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Sports Med 2005; 35 (1): 43-53
R
EVIEW
A
RTICLE
0112-1642/05/0001-0043/$34.95/0
2005 Adis Data Information BV. All rights reserved.
The Impact of Instability Resistance
Training on Balance and Stability
Kenneth Anderson and David G. Behm
School of Human Kinetics and Recreation, Memorial University of Newfoundland, St John’s,
Newfoundland, Canada
Contents
Abstract .....................................................................................43
1. Mechanics of Posture .....................................................................44
1.1 Proprioceptive and Peripheral Control ..................................................44
1.2 Central Processing: Anticipatory Postural Adjustments ....................................45
2. Effect of Stable Resistance Training on Balance ..............................................46
3. Stabilising Function of Muscle...............................................................47
3.1 Local and Global Trunk Stabilisation ....................................................47
4. Effect of Instability Training on Trunk Musculature .............................................48
5. The Effects of Instability on Muscle Force and Activation ......................................49
6. Resistance Training ........................................................................50
7. Conclusions ..............................................................................51
The most predominant literature regarding balance has emphasised the physio-
Abstract
logical mechanisms controlling stability. Topics range from extrinsic factors
(environment) to intrinsic factors (i.e. muscle coordination, vestibular response).
Balance is achieved through an interaction of central anticipatory and reflexive
actions as well as the active and passive restraints imposed by the muscular
system. However, less research has attempted to document the effects of balance
on performance measures (i.e. force, power). Furthermore, short- and long-term
adaptations to unstable environments need more substantial research. While force
and other performance measures can be adversely affected by a lack of balance,
the transferability of instability training to activities of daily living and sport is not
precisely known. The applicability of instability and resistance training using
unstable platforms or implements may have strong relevance in a rehabilitative or
athletic setting. Therefore, a comprehensive review of the literature in this area
may possibly be of benefit to practitioners who deal with the general population,
athletes or persons debilitated by balance and/or stability disabilities.
There has been an increasing awareness of the ment system. The mechanisms of human motion
importance and relevance of the specialised and have largely been studied under simplified move-
integrated action of the muscular system in main- ment conditions.
[1-4]
By analysing single-joint
taining posture and optimal function of the move- movements, such principles as force production,
44 Anderson & Behm
force or torque due to joint positions, muscle ments of body position to keep the centre of gravity
mechanics and the synchronisation of muscle activi- over the base of support. The smaller the base, the
ty, studies have described the basic features of more accurate such adjustments must be to maintain
human movement. In the more complex motion of balance. It is proposed that the differential effect of
bi-articular movements, findings have not precisely postural instability on balance could be accounted
substantiated those results found with single-joint for by two main mechanisms: (i) those related to the
movements.
[5]
The complexity of multi-articular alteration of proprioceptive messages at the periph-
movements underlies the difficulty in formulating eral level; and (ii) those related to the central
training exercises and programmes to improve per- processing.
[6]
formance requiring dynamic balance.
Neuromuscular mechanisms play an important
1.1 Proprioceptive and Peripheral Control
role in balance not only when motionless but also
Optimal control of balance in upright posture is
during movement. The human body is not a rigid
an essential requirement for sport, daily activities, or
unit, as it is capable of changing shape, thereby
for the prevention of injury. Stabilisation of postural
complicating some of the simple principles of bal-
equilibrium is achieved by continuous afferent and
ance normally applied to inanimate objects. Moving
efferent control strategies within the sensorimotor
a human body through three dimensions at differing
system with feedback from somatosensory, vestibu-
velocities and while experiencing varying torques
lar and visual inputs.
[7]
For example, viscoelastic
and forces places great demands on the strength,
forces inherent to the ankle muscles correct per-
endurance and coordination of the system. A less
turbations to the upright posture in humans provided
than efficient neuromuscular system may not adapt
the ankle rotation is small.
