Van Hooren Bosch 2016 Muscle slack

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Influence of Muscle Slack
on High-Intensity Sport
Performance: A Review
Bas Van Hooren, MSc and Frans Bosch, BSc
Fontys University of Applied Sciences, School of Sport Studies, Eindhoven, the Netherlands
A B S T R A C T
RAPID FORCE DEVELOPMENT IS
OF PARAMOUNT IMPORTANCE
FOR MOST SPORTS. AN OFTEN
OVERLOOKED PERFORMANCE
LIMITING FACTOR IS MUSCLE
SLACK, WHICH IS REPRESENTED
BY THE DELAY BETWEEN MUSCU-
LAR CONTRACTION AND RECOIL
OF THE SERIES ELASTIC ELE-
MENTS. WE WILL REVIEW ACUTE
AND LONG-TERM EFFECTS OF
APPLYING COCONTRACTIONS,
COUNTERMOVEMENTS (CMs),
AND EXTERNAL LOAD ON MUSCLE
SLACK. COCONTRACTIONS MAY
BE AN EFFECTIVE SOLUTION TO
REDUCE THE DEGREE OF MUSCLE
SLACK. MOREOVER, CMs AND
EXTERNAL LOADMAY NEGATIVELY
INFLUENCE THE CAPABILITY TO
DEVELOP COCONTRACTIONS
ANDHENCEMAY BE DETRIMENTAL
TO HIGH-INTENSITY SPORT PER-
FORMANCE THAT IS USUALLY
PERFORMED WITH LITTLE OR NO
EXTERNAL LOAD.
INTRODUCTION
I
n most sports, the time to develop
force is limited. For example, a de-
fending soccer player will try to
prevent an attacking player from
scoring by limiting the time available
to perform the kick. In addition, dur-
ing volleyball and basketball, some
jumps are performed under time pres-
sure, and in these situations jump
height may be compromised (19). In
many athletic movements such as
sprinting, javelin throwing and shot
putting, the time period in which
force can be developed is only about
300 milliseconds and usually even
much shorter (97). For example, dur-
ing linear top speed sprinting, the
ground contact time is only about
100 milliseconds, while it can take
up to 900 milliseconds to develop
maximum force (3). Therefore, in
most sports, the capability to rapidly
develop force is critical for maximiz-
ing sport performance. In addition,
during some daily activities, the capa-
bility to rapidly develop force is also
important. For example, falling (and
associated injuries) in elderly people
may be prevented by enhancing their
capability to rapidly develop force
after a sudden loss of balance.
PROCESS OF FORCE
DEVELOPMENT
Maximum force development takes
time (3) and because time is limited
during most sport actions, reducing
the time to reach maximum force will
likely improve performance. However,
to effectively reduce this time, it is
important to first understand the pro-
cesses that limit rapid force develop-
ment. If the limiting processes have
been identified, a next step is then to
investigate whether and how these
processes can be altered by training.
Therefore, the purpose of this review
is to discuss these processes and to
elucidate on whether and how they
can be altered by training.
MECHANISMS OF FORCE
DEVELOPMENT
During force development, 6 consecu-
tive mechanisms can be distinguished
(Table, Figure 1). We will first briefly
define these mechanisms and then
expand on their influence.
First, a relevant stimulus is detected.
This stimulus contains perception or
is processed into perception. Subse-
quently, the central nervous system
sends a gross signal to activate the
muscles. These 2 processes will be
described as the premotor reaction
time (steps 1 and 2). The signal ar-
rives at the neuromuscular junction
and propagates across the muscle
membrane to activate chemical pro-
cesses which lead to shortening of
the contractile element (CE) of the
muscle. The delay associated with
these processes is termed the electro-
chemical delay (ECD; step 3). The
contraction of the CE aligns, or
straightens, the muscle-tendon unit
(MTU) that before activation hung
in a relaxed position between the
attachment points of the muscle.
The time required for straightening
the muscle is known as the mechan-
ical delay (step 4). In the scientific
Address correspondence to Bas Van Hooren,
MSc, basvanhooren@hotmail.com.
KEY WORDS :
rate of force development; electrome-
chanical delay; resistance training;
explosive sport performance; high-
speed running; muscle-tendon unit
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literature, steps 3 and 4 are usually
combined and described as the elec-
tromechanical delay (EMD). As soon
as the MTU is aligned, the series
elastic element (SEE, the tendon
and aponeurosis) begins to stretch.
SEE stretching will be defined as
compliance (step 5). Once CE force
production diminishes, the SEE re-
coils and this is termed the catapult
effect (step 6). In this review, the
delay between the start of CE con-
traction (step 4) and SEE recoil
(step 5) will be termed muscle slack,
and the processes involved in this
delay will be described.
Reducing the time to move through
these steps will benefit high-intensity
sport performance. For example,
reducing the premotor reaction time
and the associated degree of muscle
slack will result in a faster action, or,
when total movement time remains
equal, provide more time to apply
force. Unfortunately, most strength
and conditioning research has focused
on the isolated force development ca-
pabilities of the muscle CE, neglecting
the influence of muscle slack and lim-
iting application of the findings to
actual sport performance. This limited
application will be evident in closed
and especially open motor skills,
where rapid force development is cru-
cial. This review though will start the
discussion at the EMD.
