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The Stretch-Shortening
Cycle: Proposed
Mechanisms and Methods
for Enhancement
Anthony N. Turner, MSc, CSCS
1
and Ian Jeffreys, MSc, CSCS*D, NSCA-CPT*D
2
1
London Sport Institute, Middlesex University, London, England; and
2
University of Glamorgan Pontypridd, Wales,
United Kingdom
SUMMARY
EFFICIENT STRETCH-SHORTENING
CYCLE (SSC) MECHANICS RESULT
IN ENERGY CONSERVATION AND
ENHANCED PROPULSIVE FORCES.
BASED ON THE CURRENT REVIEW
OF RESEARCH, ELASTIC ENERGY
THROUGH TENDON RECOIL AND
AN INCREASE IN ACTIVE STATE
BECAUSE OF AN INCREASE IN THE
WORKING RANGE SEEM THE
MOST PLAUSIBLE EXPLANATION
FOR THE SSC MECHANISM. USING
THE RESEARCH PRESENTED, THE
SSC MECHANISM MAY BE BEST
DEVELOPED USING THE PLYO-
METRICS PYRAMID ENSURING
THAT THE ATHLETE IS TECHNI-
CALLY COMPETENT AT EACH
STAGE BEFORE PROGRESSING IN
INTENSITY AND COMPLEXITY.
STRENGTH TRAINING AND SSC
TRAINING SHOULD BE PER-
FORMED CONCURRENTLY. SSC
DRILLS SHOULD BE BASED ON
STRENGTH CAPACITIES AND
SPORT-SPECIFIC VARIABLES.
INTRODUCTION
It is well established that a vertical
jump preceded by a countermove-
ment (i.e., a prestretch) will increase
vertical displacement above a squat
jump (one with no prestretch) (10).
Investigations have revealed improve-
ments in the range of 18–20% (15) to
20–30% (13) and a difference in max-
imum jump height of approximately
2–4 cm (9). Moreover, by increasing
the load applied and the rate of loading
during the countermovement, for ex-
ample, after a run-up or a depth jump,
jump height may further increase
(4,8,77,78). This phenomenon is a con-
sequence of what is termed the stretch-
shortening cycle (SSC), which describes
an eccentric phase or stretch followed
by an isometric transitional period
(amortization phase), leading into an
explosive concentric action. The SSC is
synonymous with plyometrics (37) and
is often referred to as the reversible
action of muscles (112). Other exam-
ples of SSC actions include the natural
parts of movements such as running
or walking or the windup movement
in throwing.
Aside from an enhanced concentric
contraction (propulsive force), efficient
usage of the SSC also affords the
athlete with a reduction in the meta-
bolic cost of movement (9,10). This
may be evidenced with data suggesting
that the energetic cost of running for
animals with heavy limbs is about the
same as those with light limbs (as
heavier limbs would increase the load
applied and the rate of loading) (40).
In addition, Verkhoshansky (101) and
Voigt et al. (102) reported that
economical sprinting (i.e., efficient
usage of the SSC) can recover approx-
imately 60% of the total mechanical
energy (40% being replenished by
metabolic processes during the follow-
ing cycle). In addition, the contribution
of nonmetabolic energy sources in-
crease with increases in running veloc-
ity (24,101,102).
The SSC is therefore essential to many
sporting movements, with performance
dependent on its efficient use within
a movement skill. Consequently, many
coaches look to incorporate training
drills such as plyometrics, which can
enhance the athlete’s use of the SSC
(68,77,85,91,93,95,97). The successful
integration of these exercises, however,
may only be achieved with an under-
standing of the underpinning mechan-
ics. Several mechanisms have been
proposed to explain the SSC phenom-
enon, with varying emphasis and con-
clusions reached across the literature.
Investigators have reported the sig-
nificance of elastic strain energy
(11,33,64,74,77), involuntary nervous
processes (12,13), increased active range
of movement (9,10), length-tension char-
acteristics (30,34), preactivity tension
KEY WORDS:
stretch-shortening cycle; elastic energy;
tendon; stiffness; spindle; Golgi tendon
organ
Copyright ÓNational Strength and Conditioning Association Strength and Conditioning Journal | www.nsca-lift.org 87
(68,95), and enhanced coordination
because of the innate action of the
prestretch (9,10). The purpose of this
article, therefore, is to review the most
current theories to discuss and define
the processes that underpin the SSC. A
second aim is to identify training strate-
gies to enhance the SSC processes to
optimize performance.
STORAGE OF ELASTIC ENERGY
During hopping, jumping, and running,
for example, our legs exhibit similar
characteristics to a spring, whereby
the leg spring compresses on ground
contact and stores energy, before
rebounding at push-off and releasing
energy (50). It is currently recognized
that the tendon is the primary site for
the storage of elastic energy (EE)
(64,74). The magnitude of stored EE
(often referred to as strain or potential
energy) is hypothesized to be pro-
portional to the applied force and the
induced deformation (112). Previous
research supports that elasticity plays
a substantial role in enhancing the
motor output in sport movements
(13,33,64,74,77) and likely explains
the 20–30% difference seen between
a countermovement jump (CMJ) and
a squat jump (SJ) (13). In addition,
Verkhoshansky (101) reported a high
correlation (r= 0.785) between the
tendon’s capacity to store EE and the
performance of distance runners.
Although recognizing the fundamental
role of EE, some investigators do not
support its significant role in enhancing
force production (9,10). Instead, they
place emphasis on its ability to reduce
the metabolic cost of movement. For
example, Bobbert et al. (10) suggests
that if propulsive displacement is equal
between 2 athletes, the athlete who
most optimally uses SSC mechanics
would incur less metabolic work. They
argue that lengthening of the series
elastic component (SEC), namely, the
tendon, occurs at the expense of length
over which the contractile elements
can produce force. This is in agreement
with Bobbert and Casius (9) who
explained that despite the SEC con-
tributing to maximum jump height
through storage of EE, it does not
explain the differences in jump height
among various types of jumps. In
summary, therefore, an inverse rela-
tionship may exist between EE release
and force production via the contrac-
tile components. Put simply, the
greater the release of EE, the greater
the reduction in cross-bridge formation
and concomitant force output from
these structures. Research by Licht-
wark and Wilson (74) may also provide
support for this theory. Their research
suggests that muscles act at high values
of efficiency by contracting fibers at
favorable speeds, which are often
different from the speed of the whole
muscle-tendon complex (MTC).