[8]
For larger displace-
well to the stresses resulting in impaired perform-
ments, active contractions are required if balance is
ance and injury. Trunk stability is an essential com-
to be maintained. The literature identifies that these
ponent for providing a solid base or core to exert or
contractions could originate from stretch or ves-
resist forces. It is still uncertain what type of training
tibulospinal reflexes, or be a voluntary response
is most effective for improving trunk and joint sta-
triggered by multimodal sensory inputs.
[9]
It has
bility in its roles in the prevention of injury and its
been postulated that a confluence of trunk and up-
contribution to balance and force output. As balance
per-leg proprioceptive input establishes the basic
and stability have a functional role for vocational
timing of automatic, triggered balance corrections,
purposes, recreation, daily tasks or injury prevention
which is then preferentially weighted by vestibular
and rehabilitation, it would be beneficial to identify
modulation in muscles that prevent falling.
[10]
While
if a particular exercise regimen and/or technique
human postural control is shared among the vestibu-
could maximise dynamic balance and the ability to
lar, visual and somatosensory systems, the vestibu-
exert forces while maintaining balance. As a number
lar system is considered the main control system for
of neuromuscular mechanisms responsible for bal-
a vertical detector.
[11]
ance exist within the human body, we must first
The afferent information is processed in the
attempt to identify and then understand these partic-
brainstem and cerebellum, followed by the initiation
ular mechanisms and the movement considerations
of motor commands to maintain balance. Unstable
involved. The objective of this paper is to review the
environments stress sensory and motor feedback
literature concerning the need or desirability of un-
loops resulting in increased body sway and muscle
stable environments for training.
activity.
[12]
Standing on an unstable support calls
upon higher levels of the control system and re-
1. Mechanics of Posture
quires an essential change in the mode of utilisation
The apparently simple act of standing motionless of incoming proprioceptive information. This theory
is actually a continuing process of minute adjust- of postural mechanisms was investigated while
2005 Adis Data Information BV. All rights reserved. Sports Med 2005; 35 (1)
Impact of Instability Resistance Training 45
standing on a rigid floor and varied amplitude ‘see- when they were performed from an initial unipedal
saws’.
[13]
Electromyographic (EMG) activity of the
posture (unstable).
soleus and tibialis anterior during standing on the
Lynn and Woollacott
[17]
found that during quiet
rigid floor and on a see-saw resembled each other.
stance without support, EMG activity was clearly
However, during standing on the more unstable see-
evident from the soleus, therefore, identifying the
saw, the amplitude of the movement in the ankle
role the soleus muscle had in maintaining standing
joint was larger and a marked modulation of the
posture. In their study, measuring EMG activity of
EMG activity of the soleus muscle was observed.
the muscles of the lower limb, they found that when
These results suggest that directionally specific tor-
the subjects were unstable, both the tibialis anterior
que changes in response to centre of gravity shifts
and soleus muscle would fire before any movement
provide important information for maintenance of
would occur. However, during the same task but
posture.
while under a stable condition (holding on to sup-
ports), the activity prior to the movement phase was
abolished. Other results with an unstable base show
1.2 Central Processing: Anticipatory
the occurrence of an early inhibition of the EMG
Postural Adjustments
activity of the triceps surae muscles in advance of
their bursting activity leading into the intended
To achieve the primary goal of a given task, the
movement. Also, the occurrence of an early increase
fundamental role of the central nervous system is to
in triceps surae EMG activity before the voluntary
coordinate the focal movement. Anticipatory pos-
activation of the tibialis anterior was also identified.
tural adjustments play an important role in maintain-
These phenomena outline a complex pattern of ac-
ing balance during task performance. As a result of
tivities, whereby a muscle’s activity is decreased
the enhanced central drive and the corresponding
just prior to fast activation of the same muscle, or is
augmented gamma motor neuron activity during
enhanced when the only intended command is the
balancing, co-contraction of the muscles involved
contraction of its antagonist. Slijper and Latash
[18]
can be implemented.
[14]
It is known that postural
reported an anticipatory increase in activity of the
adjustments of the trunk or legs may be initiated
tibialis anterior, biceps femoris, erector spinae and
prior to the onset of voluntary movements of the
rectus abdominus when experiencing unstable
trunk or upper limb.