THE ELECTROMECHANICAL
DELAY
Shortening of the EMD can poten-
tially enhance high-intensity sporting
performance (40,52,90,95). Primarily
in older studies (8,78) the term motor
time is also used to describe EMD. The
EMD is the time interval between acti-
vation of the muscle fibers (i.e., arrival
of the action potential at the neuro-
muscular junction) and the onset of
Table
Process of force development in 6 chronological steps
Step Description Old Terminology New Terminology
1 Meaningful information (i.e., affordances) present in the environment is processed
by the senses.
Premotor time Premotor time
2 As a result of this information, the central nervous system sends a gross signal to
the muscles. This signal is shaped to a clearer signal for the movement on the
spinal cord.
Premotor time Premotor time
3 The signal arrives through the a-motor neuron at the neuromuscular junction and
propagates across the muscle membrane to active chemical processes which
lead to a contraction of the CE of the muscle.
EMD ECD
4 The contraction of the CE takes up the slack in the MTU, which initially hangs in
a relaxed position between the attachment points of the muscle.
EMD Muscle slack
5 As soon as the slack is taken out of the MTU, the SEE is stretched. More stretch will
stiffen the SEE, and hence more force will be transmitted to the attachment
points of the muscle.
Compliance Muscle slack
6 Once the force production of the CE reduces, the SEE recoils. The recoil amplifies
the power production.
Catapult effect Catapult effect
CE 5 contractile element; ECD 5 electrochemical delay; EMD 5 electromechanical delay; MTU 5 muscle-tendon unit; SEE 5 series elastic
element.
Figure 1. Schematic representation of the time course of different processes during a squat jump in reaction to a simple visual
stimulus. The duration of the premotor time and mechanical processes is very much related to the context in which the
movement is performed. EMD 5 electromechanical delay.
Influence of Muscle Slack on Sport Performance
VOLUME 38 | NUMBER 5 | OCTOBER 201676
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force production as detectable on for
example a force platform (9). Theduration of the EMD is influenced by
electrochemical processes (i.e., the
propagation of the action potential
across the muscle membrane and the
excitation-contraction coupling) and
mechanical processes (e.g., alignment
of the MTU). The EMD is a concept
that lacks a clear definition because it
contains several processes that are to
a certain extend independent of each
other. Measurements of the EMD
therefore do not provide information
about the relative contribution of each
of the processes involved. To under-
stand the overall process, it is impor-
tant to distinguish between the
subprocesses. For example, the influence
of the mechanical processes can be large
or small, depending on the situation in
which the measurement is made (e.g.,
fatigued or nonfatigued, length of the
MTU, the movement pattern executed
and measuring equipment)
(12,76,84,98). Therefore, this review will
divide the EMD into the electrochem-
ical processes; the ECD and mechanical
processes; the mechanical delay.
The electrochemical delay. The ECD
starts when the action potential ar-
rives at the end plates of the motor
neuron and ends when the CE begins
to contract. The duration of the ECD
is very brief, with most studies
reporting a duration of approximately
3–6 milliseconds (41,58,70,76),
although a longer delay of approxi-
mately 20 milliseconds has also been
reported (2). These discrepancies may
be related to different methodological
approaches. Because of their brevity,
only small performance gains can be
made by reducing the duration of
these processes.
Fiber type distribution is usually con-
sidered very important for rapid force
production. Indeed, type II fibers typi-
cally have a shorter ECD and a higher
contraction speed compared with type
I fibers due to a faster excitation-
contraction coupling (77).
The mechanical delay. The processes
that occur during the mechanical delay
are poorly understood. Some pro-
cesses that may occur are the uptake
of slack in the CE and SEE, alignment
of the MTU, and changes in
3-dimensional muscle shape (Figure 2).
The order in which these mechanisms
occur may overlap and differ between
an active CE contraction and passive
lengthening of the MTU. Moreover,
during some movements, the CE can
be shortening, while large external
forces simultaneously stretch the
MTU, for example, the gastrocnemius
medialis during the ground contact in
high-speed running. In this case, the
order in which these mechanisms
occur may differ yet again. The follow-
ing order may occur during an active
CE contraction:
� The take up of slack in the contrac-
tile element
In relaxed muscles, both the CE and
the SEE can be slack (37,38), which
means they produce no passive elastic
force (42). This slack has to be taken up
before force can be transmitted to the
bones. CE slack is probably taken up as
it begins to contract.
� Alignment of the muscle-
tendon unit
The MTU includes both the in series
arranged passive and active tissues
between the attachment points. Slack
of the SEE and CE is the slack that
usually is measured or modeled for
a muscle that is aligned in a straight
line between the attachment points.
However, the MTU may initially hang
in a relaxed arched position between
the attachment points of the muscle,
and therefore the length of the total
MTU will be more than the distance
between the attachment points. As
a result, total MTU slack may be larger
than the combined slack of the CE and
SEE measured in isolation (Figure 3).
After CE slack is taken up by CE con-
traction, the MTU is aligned between
the proximal and distal MTU attach-
ment points.
� The take up of slack in the series
elastic element
Figure 2. Schematic representation of the muscle action during a push-off (left) and the time course of force production (right). The
horizontal double arrows indicate where muscle slack dominates the push-off. The increasing size of the vertical arrow in
the left images represents the increase in force production and force application.