Therefore, through the use of EE,
some fibers are deactivated during
periods of shortening. They do suggest,
however, that during maximal efforts,
EE can enhance force production. The
correlation reported above by Ver-
khoshansky (101), therefore, maybe
because of the fact that improved
performance in long-distance events
with respect to the SEC is because of
its ability to conserve energy and
therefore provide an efficient energy
release/conservation system as op-
posed to one that increases force per
stride. Although this does seem plau-
sible, many investigators still cite that
EE can indeed improve force output.
Perhaps, however, this occurs during
maximal rather than submaximal force
outputs when energy conservation is
not a priority. For example, the SEC
may actually increase force per stride
during the 100-m sprint or increase
competitive jump displacement in the
long jump or high jump. In addition,
many authors report that the contrac-
tile components, when not lengthening
or shortening, are in fact undergoing an
isometric action, which is optimal for
force generation during SSC activities
(39,63,105). The role of EE appears
contentious and will be discussed
further in this article along with
additional mechanisms that may fur-
ther explain the increase in power
output after an SSC.
It should be noted that there is a point
of diminishing returns whereby once
the eccentric loading (stretch phase)
reaches a critical threshold, the sub-
sequent concentric contraction exhib-
its no further increase in force/EE
return and may even result in reduced
force output. This is likely because the
change from the eccentric contraction
into the propulsive concentric contrac-
tion (i.e., amortization phase) takes too
long (106). This may be a consequence
of involuntary neural inhibition (dis-
cussed later in this article) ultimately
causing the EE to be released and lost
as heat energy during the amortization
phase (69). In addition, Wilson et al.
(106) found that the SSC had a half-life
of 0.85 seconds and that by 1 second,
the benefits diminished by 55%. This
investigation, however, examined the
countermovement within a bench
press (of male weightlifters) and thus
results may be more indicative of
upper-body mechanics. Therefore, if
EE is to be optimally used, the load
experienced within the eccentric phase
should be within the limits of the
athlete and the amortization phase
should be as rapid as possible. It is
well recognized that these variables
differ among athletes and, most im-
portantly, are measurable and trainable
(37,87). This can further be exampled
by reviews from Newton and Dugan
(87) and Flanagan and Comyns (37)
who reported that untrained individu-
als generally attain higher vertical
displacements after the CMJ compared
with drop jumps. As the individual’s
SSC mechanics develop, however, this
reverses and higher values are achieved
during drop jumps. Moreover, with
progressive training, the athlete can
continue to increase the drop height
with concomitant increases in jump
height.
MECHANICAL MODEL
To further understand the role of EE,
it is important to consider the me-
chanical model of muscle function
devised by Hill (49). Hill (49) suggests
that force can be analyzed as the
summation of 3 components: a con-
tractile component, namely, actin and
myosin; a parallel elastic component
(PEC), comprising the sarcolemma
and muscle fascia; and a SEC,
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88
Mechanisms and Training Strategies of SSC
comprising the cross-bridges, struc-
tural proteins, and tendons. The PEC
is responsible for force exerted by
a relaxed (passive) muscle when it is
stretched beyond its resting length. Its
contribution of stored mechanical en-
ergy is considered small and conse-
quently so too is its contribution to
propulsive force. Conversely, the SEC
is put under tension by the force
developed in actively contracted
muscles. Because an actively con-
tracted muscle resists stretching, par-
ticularly if the stretching is imposed
rapidly (28,29,63), the SEC results in
considerable storage of energy.
Most research describes the tendon as
the workhorse of the SEC (64,74);
however, the additional components
have still been credited with signifi-
cance. The following sections thus pro-
vide a brief review of the components
oftheSECthatmaycontributetothe
enhanced power output after the SSC.
CONTRACTILE FILAMENTS AND
STRUCTURAL PROTEINS
The time span for cross-bridge mainte-
nance has been estimated to be in the
region of 15–120 milliseconds (96). The
amortization period therefore needs to
be minimal to augment energy return
from these structures (96) because
energy is lost at the instance of de-
tachment (37). To the contrary, Fleck
and Kraemer (37) hypothesize that the
period of amortization is simply too
long to allow any significant contribu-
tion from cross-bridge maintenance,
which they estimate to be in the region
of 30 milliseconds. They therefore
suggest that alternate mechanisms are
responsible for the SSC phenomenon.
Bosco et al. (14), however, suggests
that there is a difference in cross-bridge
life times between fast-twitch (FT) and
slow-twitch (ST) fibers, suggesting that
they exhibit different viscoelastic prop-
erties. In agreement, Siff (96) explains
that muscles that are rich in FT fibers
would benefit from a rapid short-range
SSC. Conversely, slower larger ampli-
tude jumps with a longer transient
period would benefit muscles rich in
ST fibers. Schmidtbleicher (94)
describes these as short and long
SSC, respectively, whereby the former
has a ground contact time (GCT) of
,250 milliseconds (e.g., a drop jump)
and the latter .250 ms (e.g., a CMJ).
It has also been suggested that the
prestretch of active muscles alters the
properties of the contractile machinery
(10), whereby cross-bridges may be-
come ‘‘stuck’’ on stretch and release
slower than when followed by an
isometric or concentric action (72). In
addition, the force produced by teta-
nized single muscle fibers may be
augmented by a prestretch (28,29).
This enhancement has been shown
to increase with the speed of prestretch
and to decrease with the amount of
time elapsed after the prestretch
(28,29,63).
Finally, the structural protein titin,
which spans half the sarcomere from
the M-line to the Z disc, has been
proposed as an explanation of passive
force enhancement (72). It has been
suggested that titin attaches to actin in
a calcium-dependent manner so that its
length becomes smaller and stiffness
increases on muscle activation when
calcium is released from the sarcoplas-
mic reticulum into the sarcoplasm
(69,72). The role of titin within the
SSC mechanics is relatively novel and
as such requires further research.