[11]
These postural adjustments
standing. In the soleus and rectus femoris, changes
appear to have the aim of minimising the equilibri-
in the background activity were less pronounced. In
um disturbances provoked by these movements.
the absence of additional support (touch or grasp),
Kornecki et al.
[15]
reported that when the support
arm muscles (wrist flexors/extensors, biceps, tri-
object was unstable, the myopotentials of all the
ceps) tended to show an increase in the background
investigated muscles preceded the instant of force
EMG activity. For these reasons, the authors specu-
application (anticipation). The stabilising muscles
late that these phenomena are anticipatory postural
of the task dominated this specific neuromuscular
adjustments, and serve the purpose of minimising
anticipation. This may be explained by the fact the
the subsequent postural destabilisation.
supporting structures must first be stabilised before
In very stable conditions, the requirements of
a motor movement can be efficiently elicited. In
stabilising posture under the action of transient,
addition, the postural adjustments in a number of
motion-related perturbations are alleviated. On the
different stances were measured and found that
other hand, in very unstable conditions, anticipatory
stabiliser muscles fired approximately 30ms prior to
postural adjustments themselves may be viewed as
movement muscle activation.
[16]
The main result
sources of perturbations, which can move the centre
was that anticipatory postural adjustments were
of mass beyond the decreased area of support. This
large when the lower limb flexions were performed
anticipatory increase in synergistic muscle activity
from an initial bipedal posture (stable) and absent
2005 Adis Data Information BV. All rights reserved. Sports Med 2005; 35 (1)
46 Anderson & Behm
was also documented using an inverted pendulum to years) with reduced balance showed that short-term
induce instability of the arm.
[1]
strength and endurance training had no restorative
effect on balance of the study cohort.
[26]
Testing to
When we move, we are usually unaware of the
determine if a strength-training programme can im-
complex neuromuscular processes that control our
prove measures of balance among adults aged ≥65
posture. The mechanical problem of maintaining
posture is particularly challenging but with internal years (n = 55) was also done.
[27]
At post-test, the
central processing within the cerebellum paralleled
exercisers demonstrated enhanced balance, although
with anticipatory postural adjustments and proprio-
none of the post-test measures were significantly
ceptive feedback (vestibular, visual and somatosen-
different from the control group. Furthermore, a
sory inputs), we are able to meet the constant de-
randomised, controlled trial compared the effects
mands for maintaining posture and balance. While
of resistance training on static balance.
[28]
The
instability resistance training should certainly tax
strength-training group exercised three times per
the proprioceptive control of posture, it has not been
week using exercise machines with balance being
established whether any positive adjustments would
measured on a force platform. Results indicate that
be mediated through anticipatory postural adjust-
double-stance measurements were unchanged after
ments (central processing). There is some literature
training. In single stance, the centre of displacement
to suggest that resistance training under stable con-
of the centre of pressure improved by 17%. Finally,
ditions can improve balance, especially in older
no significant between-group differences for one-
adults.
leg blind balance time suggests that strength training
alone does not appear to enhance standing balance
2. Effect of Stable Resistance Training
in active, community dwelling older adults.
[29]
on Balance
In a study of younger healthy subjects, partici-
pants were assigned to either strength- or balance-
The effect of resistance exercise on muscle
training regimens.
[7]
After 1 month, the training was
strength and size has been clearly documented,
[19-22]
exchanged between groups. At the first follow-up,
but evidence suggests that resistance training, absent
balance training led to significant increase in the
of balance training, also has a positive effect on
performance outcomes of the balance-training
balance. It was found that strength exercises contrib-
group, whereas the strength-training group did not
ute to better balance and gait in women aged ≥57
(p < 0.001). However, at the second follow-up,
years.
[23]
In a separate study, the mean increase in
scores of both groups were significantly increased
balance scores in a balance-training group was
when compared with baseline.
146% and 34% in the strength-training group (p <
Motor-skill training, including balance training,
0.01).