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As mentioned previously, the SEE
can be slack in relaxed muscles. When
the MTU is aligned between the
attachment points, further contraction
of the CE will reduce SEE slack. In
computational musculoskeletal mod-
eling studies, slack is usually modeled
by setting tendon slack length (96).
This parameter represents the length
at which the tendon begins to gener-
ate force. Most of these studies assume
that the tendon falls slack at the same
length as the entire MTU, and there-
fore these models have used only ten-
don slack length to represent MTU
slack length. However, the length at
which the SEE and CE fall slack can
differ. For example, a recent study
found that slack length of the gastroc-
nemius medialis fascicles and Achilles
tendon occurred at different joint an-
gles (42), possibly because subcutane-
ous adipose tissue and/or the parallel
elastic elements first take up slack in
the tendon (39,42). Therefore, tendon
slack length may not give a good indi-
cation of total MTU slack length.
Assuming that the MTU falls slack
at the same length as the SEE may
lead to errors in these computational
models. In addition, recent research
has shown that CE slack length differs
between synergistic muscles (39),
making generalizations of slack
lengths even more limited. Also com-
putational musculoskeletal modeling
studies have been found to be very
sensitive to tendon slack lengths, but
the value used to represent tendon
slack length is based on arbitrary esti-
mates of which the accuracy cannot
be determined (18,61). This is because
experimental data, for example, based
on ultrasound measures, have only
recently become available for some
muscles (37,38). Finally, some models
require a minimum amount of activa-
tion, which prevents the MTU from
going slack.
� Changes in the 3-dimensional mus-
cle shape
Changes in 3-dimensional muscle
shape include mechanisms linked to
the muscle bulging out, variable fascial
curvature and changing pennation
angle. This article will not describe
these mechanisms in detail.
� Stretch of the series elastic element
The SEE is stretched until the force
required to bring about more stretch
is higher than the force needed to
move the joints.
These 5 mechanisms can differ
greatly between movements, and
they are therefore very difficult to
model, especially for high-intensity
sport movements. Their influence
on performance can also differ
greatly, and this makes it difficult to
interpret the findings of studies that
have investigated the duration of the
EMD. For example, if the mechanical
delay is 100 milliseconds, it is not
known how much of this time is
needed to take up slack in the CE
and SEE and how much of this time
represents compliance of the SEE,
although slack may have a large
influence on the duration of the
mechanical delay at very short
MTU lengths (58,68,76). For exam-
ple, Sasaki et al. (76) found the
mechanical delay of the elbow flexors
to be approximately 8 milliseconds at
the most extended position (408 joint
angle), whereas it increased to
approximately 20 milliseconds at
the most flexed position (i.e., 1308
joint angle). In addition, there was
no significant change in the duration
of the mechanical delay when the
MTU was lengthened beyond slack
length, indicating that more slack
lead to an increase in mechanical
delay. Furthermore, it has been
shown that the way in which the
EMD is measured can have a large
influence on the duration (12). For
example, the time of the mechanical
delay is very much related to the con-
text in which the movement is per-
formed, and therefore large variations
in EMD duration have been found,
rangingfrom about 6 milliseconds
(41,47,70) to more than 100 millisec-
onds (11,84,85). However, in the lat-
ter studies, the duration of ECD and
mechanical delay was not directly
measured, and the duration of the
mechanical delay is therefore based
on the assumption that the ECD is
approximately 6 milliseconds (41,70).
Figure 3. Schematic representation of the dangling position of the muscle-tendon unit and slack in the contractile element (CE)
and series elastic elements (SEEs).
Influence of Muscle Slack on Sport Performance
VOLUME 38 | NUMBER 5 | OCTOBER 201678
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Because ECD is very brief, it is usually
assumed that the uptake of slack
and SEE alignment in vivo is respon-
sible for the majority of EMD
(2,9,35,68,98). Indeed, a large contribu-
tion of the SEE (i.e., tendon and apo-
neurosis of the gastrocnemius medialis)
to the EMD has recently been con-
firmed using very high–speed ultra-
sound (4 kHz) in vivo (70). However,
another study did not find any differ-
ence between the influence of the elec-
trochemical and mechanical processes
during biceps brachii actions (41).
These contrasting outcomes are likely
due to differences in the muscle and/or
tendon structure between the studied
muscles (58), or they might be attrib-
uted to differences in mechanisms 1, 2,
and 3 of the mechanical delay.
Because the structure of the MTU and
the joint angles (76) have a large influ-
ence on muscle slack, the results of
studies of isolated muscles have limited
transfer to movement patterns that
involve multiple muscles and joint an-
gles. Nevertheless, since the duration of
EMD, and especially the uptake of
slack and MTU alignment, can be long,
it is likely that this limits high-intensity
sports performance in both relatively
untrained and elite athletes. Because
several studies found a simultaneous
reduction in the duration of the
EMD and improvements in rapid force
development (40,52,90) or vertical
jump height (95), the duration of
ECD and mechanical delay may be
reduced to enhance performance.
However, because ECD is very brief,
only small performance gains can be
made by reducing the duration of these
processes. In contrast, because of the
sometimes large duration of the delay
between muscular contraction and
SEE elongation, significant time can
potentially be gained by reducing this
duration.