TENDONS
Tendons are considered the key site for
energy storage within the SEC because
of their ability to extend and store
energy and recoil and release energy
(64,74). Kubo et al. (64) suggests that
the EE stored in tendons is a key
mechanism underpinning the SSC
phenomenon. This is in agreement
with Lichtwark and Wilson (74) who
suggest that tendon recoil is responsi-
ble for both increasing power output
and conserving energy during locomo-
tion. The elastic properties of a tendon
are therefore very important for power
production and efficiency.
In contrast to the tendon, muscle tissue
is not efficient at energy storage and
return. However, because muscle and
tendon are arranged in series, they are
subjected to the same forces (112). The
distribution of stored energy among
these tissues therefore is dependent
on their deformation, which in turn is
a function of stiffness or its inverse
value compliance (112). Put simply,
whichever tissue structure stretches
the most will store the most EE. For
example, during passive stretch, the
stiffness of the PEC is less than 100
times that of the tendon and thus
the majority of deformation occurs in
the PEC (71). Conversely, during active
movement, the stiffness of the muscle
tissue and its surrounding PEC far
exceeds that of the tendon, therefore
reversing the storage site for EE (110).
As a consequence of the tendon’s sup-
erior ability to store and release energy,
a goal of all athlete training programs
should be the optimal transfer of
potential energy arising from a pre-
stretch being delivered to these struc-
tures. This transfer, however, can only
be optimized through the development
of muscle stiffness throughout the
prestretch. Stiffness and compliance
are therefore key terminology when
explaining the efficiency of the SSC
and the enhanced power output noted
in trained athletes. The following
sections thus provide an overview of
the stiffness-compliance continuum
and the performance-related benefits
of muscle stiffness.
STIFFNESS AND COMPLIANCE
Zatsiorsky and Kraemer (112) explain
that while the stiffness of a tendon is
constant, the stiffness of a muscle is
variable and depends on the forces
exerted (i.e., the muscle is compliant
when passive and stiff when active).
Through training, particularly plyo-
metrics (68,77,85,91,93,95,97), it is
possible to develop high forces and
maintain high levels of stiffness in
muscles, exceeding that of tendons.
In such a scenario, whereby the muscle
does not stretch, the tendon is forced
to. As discussed above, this is optimal
with elite athletes demonstrating a
superior ability to store EE primarily
in their tendons (2,24,46,50).
Strength and Conditioning Journal | www.nsca-lift.org 89
Leg stiffness can be defined as the ratio
of maximal ground reaction force to
maximum leg compression during the
mid stance phase (50) or by dividing the
change in force by the change in length
(55). Komi (58) suggests that higher
stiffness levels of lower limb muscles
during SSC exercises increase the
amount of stored and reused EE. Also,
Bojsen-Moller et al. (11) found that
power, force, and velocity parameters
obtained during jumping had a signifi-
cant and positive correlation to tendon
stiffness. In agreement, leg stiffness has
been shown to augment with an in-
crease in jump height and hopping
height (3,31,32), and knee joint stiffness,
that is, the ability to resist flexion, has
been shown to be crucial to performance
after a drop jump (53). Arampatzis et al.
(3) also demonstrated that GCT and
ankle joint stiffness were inversely re-
lated during drop jumps. Similarly,
Kuitunen et al. (67) also demonstrated
that subjects with the highest stiffness in
the ankle joint had the shortest GCT at
all running speeds and that these times
are also related to decreased flexion
in joints such as the knee and hip.
Furthermore, a positive correlation ex-
ists between rate of force development
(RFD) and connective tissue stiffness in
the lower body (11) and the upper body
(106). Heise and Martin (46) and
Dalleau et al. (24) reported a further
positive correlation between leg stiff-
ness and running economy, concluding
that economical runners possessed
a running style that was stiffer during
ground contact. Therefore, in agree-
ment with Wilson and Flanagan (105),
there appears to be a strong relationship
between the amount of stiffness in a
human system and various parameters
of sports performance.
DEVELOPING MUSCLE STIFFNESS
Leg stiffness largely depends on ankle
stiffness (3,31,32), and joint stiffness in
general depends on antagonistic co-
contraction (17,54,89). These in turn
are regulated by muscle tension at
landing (preactivation; 50) and the
concerted actions of the involuntary
reflexive neural processes (discussed
later in this article). For example, co-
contraction between the plantarflexor
and dorsiflexor muscles and the knee
extensor and knee flexor muscles will
increase joint stiffness throughout the
whole leg in preparation for ground
impact (54). In agreement, Kuitunen
et al. (67) found that as running speed
increased, preactivation of the plantar
flexors and knee extensors increased,
increasing MTC stiffness and the
ability to tolerate and absorb high-
impact loads at the beginning of the
contact phase. Moreover, the preacti-
vation of the triceps surae muscle
coupled with the stretch reflex and
Golgi tendon organ (GTO) inhibition
will ensure high muscular stiffness at
ground contact to support and propel
the body (59). Finally, Gollhofer et al.
(43) and McBride et al. (77) found that
increase in preactivation and eccentric
phase muscle activity increased con-
centric work output.
In a study by Hobara et al. (50), it was
shown that power-trained athletes (e.g.,
sprinters) show higher leg stiffness and
ankle stiffness than endurance-trained
athletes and untrained individuals. Fur-
thermore, these athletes exhibit less
GCT and longer aerial time during
hopping. This is in agreement with
Arampatzis et al. (2) who also reported
that, as well as exhibiting higher max-
imal plantar flexion moments, sprinters
also have higher stiffness in the triceps
surae tendon and that a significant re-
lationship exists between the two (r=
0.817). The higher tendon stiffness is
likely to reduce the probability of
tendon strain injuries (excess elonga-
tion may lead to partial or complete
tendon ruptures), which would occur
as a consequence of stronger muscles
(2,83). In addition, the increased ten-
don and muscle stiffness may increase
joint stability through its resistance to
joint displacement (16). Therefore, it
may be concluded that both muscle
and tendon tissue show plasticity to
sufficient external mechanical loads.