[24]
A prospective, blinded, randomised trial of
increases the sensitivity of feedback pathways and
moderate intensity strength exercise was conducted
shortens the onset times of the selected muscles by
on 132 older adults.
[25]
They found that gait stability
improved significantly more in the resistance exer- improving the sensitivity of the position sense of
cise group than in the control group. These results
both agonistic and antagonistic muscles.
[7]
The mus-
show that even moderate strength gains (17.6%)
cle, as the termination of the final pathway of the
may benefit gait and balance, thus providing a sound
sensorimotor system, particularly contributes to the
basis for the encouragement of low-intensity
maintenance of body balance. It has been document-
strength training for individuals with functional lim-
ed that resistance training that increased muscular
itations.
strength also increased stability and coordination.
[30]
Improvements in task performance were accompa-
A number of studies were conducted to test if a
nied by changes in the pattern of recruitment of the
training programme can restore balance in older
muscles that were the focus of the resistance-train-
individuals.
[26-29]
The effect of strength and endur-
ance training on balance in older adults (aged 65–85 ing programme. Specifically, the trained muscles
2005 Adis Data Information BV. All rights reserved. Sports Med 2005; 35 (1)
Impact of Instability Resistance Training 47
were recruited in a more consistent fashion after tractions plays a significant role in stabilising the
training. However increased strength does not guar- shoulder joint.
[32]
antee improved balance. Kollmitzer et al.
[7]
showed
This coordination leads to the extraordinary com-
that strength training actually reversed any benefit
plexity of the nervous system, of which one of the
from the balance training in their study. Training of
most important tasks is to control the stabilisation of
the muscles that contribute to posture may reasona-
the joints’ degrees of freedom unused in a given
bly change not only muscle force output, but also the
motor task by stimulation of antagonistic muscles.
coordination of synergistic and antagonist muscle
Behm et al.
[33]
studied agonist/antagonist relation-
activation. Thus, depending on the specific training
ships of the leg extensors with instability. During leg
programme and population in question (i.e. a frail,
extensions, antagonistic hamstring activity in-
aged individual may benefit more from the benefi-
creased by 29.1% (p = 0.05) under unstable versus
cial effects of increased force output for responding
stable conditions. Milner et al.
[34]
provided evidence
to perturbations), the role of resistance training on
from comparison of forceful flexor and extensor co-
balance is unclear.
contractions that EMG was always less during maxi-
mal co-contractions than it would be as a prime
mover. Slijper and Latash
[18]
demonstrated that dur-
3. Stabilising Function of Muscle
ing standing on an unstable board, there was a
Although much is known about how muscles
tendency for an increase in the background muscle
maintain static equilibrium, little is known how they
activity, which would increase the resting stiffness
maintain dynamic balance when exerting an exter-
or tonus of the muscle in response to the unstable
nal force. Exerting external forces while attempting
platform. Adding manual support typically resulted
to maintain dynamic balance is tantamount to suc-
in decreased background activity. The cost of cop-
cess in the majority of sports and a necessity in the
ing with instability is an increase in co-contractions
activities of daily living (i.e. carrying shopping
resulting in a decrease in external force. However, it
bags, babies). Exerting a force with the upper ex-
can be argued that the task would not be able to be
tremity upon an external object requires coordina-
performed without this co-activation.
tion of the neuromuscular system. This coordination
Thus, this stabilisation process consists of estab-
would ensure the inhibition of some muscles to
lishing active muscular constraints to minimise the
achieve simultaneously coupled motions to take
degrees of freedom within a joint or series of joints
place in other joints. Sporrong et al.