COMPLIANCE EFFECT
Because the processes involved in the
mechanical delay and stretching of the
SEE partly overlap, they have a com-
bined effect within muscle slack. Align-
ing and elongating the SEE can be
compared to using an elastic tow cable
to pull a car. However, it should be
noted that this comparison is a very
simplified representation that does
not include 3-dimensional effects such
as muscle gearing ratio (1) and lattice
spacing (94). If the car doing the pull-
ing starts moving, the other car will not
instantly move. First, the elastic tow
cable will be straightened. Thereafter,
the cable will stretch until the force
required to stretch the cable is higher
than the force needed to pull the other
car. It is only at this point that the car
will start moving. Similarly, initially on-
ly a small amount of force is needed to
take slack out of the MTU and to align
the MTU (or elastic tow cable in the
example). However, as the SEE gets
stretched, an increasing amount of
force will be needed to bring about
more stretch (Figure 4).
An increase in the amount of stretch
will result in an increased stiffness of
the SEE. Therefore, this stiffness is
a consequence of the interaction
between the CE and SEE. In addition,
the speed of force production will also
influence the SEE stiffness. A rapid
increase in force will lead to a higher
stiffness because of the viscoelastic
properties of the SEE (51). Therefore,
muscle slack and the capability of the
CE to rapidly develop force are inter-
dependent. An increased SEE stiffness
will enhance rapid performance
because the transmission of force from
the CE to the attachment points of the
muscle will be faster.
SEE stiffness can be altered by train-
ing because of exercise-induced adap-
tations in the mechanical, material,
and morphological properties (6).
Although a decrease in SEE stiffness
following training has been reported
(31), most studies among untrained
or recreationally trained individuals
found an increased SEE stiffness as
a result of training both with and with-
out external load (25,31,52,82,91,95).
Therefore, training (also with the
addition of external load) may be ben-
eficial for SEE stiffness as a result of
structural changes. However, these
structural adaptations occur over time,
and therefore these cannot immedi-
ately reduce muscle slack.
CATAPULT EFFECT
The SEE has several functions. One of
these functions is to increase the
power output beyond what can be
reached by the CE in isolation. If the
Figure 4. Compliance and stiffness. As the length of the series elastic elements (SEEs)
increases (more stretch), an increasing amount of force will be needed to
stretch the SEE further. SEE b has a higher stiffness than SEE a as a result of
training and as a consequence, the force transmission to the bone will be
faster.
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force produced by the CE diminishes,
the SEE will recoil and provide extra
power (54,72). This SEE recoil is called
the catapult effect. It is often thought
that a stretch and recoil of the SEE
only occurs during movements with
a countermovement (CM) (e.g., a coun-
termovement jump) or where large
external forces stretch the SEE (e.g.,
ground contact during sprinting).
However, during movements such as
a squat jump, the SEE is initially
stretched before recoiling as the force
in the CE decreases (24). Moreover,
even in isokinetic strength measure-
ment, there is a dynamic interaction
between the CE and SEE (36).
STRATEGIES TO REDUCE MUSCLE
SLACK
In the remaining part of this review,
we will discuss the acute and long-
term effects of 3 strategies that may
be used to reduce the influence of
muscle slack on high-intensity sport
performance:
� The use of pretension through
cocontractions.
� Using a CM.
� Using external load.
Furthermore, the way in which these 3
strategies influence each other will be
outlined. This mutual influence is
important in identifying training inter-
ventions that have a positive influence
on high-intensity sport performance.
COCONTRACTIONS
The first possibility for reducing mus-
cle slack is creating pretension by
simultaneously contracting agonistic
and antagonistic muscles around a joint
(i.e., coactivation and cocontraction).
Acute effects of cocontractions
on muscle slack. By cocontracting
muscles before joint motion starts,
a certain amount of slack is taken
out of the MTU, the MTU is aligned,
and possibly some of the compliance
is overcome. For example, after all
slack is taken out of the MTU, fur-
ther contraction of the CE may
stretch the SEE, which increases
SEE stiffness and reduces the effect
of SEE compliance. When the
antagonist muscle relaxes, the agonist
will be able to produce force with
a reduced influence of muscle slack.
As a consequence, force production at
the attachment points starts from a high
plateau and performance will benefit
(87). For example, it is well possible that
the MTU of the gastrocnemius would
be slack right before ground contact in
high-speed running or right before
ground contact in a drop jump if there
was no contraction of the CE before
ground contact (37,38). In this case,
the ground contact would initially serve
to take out slack and only whenall slack
is taken out, force could be applied to
accelerate the body. Fortunately, the CE
is activated before ground contact, and
therefore less slack has to be overcome
during the initial ground contact and
more time can be used to accelerate
the body. In addition to the possibly
beneficial effects on performance, co-
contractions may also offer protection
against injuries (34). For example, co-
contractions may result in a shorter
mechanical delay and more rapid force
development, which results in a faster
correction after an unexpected pertur-
bation. For example, an unexpected
inversion in the ankle joint during the
ground contact of high-speed running
may be corrected faster when the
muscles are pretensed because of the
preflex capabilities of the MTU.