Both undergo hypertrophy and share
the characteristic of enhanced collagen
synthesis resulting in stiffer muscles
and also stiffer tendons. It may be
deduced, however, that this increased
tendon stiffness reduces elongation and
impairs elastic strain energy. In sup-
port, investigations by Kubo et al. (64)
reveal that tendon stiffness in the knee
extensors is inversely correlated with
countermovement prestretch augmen-
tation. This is conceivable because
prestretch augmentation has been
shown to be significantly greater in
individuals with compliant tendon
structures because this allows for
greater EE storage under a given extent
of force (18,64,83) Conversely, with
a stiff tendon, all the shortening must
occur in the muscle tissue, which under
the high velocities of the SSC is not
optimal for contractile force (74).
In summary, despite stiffness possibly
increasing in both tissues (7), the ratio
of muscle to tendon stiffness increase
may be such that optimal efficiency is
maintained based on anatomical loca-
tion and continued imposed demands.
For example, Lichtwark and Wilson
(74) suggest that tendon stiffness is
optimal to achieve the highest effi-
ciency in both walking and running.
Also, the human Achilles tendon can
strain up to 10.3% during 1-legged
hopping (73), whereas the anterior
tibialis tendon has a maximum re-
corded value of 3.1% (75). In addition,
Markovic and Jaric (76) also explain
that the muscular system is adapted to
provide maximum mechanical output
under accustomed loads of daily living.
Therefore long-term exposure to high-
er or lower loads would then result in
shifting the optimal loading for maxi-
mal mechanical output. Similarly, it has
also been reported that the stiffness of
the lower limbs may be limited to
prevent injury, whereby high stiffness
may increase stress to the anatomical
structures during ground contact
phases (16,24,80).
In support of the research by Markovic
and Jaric (76), human tendon stiffness
has been shown to increase only after
resistance training using heavy loads
(61,66,92) and isometric squats (61,66).
Therefore, any increase in muscle
strength would be offset by stiffer
tendons (65). Conversely, jumping,
sprint training (86,99), and drop jump
VOLUME 32 | NUMBER 4 | AUGUST 2010
90
Mechanisms and Training Strategies of SSC
protocols (62) have produced no
significant changes in tendon proper-
ties. This led investigators to conclude
that an exercise protocol centered on
ballistic contractions (rapid accelera-
tion against resistance) cannot change
the tendon properties and that adapta-
tions leading to increases in joint
stiffness are therefore related to the
significant changes in mechanical
properties of the muscle (e.g., active
cross-bridges) not the tendon (65). Siff
(96) also hypothesized that increases in
strength and stiffness are achieved with
increased loading and increased rate of
loading. Therefore, training should
emphasize exercises with high accel-
eration methods. Taken collectively,
the research may allow for the follow-
ing deduction. Strength training should
precede plyometric training to develop
sufficient strength in the muscles and
tendons and reduce the probability of
tendon injuries. After strength training,
however, plyometrics (and ballistics
alike) must be performed to create
a more favorable environment for
structural adaptations in the muscle
only, whereby increases in the muscle
to tendon strength ratio are noted and
stiffness becomes optimal for force
production/maintenance. The above
literature summarized the role of EE in
the increased propulsive force noted
after SSC actions. The following sec-
tions focus on additional mechanisms,
namely, the neurophysiological model,
active state development, isometric
muscle actions, and length-tension
characteristics.
NEUROPHYSIOLOGICAL MODEL
It has been suggested that the muscle
spindle may be responsible for the
potentiation after a prestretch because
of its initiate reflex recruitment of
additional motor units or increased
rate of firing of already recruited units
(12,16). This mechanism may partly
contribute to the development of
a high level of active state, enabling
the muscles to generate larger forces
and thus more work during the
concentric phase before the start of
push-off (10). Electromyographic
(EMG) analysis, however, does not
support this hypothesis. Many studies
have reported a nonsignificant change
in EMG activity after a prestretch
when compared with a non-prestretch
action (33,65,98). It may be concluded,
therefore, that reflex activity does not
account for the increased force caused
by the SSC (9,10,65). It is also in-
teresting to note that myoelectric
activity does not change during max-
imal vertical jump performance with or
without additional loads (96). This
suggests that ballistic actions maxi-
mally activate motor units regardless of
muscle shortening velocity and force
production during the concentric
phase (33,65,77,98,103).
Neural reflexes, however, despite pos-
sibly not explaining the difference in
vertical displacements of various
jumps, are likely intimately involved
in the final power output after SSC-
type activities. The nervous stimulation
to the muscle during the eccentric
phase of an SSC is modified by the
concerted actions of the muscle spindle
and GTO (68,95). Zatsiorsky and
Kraemer (112) summarized this re-
lationship with the following example:
during landing after a drop jump, the
stretch applied to the leg extensor
causes the muscle spindle to contract
that muscle. However, the sudden high
muscular tension causes the GTO to
simultaneously inhibit its activity. This
drop in muscle tension prevents the
muscle and tendon from incurring
damage. Therefore, if athletes are not
accustomed to these exercises and
loads, the activity of the extensor
muscles during takeoff is inhibited by
the GTO. In support, Schmidtbleicher
et al. (95) reported that in subjects
unaccustomed to intense SSC move-
ments, EMG activity was reduced from
50 to 100 milliseconds before ground
contact and lasted for 100–200 milli-
seconds. After plyometric training,
however, it is possible to reduce the
inhibitory effects (disinhibition) of the
GTO, whereby the athlete is able to
sustain high landing forces without
a decrease in exerted muscular force
(68,95). The intensity (e.g., dropping
height or load) may then be increased.
For example, Kyrolainen et al. (68)
reported that after 4 months of SSC
training, the preactivity of muscles
increased and this change led to
increased MTC stiffness.
When this data are examined alongside
the above theories underpinning the
efficacy of muscle stiffness, additional
conclusions can be proposed. When an
athlete is trying to generate maximal
muscular effort and attain high levels of
muscle stiffness, there becomes a trade-
off between the 2 reflexes and voli-
tional muscle activation. It is evident
that it is not simply a case of the athlete
contracting as hard as possible. Ulti-
mately, the final power output is
regulated by subconscious neural re-
flexes. Moreover, the intensity of each
reflex, which is not constant, deter-
mines the final outcome (112). There-
fore, the objective of drop jumps, for
example, may be to expose the athletes
to fast muscle stretching rather than
to immediately generate large forces
(112). Although nervous reflexes may
not explain the potentiation after the
SSC, they may be able to limit it. As
suggested above, however, it may take
up to 4 months of plyometric training
to inhibit the GTO and enable the
potentiation of the muscle spindle, if
indeed it does occur.