[31]
studied how
and results in stabilisation of the excessive mobility
handgrip exercises/activities increased shoulder
of external objects. Trunk or core stabilisation is
muscle activity. Subjects were asked to perform
essential for maintaining static or dynamic balance
isometric contractions against a handgrip dynamom-
and especially when attempting to exert forces upon
eter. In the supraspinatus and infraspinatus muscles,
external objects. A lack of trunk stabilisation may
there was a positive correlation between the degree
also be a major contributor to the back pain endemic
of the shoulder muscle activity and the intensity of
within our society.
handgrip exertion. This could have practical value in
the rehabilitation of the hand, as not only the mus-
3.1 Local and Global Trunk Stabilisation
cles of the hand and forearm should be rehabilitated,
but also the stabiliser muscles of the shoulder as well Functional joint stability and its effect on balance
for maximal rehabilitation of grip strength. Further- are dependent on integrated local and global muscle
more, muscle activity occurred simultaneously in function.
[35-37]
Comerford and Mottram
[36]
have pro-
muscles producing a movement and in antagonistic posed a classification system for muscle function.
muscles of a number of different shoulder move- They have defined and characterised vertebral mus-
ments (i.e. flexion, extension, adduction, abduc- cles as local stabilisers, global stabilisers and global
tion), showing how coordination due to muscle con- mobilisers. They identify that role of the local sta-
2005 Adis Data Information BV. All rights reserved. Sports Med 2005; 35 (1)
48 Anderson & Behm
bilisers is to maintain low force continuous activity Arokoski et al.
[35]
identified that the stability of
the spine was increased with either increased flexor-
in all positions of joint range and in all directions of
extensor muscle co-activation forces or increased
joint motion. Their activity usually increases in an-
intra-abdominal pressure. Deep local stabilising
ticipation to a load and/or movement, thus providing
muscles, especially the multifidus and the transverse
joint protection and support. Global stabilisers gen-
abdominus muscles, mainly contribute to stability.
erate torque and provide control over some motions.
In the investigated exercises, the researchers found
Global mobilisers are required to have adequate
the lumbar multifidus muscle function patterns ap-
length to provide full range of motion around a joint
peared to be coupled with longissimus thoracis mus-
without causing overstrain elsewhere in the move-
cles, thus the local and global back muscle function
ment system; however, they do have a stability role
showed similar activation patterns and simultaneous
under high load or strain. The normal function of the
function.
local muscle system is to provide sufficient segmen-
It has been suggested that back muscle contrac-
tal stability to the spine. The global muscle system
tions as low as 25% of maximal voluntary contrac-
provides general trunk stabilisation and enables the
tion (MVC) are able to provide maximal joint stiff-
static and dynamic work necessary for daily living
ness.
[39]
A low percentage of maximal voluntary
and sport activities.
[38]
The multifidus has been iden-
isometric contraction from the trunk musculature
tified as a local stabilising muscle that acts simulta-
thus stabilises the spine during normal movements
neously with the global muscles (longissimus tho-
and motor control, not just muscle strength is impor-
racis, rectus abdominus).
[35]
One definition of global
tant to dynamic stability training.
[40]
Furthermore,
muscles is that they are muscles that act on the
because lumbar stabilising multifidus muscles are
spinal column via the rib cage (i.e. erector spinae,
mainly composed of type I muscle fibres,
[41]
only
rectus abdominus), which control the overall re-
relatively low loads (approximately 30–40% MVC)
sponse, and local muscles that are attached directly
are needed to improve their effectiveness.
[42]
There-
to the lumbar spine.
[37]
fore, resistance training under unstable conditions
In a study of muscle recruitment patterns during
with relatively high loads may not automatically
asymmetric lifting in healthy individuals, it was
provide a suitable training environment for vertebral
found that the left and right internal obliques, rectus
stability and dynamic balance. On the other hand,
femoris and multifidus showed symmetrical co-con-
the integration of balance and resistance training
traction in all variants of the lifting activities.
[37]
In
may augment motor control while increasing muscle
contrast, significant left/right differences were ob-
activation, strength and stiffness.
served in the external oblique, gluteus maximus,
illiocostalis lumborum pars thoracis and latissimus
4. Effect of Instability Training on
dorsi. These results show a symmetrical activation
Trunk Musculature
of the local muscles during the performance of low
load, asymmetric lifting tasks, which suggest that
It is proposed that training under unstable condi-
these muscles play a stabilising role during these
tions will stress the neuromuscular system to a
manoeuvres. The global muscles, however, show
greater extent than traditional resistance training
asymmetric patterns of activation during the same
methods using more stable benches and
tasks, supporting their role of global stabilisers and
floors.