However, there may be a trade-off
between cocontractions and rapid
force development. Milner et al. (66)
found less muscle activation during
cocontractions compared with the
sum of agonist and antagonist activa-
tion alone, which they attributed
to reciprocal inhibition. Therefore,
excessive cocontractions may hamper
rapid force development. In addition,
asynchronous cocontractions may
result in a loss of energy and a lower
net force production. For example,
although muscles reached maximum
torque faster after an unexpected
inversion of the ankle joint when the
subjects used pretension compared
with no pretension (48), another
study among untrained individuals
found that pretension before a ballistic
action resulted not only in a shorter
EMD but also in a lower rate of torque
development (83). Therefore, when
used effectively, cocontractions may
be an appropriate strategy to reduce
the influence of muscle slack. How-
ever, when used inappropriately, they
may hamper performance.
Cocontractions in endurance sports.
Creating cocontractions may not
seem important for all sporting ac-
tions. One could suppose that an ath-
lete participating in an endurance
sport like cycling may have enough
time to develop force. However,
reducing muscle slack in endurance
sports like cycling is important as
cycling with a high cadence requires
activation of the muscles before force
application on the pedals to prevent
a slow force development due to mus-
cle slack. For example, counterintui-
tively, the quadriceps have to be
activated when the pedal is moving
upward, so that muscle slack
is minimized when the downward
stroke is initiated. If the muscles are
not activated at the initiation of the
downward stroke, the result will be
a brief moment where no force is
applied (69,86) and as a consequence,
cycling speed will slow down. This
problem increases as cadence rises.
Activating (cocontracting) involved
muscles earlier at higher pedaling fre-
quency will reduce EMD to a certain
extent. Sarre and Lepers (75) specu-
lated that a strategy of even earlier
activation of the muscles would per-
haps further decrease the EMD, but
because muscle activation costs
energy, it would also be detrimental
to the efficiency of the movement.
It is well possible that cocontractions
can never completely take up all
muscle slack and hence, without the
use of an external load providing
a stretching force to the MTU, muscle
slack will always limit performance to
some extent. For endurance sports,
there will always be a trade-off
between developing sufficient cocon-
tractions to minimize muscle slack
while simultaneously minimizing the
associated energy costs.
Influence of Muscle Slack on Sport Performance
VOLUME 38 | NUMBER 5 | OCTOBER 201680
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Long-term effects of cocontractions
on muscle slack. As cocontractions
may reduce muscle slack and hence
enhance rapid force development, it
would be interesting to know which
training methods and/or exercises
can improve the capability to create
pretension by cocontractions. Several
studies have investigated how preten-
sion can be trained (10,53,56,57). In
addition, longitudinal studies have inves-
tigated the effect of training on the
EMD during an involuntary (17,31–33,
43,60,82) and voluntary contraction
(7,21,40,43,52,79,80,82,90,91,95,98).
Although several studies showed the
EMD to be shorter following training,
they did not clearly differentiate
between the mechanisms affecting
the change in performance and hence
cannot be used to determine the
impact of training on the capability
to create pretension. Therefore, the
training interventions effective for cre-
ating optimal cocontractions and pre-
tension remain unknown.
COUNTERMOVEMENT
The second possibility that may be
used to reduce the impact of muscle
slack is the performance of a CM.
Acute effects of a countermovement
on muscle slack. When the attach-
ment points of a muscle move closer
together, muscle slack will increase,
and therefore the duration of the
EMD will increase (37,58,76). By per-
forming a CM, for example during the
downward phase of CM jump, the
attachment points of the quadriceps
will move away from each other. This
will take up slack, line up the MTU,
and stretch the SEE, hereby reducing
muscle slack (23,24,38,45,55). There-
fore, immediately after the CM, force
can be transmitted directly to the
bone. As a consequence, some re-
searchers and strength and condition-
ing professionals suggest that using
a CM is a good strategy to reduce
the negative influence of muscle slack
on the high-intensity sport perfor-
mance. In addition, it is usually
thought that a CM improves rapid
force production because a reflex is
triggered by stretching the muscle fi-
bers. The effectiveness of a CM may
however be the result of (slow) take up
of slack rather than stretch of the mus-
cle fibers. However, an action with
CM takes longer than one without
CM (Figure 5). For instance, a throw
using a light medicine ball takes about
310 milliseconds without CM and 500
milliseconds with CM (81). Further-
more, data from several studies indi-
cate that CM jump times range from
500 to 1,000 milliseconds (measured
from the initiation of the downward
movement until toe-off ), whereas
squat jump movement times range
from 300 to 430 milliseconds (mea-
sured from the initiation of the
upward phase until toe-off ). During
most sporting actions, there is not
enough time to perform a CM, and
therefore it is not a very useful strategy
to reduce the effect of muscle slack.