WORKING RANGE
Investigations by Bobbert et al. (10)
and Bobbert and Casius (9) and a
comprehensive review by Van Ingen
et al. (100) reported that the greater
jump height seen in the CMJ com-
pared with the SJ was exclusively
because of the fact that greater active
state could be developed during the
prestretch. This results in an increased
impulse (force 3time) and thus a
greater change in vertical velocity of
the body. They suggested that this may
be because of the greater moments
occurring at the hip, knee, and ankle
joints, enabling the production of
greater force and work over the first
part of the propulsive phase. This also
led to an increase in vertical velocity
over the entire concentric range (100)
and greater ground reaction force at
Strength and Conditioning Journal | www.nsca-lift.org 91
the start of push-off in the CMJ
compared with other jumps (10).
The time delay in reaching peak force
is in part because of the finite rate of
increase in muscle stimulation by the
central nervous system, the propaga-
tion of the action potential on the
muscle membrane, time constants of
calcium release and cross-bridge for-
mation, and the interaction between
contractile elements and the SEC
(10,84,111). In addition, the slack,
caused by the crimped orientation of
tendon fibers (71) (described by the toe
region of the tendon force-deformation
curve), must be stretched out of the
SEC before it will transmit any force to
the skeletal system (49). This factor is
reportedly a significant contributor to
the time to develop peak force (111).
Collectively, this delay in reactivity is
known as the electromechanical delay
(EMD) and describes the interval
between the time of onset of muscle
activity and the time of onset of
mechanical output (19,84). This time
delay is consequential to commencing
movement from zero to low muscular
tension, and its negative effects can be
reduced by enabling the muscle to
build up a maximum active state before
the start of the propulsive phase
through either an isometric contrac-
tion (e.g., preloading) or a counter-
movement (10). Moreover, it has been
hypothesized that increases in MTC
stiffness would decrease the EMD,
allowing muscles to generate tension
more rapidly and counteract deleteri-
ous forces at joints (105).
In support of an active range, Bobbert
et al. (10) found a direct relationship
(r= 0.88) between the time to
stimulate the gluteus maximus and
vertical ground reaction force in
a non–countermovement jumping task.
In addition, with the use of a simulated
spring model, Bobbert and Casius (9)
further demonstrated that by increasing
the rate of muscular stimulation during
an SJ, the difference in vertical ground
reaction force was reduced because this
in turn reduced the distance covered at
a submaximal active state. They there-
fore concluded that the difference
between CMJ and SJ heights would
vary among individuals depending on
their ability to develop force. A small
difference would be seen in those who
could develop force quickly, whereas
a large difference would be seen inthose
who developed forces at slower rates.
The authors attributed this to the faster
cross-bridge cycling rates of FT fibers
and hence the ability to build maximal
force at greater rates than ST fibers. To
the contrary, however, Bosco et al. (14)
found no difference in jump heights
after short SSCs between individuals
with predominantly FT or ST muscle
fiber composition. However, and in
agreement with Van Ingen et al. (100),
the investigators reported that individ-
uals with predominantly ST muscle
fibers benefited most from longer SSCs
such as the CMJ. Finni et al. (33)
provided further contention to the
above theories by reporting that partic-
ipants jumped higher when using
a CMJ compared with a condition that
allowed for the build up of maximal
isometric force before the vertical jump.
They therefore concluded that other
factors, such as EE, must contribute
to the enhanced performance after a
prestretch.
Research supporting the significance of
the development of active state, how-
ever, may gain further credibility when
examined concurrently with research
concerning the time available to de-
velop peak force (i.e., RFD). Aagaard (1)
hypothesized that in skilled athletes,
this takes between of 0.25 and 0.4
seconds, with force linearly increasing
throughout. Other researchers, how-
ever, suggest that maximum force de-
velopment may require 0.6 to 0.8
seconds (27,58). A prestretch therefore
provides additional time over which
force can be developed, ensuring that
by the time of the concentric contrac-
tion, the accumulated force is above that
of the starting force of the SJ, leading to
more powerful propulsion. Fundamen-
tal to coaches, RFD is trainable, with
advancedathletes from power-orientated
sports reaching peak values quicker
(112) and perhaps explaining their
increase in jump heights.
MUSCLE-TENDON
INTERRELATIONSHIP AND
ISOMETRIC CONTRACTIONS
Kubo et al. (60) examined the MTC of
the human medial gastrocnemius dur-
ing ankle dorsiflexion to plantarflexion.
The movement consisted of dorsiflex-
ion at 2 different speeds of lengthening,
followed by plantarflexion. The inves-
tigators revealed that the tendon length
increased to a greater extent in the fast
lengthening condition. No significant
change was found in the fascicle length
during the first half of plantarflexion,
whereas the tendon rapidly shortened.
Both the tendon and muscle rapidly
shortened in the second half of plan-
tarflexion, suggesting that during the
transition from prestretch to shorten-
ing, the muscle contracted isometri-
cally. These findings are in agreement
with Fukunaga et al. (38) who found
that during the stance phase of walk-
ing, the medial gastrocnemius muscle
contracted isometrically, whereas the
tendon lengthened by 7 mm. During
the push-off phase, however, both the
tendon and muscle rapidly shortened.
The isometric action may be of benefit
because it likely avoids the lowered
force output that occurs with increas-
ing velocity and can also far exceed the
force output of concentric contractions
(27). Fukunaga et al. (38) and Wilson
and Flanagan (105) hypothesized that
before the isometric action, muscles
independently lengthen toward their
optimal length-tension relationship
to increase concentric force output.
This is in agreement with Lichtwark
and Wilson (74) who suggested that
muscles contract at favorable speeds,
which may differ from the MTC. In
further support, Ishikawa et al. (56)
found that the medial gastrocnemius
only lengthened during the early
stance phase of walking, whereas the
soleus continued to lengthen until the
end of the stance phase, when both
muscles rapidly shortened at toe-off.