[12,14,43,44]
Strength gains can be attributed to
prime movers. In addition to these findings, it was
both increases in muscle cross-sectional area and
identified that the multifidus, transverse abdominus
improvements in neuromuscular coordination.
[45]
A
and the internal obliques act as part of the local
number of researchers have reported that neural
stabilising system; whereas the longissimus
adaptations play a vital role in strength gains in
thoracis, rectus abdominis and external obliques
the early portion of a resistance training pro-
constitute a part of the global stabilising system.
[35]
gramme.
[45,46]
In addition, it is suggested that the
2005 Adis Data Information BV. All rights reserved. Sports Med 2005; 35 (1)
Impact of Instability Resistance Training 49
specific neural adaptations occurring with resistance significantly fewer recurrent sprains. Also, signifi-
cantly fewer patients in a wobble-board training
training are not increased recruitment or activation
group had functional instability of the ankle com-
of motor units, but an improved coordination of
pared with a no wobble-board training group.
[44]
The
agonist, antagonists, synergists and stabilisers.
[47,48]
authors concluded that training on a wobble board
With the current interest in stability training for
was effective in reducing residual symptoms follow-
the injured low back and home fitness equipment
ing ankle ligament trauma compared with no train-
available to the consumer, the use of labile (movea-
ing.
ble) surfaces to challenge the motor control system
Behm and colleagues
[52]
had subjects perform a
is becoming more popular. However, this could be
variety of typical trunk strengthening exercises
of concern as little is known about the effects of
under stable and unstable (Swiss ball) conditions.
these unstable surfaces on muscle activity.
Overall, activation of the lumbo-sacral and upper
To maximise functional performance, individu-
lumbar erector spinae as well as the deep abdominal
als should attempt to train in an environment that
stabilisers was significantly greater with the unsta-
mimics their real-world situation. Often in sport and
ble condition. Thus, the very few investigations of
activity, the individual is not in a stationary, stable
unstable exercises on trunk muscle activation seem
position, therefore, numerous training aids have
to concur that the introduction of instability into the
been developed to simulate these real-world situa-
activity increases the extent of muscle activation.
tions. One of these training aids is the ‘Swiss ball’.
Siff
[49]
found that the Swiss ball provided a wider
5. The Effects of Instability on Muscle
range of movement (with an optimal starting posi-
Force and Activation
tion from a few degrees of active trunk extension),
which he described as preferable to similar actions
Unstable platforms or loads can lead to increased
performed in most circuit training gyms. The impor-
stabilisation functions at the expense of motive or
tance of ‘Swiss balls’ in a rehabilitative setting has
force functions.
[48,53]
Thus, even with substantial
been documented in the re-education of postural
force decrements, overall muscle activation may
muscles and to facilitate movement and postural
remain virtually unchanged. Kornecki et al.
[15]
found
reactions in neurologically impaired patients.
[50]
that contributions of stabilising muscles increased
on average by 40% when the handle changed from
Vera-Garcia et al.
[51]
tested the type of surface
stable to unstable during pushing movements They
(stable or unstable) on the muscle mechanics of the
showed that the process of muscular stabilisation of
abdominal wall. They indicated that performing
the investigated joint caused, on average, 30% drops
curl-up exercises on an unstable surface increased
in force, velocity and power (p < 0.01). Instability-
abdominal muscle activity. EMG analysis showed
induced muscular stabilisation of the wrist joint
the rectus abdominus muscle activity on a stable
caused a significant increase in the EMG contribu-
surface was 21% of the MVC and external oblique
tions of the stabilising muscles and a visible drop in
muscle activity was 5% of MVC. For the curl-up on
the contributions of the muscles that realised motor
an unstable ball, rectus abdominus activity was 35%
functions, which in turn bring about a significant
of MVC and external oblique muscle activity was
loss of maximum force, velocity and power pro-
10% of MVC. This study suggests a much higher
duced against an external object.
demand on the motor control system when perform-
ing abdominal exercises on labile equipment. In
A number of other authors have examined the
addition, a stability ball training group’s perform-
function of limb stabilising muscles. It was found
ance in trunk flexion and back extension improved
that both the short and long heads of the bicep have
significantly more (p < 0.05) than either the tradi-
similar functions as anterior stabilisers of the gle-
tional group or the controls.