Long-term effects of countermovement
training on muscle slack.Although the
difference between a CM jump and
squat jump can be small (4), an action
with CM will almost always result in
a better performance than one per-
formed without CM. For example,
jump heights reached during a squat
jump are lower compared with
a CM jump (5), and a ball throw
without CM results in a lower ball
speed and hence a smaller throwing
distance than a throw with CM
(assuming an equal throwing angle)
(71,81). These findings often make re-
searchers and practitioners jump to
the conclusion that producing better
results using a CM will automatically
lead to better results during compe-
tition. However, this may not be the
case because it is possible that prac-
ticing CMs leads to an increase in
muscle slack because the athlete’s
ability to perform cocontractions
may be reduced as a consequence
of the supporting effect of CM. The
athlete gets used to the CM reducing
muscle slack and hence does not cre-
ate pretension to minimize the mus-
cle slack (i.e., the central nervous
system becomes lazy). Although direct
evidence to support this reasoning is
lacking, some indirect evidence sup-
ports it. For example, untrained and
recreationally trained individuals have
been found to increase the amplitude
of the CM as a result of CM jump
training (13,63–65). This larger ampli-
tude increases the duration of theCM
(49,64,73) and because the available
time during most sporting actions
is only very brief, this training approach
may actually be detrimental to
performance.
Figure 5. Time course of the vertical ground reaction force (y axis) during a coun-
termovement jump (CMJ) and a squat jump (SJ). Although a CMJ results in
a higher jump, it also takes more time to perform (x axis).
Strength and Conditioning Journal | www.nsca-scj.com 81
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If CM indeed negatively alters the
capability to develop pretension, then
this will likely affect highly trained in-
dividuals more than untrained or rec-
reationally trained individuals because
untrained or recreationally trained in-
dividuals may experience more posi-
tive adaptations as a consequence of
training (e.g., increased neural drive
and cross-sectional area (16)), whereas
these adaptations may already be well
developed in highly trained individuals.
As a consequence, for untrained or rec-
reationally trained individuals, poten-
tially negative adaptations resulting
from CM training will be masked by
the positive adaptations. This masking
effect might lead to the incorrect con-
clusion that untrained individuals ben-
efit from CM training also in the long
run, while it actually may be detrimen-
tal to their long-term performance.
Indeed, hardly any athlete (untrained
or highly trained) will benefit from
more muscle slack. Every athlete can
perform a large CM, but all athletes
have to learn to minimize the ampli-
tude and duration of the CM.
Therefore, we recommend minimizing
CM within training for those sporting
movements where a CM will always
have a negative influence on the per-
formance (e.g., a swim start, sprint
start, and rugby scrum). During other
sporting actions, there may be plenty
of time to perform a CM. For example,
not all jumps in volleyball are per-
formed under time pressure, and dur-
ing tennis a player can sometimes
make a larger amplitude backswing.
However, minimizing CM during
training for these activities is suggested
as making a bigger CM is never a prob-
lem and reducing CM always is.
Cocontractions and countermove-
ments in running. Sprinting is charac-
terized by elastic muscle activity,
which means that for instance, the
SEE in calf and other muscles is
stretched at foot strike. Foot strike
effectively triggers a CM (i.e., ankle
dorsiflexion), which decreases the
amount of muscle slack that needs to
be overcome, and therefore one may
think that cocontractions are not
important during sprinting. However,
because the ground contact time, and
hence time to apply force is very brief,
muscles have to develop pretension
before initial ground contact. This pre-
tension is created partly by CM during
the flight phase. For example, knee
extension may act as a CM to reduce
the amount of muscle slack in the
hamstrings and hip extension may act
as a CM for the rectus femoris. These
CM may not create enough stiffness
on ground contact, and therefore co-
contractions in the involved muscles
need to be built into the movement
pattern. A proper technical execution
of this tensing action before ground
contact is complex and requires a great
deal of practice. Better sprinters are
able to produce more force during
the short ground contact (67,92,93),
probably, at least partly as a result of
less muscle slack and better stiffness.
The major problem in sprinting tech-
nique may therefore be in the flight
phase rather than in the stance phase.
The ground contact during middle-
and long-distance running is longer
when compared with sprinting, but
still too short to build up maximum
force. Therefore, during endurance
running, it is also important to create
cocontractions. However, there is
again a trade-off between minimizing
muscle slack and minimizing energy
costs. As a consequence, one might
expect elite runners to develop more
cocontractions (i.e., more electromyo-
graphic activity) when compared with
nonelite runners just before ground
contact. However, elite Kenyan dis-
tance runners have been found to have
less activity of both the agonist (gas-
trocnemius medialis) and antagonist
(tibialis anterior) when compared with
national-level Japanese distance run-
ners 100 milliseconds before ground
contact and during the ground contact
phase in which the SEE is stretched
(74). These, perhaps contradictory,
findings may be explained by a higher
SEE stiffness and greater isometric
muscle actions, making retraction of
swing leg before ground contact more
effective and requiring less muscle
activation.
EXTERNAL LOAD
The use of external load such as bar-
bells, dumbbells, and elastic bands is
a third possibility for reducing mus-
cle slack.
Acute effects of external load on mus-
cle slack. Generally, when resistance is
added to a movement such as a jump,
slack will be taken up, the MTUwill be
aligned, and the SEE will be stretched
by the extra gravitational forces (20).
This will reduce the influence of mus-
cle slack (Figure 6). If no external load
is used, or if the body weight is reduced
by assisting the movement with elastic
bands, the athlete needs to create more
cocontractions to reduce muscle slack.