LENGTH-TENSION RELATIONSHIP
As mentioned above, muscle or facial
length may be another mechanism
involved in increasing force output after
an SSC. Finni et al. (33) demonstrated
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92
Mechanisms and Training Strategies of SSC
that the vastus lateralis generates more
force with a prestretch compared with
no prestretch, yet there is no difference
in EMG activity (as discussed above).
The force enhancement may therefore
be related to a longer muscle or fascicle
length before the concentric phase,
placing the muscle in a more advanta-
geous position on the length-tension
diagram to produce force (36). In
agreement, Ettema et al. (30) explained
that during the start of propulsion in
both the CMJ and SJ, the muscle fibers
are on the descending limb of their
length-tension relationship. However,
in the CMJ, because of stretching of
the SEC, they are less beyond their
optimal length and are therefore able to
produce greater force over the first part
of their concentric range.
METHODS TO ENHANCE SSC
MECHANISMS
Because the SSC clearly plays an
important role in performance in many
sports, then developing this capacity
via effective training practices is crucial.
Reportedly, the optimal method to
train SSC motor skills is plyometrics
(68,77,85,91,93,95,97). The following
sections outline how plyometric exer-
cises can be progressively integrated
into an athlete’s training program and
also outline appropriate methods of
performance evaluation. The practical
suggestions herein will be made based
on the evidence for the SSC mecha-
nisms highlighted in the preceding
sections.
Plyometrics cover a wide range of
jumping, hopping, and bounding-based
exercises that have the fundamental
aim of enhancing SSC performance.
Although appearing relatively simple
tasks, for example, a CMJ or a drop
jump, plyometric exercises are in fact
very complex and fundamental move-
ment skills. As such, appropriate time
should be allocated to the development
of these skills, and the strength and
conditioning coach should ensure that
the athlete displays mastery in these
before progressing to additional drills.
This requires a progressive system of
exercises to be set up, through which an
athlete can pass to ensure that they have
the required technical mastery to be
able to perform the entire gamut of
plyometric exercises in a fashion that
both maximizes performance gains and
also minimizes injury risk.
Ideally, in terms of maximizing perfor-
mance, plyometric training should be
preceded by strength training to reduce
the risk of injury to the MTC and
increase the quality and quantity of type
II fibers. The latter point is of signifi-
cance because of the high correlation
between the percentage of type II fibers
and peak power output (23) and is
therefore likely to increase the athletes’
net potential to develop power (58). As
a consequence of the size principle of
motor unit recruitment (47), strength
training, that is, $85% 1 repetition
maximum (RM), #6 repetitions, 2–6
sets, 2- to 5-minute rest (6), is required to
recruit these type II fibers (45).
Although this sequence of strength
training preceding plyometric training
is undoubtedly physiologically sound, it
may not optimize sequential develop-
ment based on a motor skill basis, and
holding back the introduction of plyo-
metric development until a sound base
of strength has been developed may not
maximize long-term development. The
reality of modern sports is that perform-
ers will compete in their sports at a very
young age. Sports, such as basketball,
football, soccer, and the like, inherently
contain a large number of SSC activities
and involve numerous jumping and
landing activities. These activities are
more often than not undertaken before
an adequate strength base has been
established, and therefore, the develop-
ment of effective landing techniques, for
example, takes on a very important role.
Additionally, and as described above,
plyometric exercises contain a large skill
component in addition to a physical
component. It therefore seems logical to
progressively develop both compo-
nents in a concurrent fashion, rather
than having to develop the skill capa-
cities from scratch once the strength
base has been established. It is, however,
prudent to ensure that plyometric
exercises are prescribed based on
the athlete’s progression of physical
capacities.
INTRODUCING PLYOMETRIC
EXERCISES—THE PLYOMETRIC
PYRAMID
In introducing any skill, there needs
to be a sequence of progression that
allows an athlete to master basic
components before moving onto more
advanced exercises. Developing a pro-
gressive system requires a basic knowl-
edge of the factors that determine
plyometric intensity. Armed with this
knowledge, exercises can be sequenced
to provide appropriate stimulus for
an athlete based on the aim of the
program and on their physical and skill
capacities at any time. Jeffreys (57) lists
the determinants of plyometric inten-
sity as follows:
1. The speed of movement, the greater
the speed the greater the intensity.
2. The points of contact, with single-
leg drills being more intense than
double-leg drills.
3. The amplitude of movement, with
greater amplitudes ground contact
forces and hence increasing
intensity.
4. The athlete’s weight (or additional
load), with the greater weight lead-
ing to higher intensities.
Additionally, exercises where an ath-
lete moves from an eccentric move-
ment to a concentric movement (e.g.,
a depth jump) are more intense than an
eccentric movement alone (e.g., a drop
land), other things being equal.
In essence, plyometric ‘‘skills’’ revolve
around 2 basic capacities, jumping and
landing. Although these are basic skills,
a failure to adequately develop these
will hinder the optimal application of
plyometric exercises, limit athletic de-
velopment, and also expose the athlete
to greater injury potential. Based on the
evaluation of plyometric intensity and
the need to develop jumping and
landing skills, Jeffreys (57) advocated
the use of the plyometric pyramid as
a method of introducing plyometric
exercises. This involves 3 categories of
exercise, all of which have a given aim
and are able to alter intensity within
each stage. Throughout these stages,
Strength and Conditioning Journal | www.nsca-lift.org 93
the main focus is on technical de-
velopment, thus ensuring that on
completion of the process, athletes
are adequately prepared for the full
range of plyometric exercises.
STAGE 1: JUMP TO BOX
This stage develops basic jumping
abilities and also, crucially, landing
ability in a controlled environment.
By excluding the time gravity has to
act and by teaching landing technique
to beginner athletes or athletes with
current landing problems, landing
forces can be minimized. Varying the
height of the box can provide a chal-
lenge to the athlete’s jumping ability,
while still minimizing landing forces.