[50]
Possible rehabilita-
nohumoral joint and their roles in stabilisation in-
tive benefits were suggested with the occurrence of
crease as joint stability decreases.
[54]
The stabilising
2005 Adis Data Information BV. All rights reserved. Sports Med 2005; 35 (1)
50 Anderson & Behm
function of scapular stabilisers while performing ferent muscles (muscle co-ordination) were identi-
fied. For example, the flexor pollicis longus EMG
push-ups on miniature trampolines was also ex-
was significantly greater during stable opposition
amined.
[55]
They found no significant difference in
pinch whereas EMG activity in the dorsal interos-
stabiliser EMG activity between stable and unstable
seus muscle was significantly greater in the unstable
conditions; however, they acknowledged the degree
condition.
of stability induced by the miniature trampolines
The literature indicates that instability affects
was likely to be insufficient to illicit an unstable
force output in numerous ways (i.e. decreased force
platform.
output, increased co-contractions, altered muscle
Anderson and Behm
[56]
also reported no signifi-
coordination). The goal of instability resistance
cant difference in EMG activity of the pectorals,
training would be to accommodate to an unstable
triceps, latissimus dorsi and rectus abdominus when
environment, and thus hopefully diminish the loss of
performing maximal isometric chest presses under
force and the extent of co-contractions.
stable and unstable conditions. However, maximum
isometric force was decreased by 60% with the
6. Resistance Training
unstable base. They suggested that the decreased
balance associated with resistance training on an
Resistance training can be used to induce
unstable surface might force limb musculature to
strength, power and endurance adaptations with a
play a greater role in joint stability. In a further study
myriad of exercises and equipment that will provide
from the same laboratory, upper lumbar, lumbo-
a spectrum of stable and unstable loads. The range
sacral erector spinae, abdominal stabilisers and
of stable and unstable platforms and loads used
soleus activation were all activated to a greater
during resistance training might also induce motor
extent when performing squats under unstable con-
learning adaptations with the goal of improving
ditions with the same submaximal load.
[57]
The au-
dynamic balance and force outputs while unstable.
thors explained that the increased EMG activity of
Several modes of training are currently available,
these muscles may be attributed to their greater
with some of the more popular methods being the
postural and stabilisation roles with the unstable
use of free-weights, weight stack machines and
condition.
isokinetic devices. Each method has associated free-
In contrast, Behm et al.
[33]
reported that unstable
doms and constraints.
leg extensor and plantar flexor activation averaged
The advantages of free weights over machines
44.3% and 2.9% less, respectively, than during sta-
are well documented.
[59-61]
The major advantages
ble conditions. Unstable leg extensor force was
arise from the ability of free-weight exercises to
70.5% less than stable force while unstable plantar
mimic the movement demands of real-life sport and
flexor force was 20.2% less than its respective stable
everyday activities from the numerous possible vari-
force. They suggested that under conditions of great
ations with free-weight exercises. This use of free
instability, as was experienced with the destabilising
weights is vital in the principles of exercise specific-
torque of unilateral leg extensions performed on a
ity (training in a specific manner to produce a specif-
Swiss ball, increased stabilisation functioning of the
ic outcome). In addition, free-weight lifting requires
muscles was insufficient to maintain balance result-
the lifter to balance and stabilise the bar/dumbbells
ing in decreased overall activation. In contrast, no
in all movement planes. Having the individual lift
statistical difference was found in the magnitude of
dumbbells unilaterally can further augment the in-
forces produced in a stable versus an unstable proto-
stability of free weights. Behm et al.
