in each session, ideally before the right leg ROM was
assessed. This would have allowed us to test the covariance effect of the left leg
74
on right leg ROM at each time point in the experiment. While we can confidently
report the covariate effect on the right leg ROM over the 6 week training protocol,
we cannot be sure of its effect within sessions.
75
6.2 Future Investigations
The main focus of our investigation was to assess the mechanical
mechanisms associated with AIS. We also assessed neural activity with surface
electrodes on the VL and hamstring muscles. Our results indicated that neural
mechanisms were present during the stretch procedure. A more specific neural
assessment technique, such as the H-reflex, could be used to test the effect of
AIS. A more detailed investigation of the neural mechanisms could also be
achieved if the time point of application of the overpressure could be identified in
the EMG data. Also, by using an electrogoniometer, the amount of agonist and
antagonist activity could be analyzed at specific joint positions during the AIS
procedure. The large acute change in ROM seen in the first test session could be
investigated further by assessing ROM at several time points in the 24 hours
following, for example one hour post and 24 hours post stretching.
Most studies vary slightly in their measurement technique and prescription
of stretch stimulus volume. It would be beneficial to use our methodology to
investigate other stretch techniques, such as static, PNF and dynamic. Then we
could directly compare the results of the AIS procedure to those more commonly
known procedures.
As previously noted, our test procedures were limited to only subjects with
tight hamstrings. A measurement technique that could assess both normal and
tight subjects would also be an area of interest for future study.
76
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