Moving from a double-leg to a single-
leg landing can further challenge the
athlete’s landing ability. The Figure
(left) illustrates an athlete demonstrat-
ing effective landing technique when
using the jump to box.
STAGE 2: JUMP AND STICK
This stage builds on the athlete’s
landing capacity developed in stage 1
and develops their ability to control
eccentric forces. Initially, exercises in
this stage can involve low-amplitude
movements, but progression can be
provided by increasing the amplitude
of movement and by moving from
double- to single-leg landings. As well,
by further developing landing tech-
nique, this stage allows the athlete to
adapt to high landing forces (eccentric
loads) through learned GTO disinhi-
bition. This stage, and the amplitudes
within, should be dictated by the
quality of the movement and not be
progressed until the athlete can stick
the landing with appropriate levels
of control and with appropriate foot
contact. Heel contact, for example, is
suggestive of GTO inhibition and
the athlete’s inability to optimally store
energy in the tendons, which is
essential to the amortization phase
(and duration of ) used in the sub-
sequent stage (35). In addition, and
described in the preceding text, this
stage also requires the development of
muscle stiffness through preactivation
tensioning and antagonist co-contraction
and may therefore take several weeks
to develop (68). The Figure (middle)
illustrates an athlete demonstrating
effective landing technique during
a drop land.
STAGE 3: SHORT-RESPONSE
JUMPS
This stage begins the true plyometric
training where the SSC is used to
enhance subsequent concentric perfor-
mance. Here, the athlete performs
jump activities of initially low ampli-
tude, where the aim is to minimize
GCT, while maintaining effective land-
ing mechanics and body control.
Again, this stage should be progressed
to involve greater amplitude of jumps
and the utilization of single-leg
Figure . Jump up to box (left), drop land (middle) step from box (right) (before drop land or drop jump). When performing these
drills, coaches should ensure that the athlete uses the appropriate foot contact and displays the correct limb alignment
(i.e., shoulders in line with the knees, helping to place the center of gravity over the body’s base of support, and ensuring
no valgus or varus movement at the knees. In addition, the shoulders should be pulled down and back with the hands to
side, ready to react.).
VOLUME 32 | NUMBER 4 | AUGUST 2010
94
Mechanisms and Training Strategies of SSC
activities. Ankling drills provide a good
example of a short-response jump. In
addition, there is research suggesting
that overall leg stiffness is correlated by
ankle stiffness (2,31,32); therefore, an-
kling may provide a prudent starting
point. The Appendix provides descrip-
tion of the ankling drill.
Athletes who have moved through
these stages should have developed the
required skills to enable them to use
the full gamut of plyometric exercises.
The appropriate exercise to elicit the
required physical adaptations can then
be progressively introduced into the
program.
START WITH THE END IN MIND
When constructing plyometric pro-
grams, coaches need to be acutely
aware of the fundamental aim of their
training and the precise physical ca-
pacities they are trying to develop. This
allows the programming variables to be
appropriately applied to elicit specific
training effects.
One key factor when considering
appropriate plyometric drills is the
GCT involved in the activity to which
the drill is aimed at. Hennessy and
Kilty (48) and Schmidtbleicher (94)
found low correlations between jump
heights after a CMJ and a drop jump,
suggesting that these SSC activities are
measuring different movement charac-
teristics. To this end, Schmidtbleicher
(94) categorized plyometric activities
as either slow SSC (.250 milliseconds)
or fast (,250 milliseconds) SSC,
depending on their GCT.
As an example, therefore, a CMJ (slow
SSC) may be more suitable to train the
acceleration phase of the 100 m, as
Plisk (87) hypothesized that the force
exerted by the front leg during push-off
is applied for .250 milliseconds in elite
sprinters, with some investigators
reporting ranges of 0.34–0.37 milli-
seconds (81,83). Similarly, ski jumping
(60), shot putting (70), and a standing
takeoff in platform diving (83) have
GCT in excess of 250 milliseconds and
would benefit from CMJ training or
derivatives of (e.g., tuck jump, split-SJ,
jump over barrier, and single-leg
progressions). Conversely, a drop jump
(fast SSC) and its derivatives (e.g.,
multiple hurdle jumps, bounding and
single-leg progressions) would be more
suitable to train top speed sprinting
(82) and the take-off phase of the long
jump (109) and high jump (24) because
these SSC actions have a GCT less
than 250 milliseconds. To this end, the
GCT and the type and direction of
forces should guide plyometric choice.
During plyometric exercises, strength
and conditioning coaches should place
emphasis on the importance of maxi-
mizing jump height (where applicable)
while minimizing GCT (105,107,108).
Although this seems logical, it has
important implications for appropriate
progression within plyometric pro-
grams. For example, where fast SSC
activities are required, the use of
additional loads, increased height of
jumps, and the like may bring about an
undesired increase in GCT and hence
change the underlying nature of the
exercise. Therefore, care should be
taken when progressing plyometric
activities so as not to negatively affect
the main aim of the exercise. This is
also the case where an attempt to make
plyometrics more ‘‘sport specific,’’ by
including balls and the like could result
in a degradation of the physical
performance, thus negating the ulti-
mate training aim of the activity, that is,
enhancing SSC performance.
EVALUATING THE SSC
MECHANISM
As GCT is an important variable in
plyometric training prescription, mon-
itoring of this important variable is
important and can be achieved using
training/testing equipment, such as
contact mats and force plates, and is
available in real time, possibly facilitat-
ing athlete motivation (34,86). More-
over, calculation of the reactive
strength index (height jumped/GCT)
during activities, such as drop jumps,
can provide strength and conditioning
coaches with a good indication of an
athletes’ SSC ability (35,36,89,107,108,
110). This is usually tested over the
following drop heights: 30, 45, 60, and
75 cm (89). As previously mentioned,
efficient SSC mechanics should result
in greater jump heights from greater
drop heights. An additional method for
monitoring prestretch augmentation is
described by McGuigan et al. (79) and
Walshe et al. (110) who compared the
CMJ with the SJ and used the
following formula: % prestretch aug-
mentation = ([CMJ 2SJ] 3SJ
21
)3
100. Alternatively, reactive strength
may simply be calculated as CMJ 2
SJ height (107). Although monitoring
the athlete’s training adaptations to
plyometrics training is considered fun-
damental, the optimal method to do
this still needs to be fully elucidated
and may simply depend on the avail-
ability of specialist equipment.