[52]
reported
col of the thumb musculature (nine separate mus-
greater trunk muscle activation when dumbbell
cles) in individuals using a modified clinical pinch
chest presses were performed unilaterally rather
meter.
[58]
Although force output remained constant
than bilaterally. Further advantages of free weights
between protocols, changes in the activation of dif- include a constant external resistance baseline
2005 Adis Data Information BV. All rights reserved. Sports Med 2005; 35 (1)
Impact of Instability Resistance Training 51
throughout the particular movement as well as the be experienced with a stability/balance programme,
ability for full body training. The movement of a it would be beneficial for fitness and health practi-
free weight is constrained by the lifter, as opposed to tioners to combine balance training modalities with
a machine, which often does not require the muscles resistance training so their clients, especially if so-
to work in the similar stabilising role.
[62]
matosensory impaired, can maximise the positive
effects of their training time. Although force outputs
In contrast to free weights, most machines create
are diminished under unstable conditions, the de-
a forced or guided one- or two-dimensional move-
creased balance associated with instability resis-
ment pattern for the user opposed to the three-
tance training may force limb and trunk musculature
dimensional movement pattern of free weights. This
to play a greater role in joint stability. The use of
forced pattern does not allow as much movement
unstable platforms as a resistance training modality
freedom in movement patterns partly caused differ-
for strength gains can be employed to allow high
ences in people’s limb lengths, bone articulations
muscle activation levels to be developed through an
and muscle attachment sites. Rubber tubing and
increased reliance on stabilising functions. As this
machines using cables that can move in three dimen-
high level of muscle activation can be achieved with
sions are more adaptable to individual anthropomet-
less resistance, this training modality may have pos-
ric differences. However, cables and rubber tubing
itive implications in progressive muscle and joint
typically offer a fast-to-slow movement pattern,
rehabilitation as well as sport-specific training.
with greater resistance and slower speed toward the
Since most sports involve a combination of stabilis-
end of the movement, which contrasts with the
ing and force producing functions (running forehand
typical slow-to-fast pattern of many sport move-
in tennis, baseball pitcher wind-up, moving slap shot
ments.
[63]
Another disadvantage of machines is that
in hockey and many others), instability resistance
they often provide resistance only at a single joint.
training provides similar challenges to the neuro-
Also, because most machines support the user, few,
muscular system. However, in order to induce maxi-
if any demands are required to stabilise and balance
mum or near maximum overload forces upon the
both the user and/or the load. However, machines do
limbs, a stable platform may be necessary. It is
offer some benefits as they often ensure the correct
recommended that a comprehensive, sport-specific,
range of motion and movement pattern, lowering the
strength-training programme incorporate exercises
likelihood of injury, especially with individuals un-
under both stable and unstable conditions.
familiar to resistance training.
[64]
Numerous authors have attempted to make the
Free-weight exercises are generally agreed upon
connection in identifying the effects of strength
by the fitness community as the most advantageous
training versus balance training on balance; howev-
method of weight training due to the positive effects
er, little research has been done regarding actually
of unstable training protocols on neuromuscular
performing this strength training on stable and un-
function.
[12,14,43]
stable platforms and its resultant effect on balance. It
is now proposed that the scientific community iden-
7. Conclusions
tify which methods and exercises offer the most
It is now evident that a large amount of resistance
benefit with reference to stability and balance while
training information exists stemming from different
offering the highest degree of carry-over into a real-
equipment for varied training regimens. However, it
world setting.
is essential to summarise and apply this knowledge
in a more functional and activity-specific model to
Acknowledgements
identify if a parallel exists for the need, practicality
and importance of stability training.
No sources of funding were used to assist in the prepara-
Through the training-induced functional in-
tion of this review. The authors have no conflicts of interest
creases in balance and muscle coordination that can that are directly relevant to the content of this review.
2005 Adis Data Information BV. All rights reserved. Sports Med 2005; 35 (1)
52 Anderson & Behm
control, muscular strength, and cholesterol in type I diabetic
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