For example, several studies have
shown that the addition of external
load to a CM jump resulted in a smaller
amplitude of the CM, whereas a larger
amplitude was observed when elastic
bands assisted during the CM jump
(22,62,88,89). In contrast, other studies
found larger or similar amplitude dur-
ing the CM jump when external load
was added compared with no load
(44,59). However, in one of these stud-
ies (59), the participants were allowed
to use an arm swing during unloaded
CM jump, which may explain the con-
tradictory findings. Another possible
reason for the larger CMwith the addi-
tion of external load is that a (too) fast
downward movement resulted in
a greater inertia which had to be over-
come, and this forced the subjects to
perform a larger CM.
Long-term effect of external load on
muscle slack. Numerous sport actions
have no or very low external resistance
that can reduce muscle slack at the start
of CE contraction. For example, at the
start of a stroke during rowing, the
water provides only very low resistance
because it moves in the opposite direc-
tion of the blade. Therefore, claims that
training with external load (i.e., tradi-
tional resistance training) will automat-
ically improve high-intensity sport
performance should be questioned.
Influence of Muscle Slack on Sport Performance
VOLUME 38 | NUMBER 5 | OCTOBER 201682
Copyright ª National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
Because the addition of external load to
a movement may decrease muscle
slack, it may not teach the athlete to
develop proper pretension by cocon-
tracting muscles. This reasoning also
lacks direct evidence but some indirect
evidence supports it. It is possible that
athletes become accustomed to exter-
nal load taking up the muscle slack
and as a consequence, the athletes cre-
ate fewer cocontractions during a CM
jump, and this will cause the amplitude
of the CM to increase. However, stud-
ies investigating the effect of external
load on the amplitude of the CM dur-
ing a CM jump show inconsistent re-
sults. This may be a consequence of
differences in the used training protocol
and/or the training status of the
participants.
Several studies among recreationally
trained individuals found an increased
CM amplitude following training with
external load (14,63,64), although the
increase in the CM amplitude was not
always the largest in the group training
with external load. Other studies
among recreationally trained individu-
als (15,46) or elite female rugbyplayers
(26) did not find increases in CM
amplitude or duration following train-
ing with external load.
The small amount of indirect evi-
dence leads to the careful conclusion
that the use of external load may neg-
atively impact on the ability to
develop pretension. However, it
should be noted that resistance train-
ing and CM (or loaded jump squats)
were combined in all studies
(14,15,26,46,63,64), and therefore the
potentially negative effect of external
load on the ability to develop preten-
sion may have been caused by the
CM. In addition, changes in the CM
amplitude do not necessarily indicate
changes in actual sporting perfor-
mance. For example, it is possible that
resistance training results in a smaller
CM, but still negatively impacts on
sporting movements without a CM
because both CM and external load
reduce muscle slack, whereas muscle
slack cannot be reduced by a CM or
external load during most sporting
movements. Therefore, future
research should investigate the sepa-
rate effects of external load and CMs
on actual sport performance using
highly trained individuals.
Nevertheless, the statement “strength
training makes athletes slower” could
very well be based on the intuitive feel-
ing that (incorrect and too much)
strength training may increase muscle
slack and as a consequence makes the
athlete slower. In addition, “athleti-
cism” could very well encompass the
capability to move with a minimum
amount of muscle slack and the capa-
bility to rapidly develop force.
LACK OF CLARITY IN SCIENTIFIC
TERMINOLOGY
In the scientific literature, it is not
always properly defined what a con-
cept means. A good example of this is
the concept of the “stretch-shortening
cycle” (SSC). Often, the SSC is simply
described as a stretch of the muscle
followed by a shortening. However, it
is not described which components of
the muscle stretch and shorten. This
can lead to wrong interpretations. For
example, it is often assumed that there is
an eccentric action of the CE of the leg
muscles during the downward move-
ment of the CM jump. Although some
studies show the CE to lengthen during
the downwardmovement (23,24), other
Figure 6. Effect of external load on muscle slack. Left: the vastus lateralis is still slack. Therefore, first slack has to be taken out of the
muscle-tendon unit (MTU), the MTU has to be aligned, and the series elastic elements have to be stretched before force
can be transmitted. Right: there is less muscle slack in the MTU of the vastus lateralis because the external load stretches
the MTU.
Strength and Conditioning Journal | www.nsca-scj.com 83
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studies show the CE to shorten (55) or
work isometrically (50). Therefore, the
downward movement does not neces-
sarily present an eccentric action of the
CE. Perhaps only during slowly exe-
cuted submaximal and large amplitude
CM jumps, the CE lengthens, whereas
it works isometrically or concentrically
during faster maximum effort and small
amplitude jumps. Future research
should therefore refer to the downward
and upward phases rather than the
eccentric and concentric phases of the
CM jump.
Another probably incorrect assump-
tion is that during sprinting, the knee
extension of the front leg during the
flight phase causes an eccentric action
of the CE of the hamstrings. As a con-
sequence, several researchers and
strength and conditioning professio-
nals use exercises thought to produce
an eccentric muscle action (e.g., the
Nordic hamstring exercise) as a core
exercise for “functional” strengthening
of the hamstrings. However, there is an
increasing body of evidence suggesting
that there is no eccentric action, but
rather an isometric action of the CE
during the swing phase in high-speed
running (27–30). The knee extension
during the swing phase will first take
slack out of the MTU, align the MTU,
and then stretch the SEE before the
SEE recoils. Functional training of
the hamstrings should therefore not
be done through eccentric training,
but in an elastic-isometric way, reflect-
ing hamstring functioning during
sprinting.