PLYOMETRICS AS PART OF A
TOTAL PERFORMANCE PLAN
As with all training modalities, plyo-
metrics should not be performed in
isolation and instead as part of a total
performance program that includes
multiple modalities. It is therefore
advised that the strength and condi-
tioning coach should include addi-
tional ballistic exercises (explosive
resisted movements in which the body
or object is subjected to full accelera-
tion) such as weightlifting movements
because these may enhance the ath-
lete’s power output throughout the
triple extension (of the hips, knees, and
ankles) (51) inherent to lower-body
SSC motor skills. These exercises are
also advocated to enhance Rate of
Force Development (RFD) (35,44,51)
specifically within the first 200 milli-
seconds (43) of force production. This
may therefore assist in the develop-
ment of active state, which as dis-
cussed, may be a fundamental tenet of
SSC activities (9,10,100). Moreover,
these may be of additional significance
because the vast majority of athletic
SSC movements occur within 0.3
seconds (111). As previously men-
tioned, strength training will also play
a part in maximizing SSC activity.
Although further research still needs
to be carried out into the optimal
application of plyometrics, some re-
search suggests that its concurrent
combination with power/ballistic
Strength and Conditioning Journal | www.nsca-lift.org 95
training (combination method) may
produce superior results across a wide
variety of athletic performance varia-
bles requiring power and speed when
compared with using either method in
isolation (22,55,90).
Additionally, SSC activity may be
affected by its prior contractile history,
and this needs to be taken into account
when planning programs. For example,
Comyns et al. (20) examined the acute
effects of 3 back squats performed
using 65, 80, and 93% 1RM on the
performance of a DJ to determine if an
optimal resistive load exists for
complex training (see the references
Docherty et al. (25) and Ebben (26) for
a review of complex training). Results
showed that all resistive loads reduced
(p,0.01) flight time and that lifting at
the 93% load resulted in an improve-
ment (p,0.05) in GCT and leg
stiffness. These results may suggest that
heavy lifting will enhance the fast SSC
mechanism (possibly through postac-
tivation potentiation) because of
a stiffer leg spring action, which in
turn may benefit performance. How-
ever, it should also be noted that
although some activities may provide
the potential to enhance SSC activity,
others may decrease it. Magnusson
(75), for example, reported that static
stretches resulted in an acute reduction
in muscle stiffness and therefore should
be avoided in warm-up activities for
sessions where SSC activity is involved.
QUALITY NOT QUANTITY
Comyns et al. (21) examined the effect
of a maximal SSC fatigue workout on
the performance of a DJ performed 15,
45, 120, and 300 seconds after fatigue.
The results indicated that the fatigue
workout significantly reduced flight
time (p,0.001) and peak force (p,
0.01) and increased GCT (p,0.05) at
the 15-second interval, suggesting that
the efficiency of the SSC mechanism
was reduced. However, the results also
showed a potentiation effect at the
300-second interval because of a signif-
icant increase in peak force and leg
stiffness (p,0.05). Significant to the
former research finding, the negative
effects of fatigue on the SSC
mechanism have also been demon-
strated after submaximal intensity
workouts (41,42,52,58,88) and after
completion of a marathon run (5).
Therefore, as the SSC mechanism may
be negatively influenced by fatigue, the
quality of movements should always be
a critical factor in assessing perfor-
mance during a session and will help
a coach gauge appropriate volumes of
plyometric activities. This knowledge
should also guide the application of
plyometrics within the annual macro-
cycle, with the activity being most
effective in cycles without excessive
fatigue.
CONCLUSION
Efficient SSC mechanics result in
energy conservation of locomotion
and enhanced propulsive forces. This
efficiency, however, is largely a conse-
quence of an individual’s ability to
transfer all stretch to the tendon
through maintenance of muscle stiff-
ness. In turn, this can only be achieved
with sufficient plyometric training,
enabling GTO disinhibition and sub-
sequent preactivation tensioning and
concomitant antagonistic co-contrac-
tion. Reportedly, the optimal method
to train SSC movement skills is
plyometrics, and appropriate drills in-
clude drop lands, whereby the body
adapts to high landing forces, and drop
jumps, whereby the focus shifts to
reducing the amortization phase and
therefore the loss of EE. In addition,
plyometric training should be preceded
by strength training to reduce the risk
of injury to the MTC and increase the
quality and quantity of type II fibers.
Finally, because of the significance of
active state, it is suggested that athletes
train RFD through the use of ballistics
such as plyometrics and weightlifting.
The potentiation effects of the muscle
spindle remain contentious because of
insignificant EMG tracings. Based on
the current review of research, EE via
tendon recoil and an increase in active
state because of an increase in the
working range seems the most plausi-
ble causes of the increase in force seen
after SSC actions. These findings are in
agreement with Wilson and Flanagan
(104) who conducted a similar review.
They further speculated that the de-
velopment of active state predomi-
nated in enhancing force output in
a long SSC, whereas a short SSC relies
more heavily on the reuse of EE.
Anthony
N. Turner is
a Senior Lecturer
andStrength&
Conditioning
Coach at Middlesex
University,
London, England.
Ian Jeffreys is
Senior Lecturer in
Strength and
Conditioning at the
University of
Glamorgan, Wales,
and the Proprietor
and Performance Director of All-Pro
Performance in Brecon, Wales.
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APPENDIX: ANKLING
During ankling, the knees should
remain straight as the athlete steps from
one foot to the other. Throughout the
swing phase, the foot should be doris-
flexed with the big toe pointing up toward
the shin. At ground contact and the
instant before, the muscles of the foot and
ankle should forcefully contract (creating
MTC stiffness) and propel the body
forward as the ankle moves into plantar
flexion. Only the ball of the foot should
contact the ground and the initial foot
strike should be directly below the body’s
center of gravity. This exercise should
initially be performed on the spot and then
progressed to moving forward. Ankling is
an inherent part of jumping and running
activities and may therefore be consid-
ered a fundamental plyometric exercise.
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