CONCLUSION AND PRACTICAL
APPLICATIONS
The time to develop force is limited in
most high-intensity sporting actions.
We propose the initial part of muscle
slack, which is the delay between
contraction of the muscle fibers and
the start of SEE stretching as an
important performance limiting factor,
especially for highly trained individu-
als. Therefore, both athletes and
strength and conditioning professio-
nals should continuously be searching
for strategies to minimize muscle slack.
We concluded that a CM was not an
appropriate strategy to reduce muscle
slack because of the extra time require-
ments associated with performing
a CM and because of the probable
increase in resultant muscle slack as
a result of the reduced capability to
create cocontractions. Using external
load in training is also an inefficient
strategy for improving the rapid force
development for those sporting activi-
ties that are performed without signif-
icant external load. The effect of
resistance training on muscle slack will
be especially problematic in highly
trained individuals because most posi-
tive adaptations may already be well
developed. The only effective direct
way to reduce muscle slack in move-
ments performed against low resis-
tance may be through creating
pretension by cocontractions. An
effective and efficient technique of co-
contractions may be complex from
a coordinative point of view and there-
fore requires a great deal of practice.
For the strength and conditioning pro-
fessional, it is important to examine
which aspects of high-intensity perfor-
mance require improvement. For
highly trained individuals, it is impor-
tant, when using external loads during
training, to search for a balance
between the possible negative effects
on the capability to create pretension
and the positive effects such as an
increased motor unit firing frequency,
motor unit synchronization, and SEE
stiffness.
Scientific research is unaware of the
mechanisms of muscle slack and the
influence of CM and external load
during the exercises and activities
described above. Also the difference
in training experience between
untrained/poorly trained athletes
and highly trained individuals are
often not taken into account when
drawing conclusions. Therefore,
when reading research studies,
appraisal of the following 3 aspects
is recommended:
� Have the study participants been
described with sufficient detail?
� Has the tested movement been
described with sufficient detail?
� Has the resistance used in training
and testing been described with suf-
ficient detail?
In the near future, the capability to
rapidly develop force may prove to
be an important battlefield for discus-
sions on training transfer between the
more classic “mechanistic” approach
and a more motor control-based
approach to resistance training.
Conflicts of Interest and Source of Funding:
The authors report no conflicts of interest
and no source of funding.
ACKNOWLEDGMENTS
The authors thank Kenneth Meijer for
his comments on a preliminary version
of the manuscript and Craig Ranson
for his feedback on the final version
of the manuscript.
Bas Van
Hooren is an
independent
strength and
conditioning spe-
cialist and
received his MSc
degree in Human
Movement Scien-
ces at Maastricht
University.
Frans Bosch is
a lecturer at
Fontys Univer-
sity of Applied
Sciences and an
elite sport
consultant.
REFERENCES
1. Azizi E and Roberts TJ. Geared up to stretch:
Pennate muscle behavior during active
lengthening. J Exp Biol 217: 376–381,
2014.
2. Begovic H, Zhou GQ, Li T, Wang Y, and
Zheng YP. Detection of the
electromechanicaldelay and its
components during voluntary isometric
Influence of Muscle Slack on Sport Performance
VOLUME 38 | NUMBER 5 | OCTOBER 201684
Copyright ª National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
contraction of the quadriceps femoris
muscle. Front Physiol 5: 494, 2014.
3. Blazevich AJ. The stretch-shortening cycle
(SSC). In: Strength and Conditioning:
Biological Principles and Practical
Applications. Cardinale M, Newton RU, and
Nosaka K, eds. West Sussex, England:
Wiley-Blackwell, 2011. pp. 209–224.
4. Bobbert MF and Casius LJR. Is the effect of
a countermovement on jump height due to
active state development? Med Sci Sports
Exerc 37: 440–446, 2005.
5. Bobbert MF, Gerritsen KGM, Litjens MCA,
and Van Soest AJ. Why is
countermovement jump height greater than
squat jump height? Med Sci Sports Exerc
28: 1402–1412, 1996.
6. Bohm S, Mersmann F, and Arampatzis A.
Human tendon adaptation in response to
mechanical loading: A systematic review
and meta-analysis of exercise intervention
studies on healthy adults. Sports Med
Open 1: 1–18, 2015.
7. Buckthorpe M, Erskine RM, Fletcher G,
and Folland JP. Task-specific neural
adaptations to isoinertial resistance
training. Scand J Med Sci Sports 25:
640–649, 2015.
8. Carlton LG, Carlton MJ, and Newell KM.
Reaction time and response dynamics. Q J
Exp Psychol A 39: 337–360, 1987.
9. Cavanagh PR and Komi PV.
Electromechanical delay in human skeletal
muscle under concentric and eccentric
contractions. Eur J Appl Physiol Occup
Physiol 42: 159–163, 1979.
10. Chimera NJ, Swanik KA, Swanik CB, and
Straub SJ. Effects of plyometric training on
muscle-activation strategies and
performance in female athletes. J Athl Train
39: 24–31, 2004.
11. Chung P and Ng G. Taekwondo training
improves the neuromotor excitability and
reaction of large and small muscles. Phys
Ther Sport 13: 163–169, 2012.
12. Corcos DM, Gottlieb GL, Latash ML,
Almeida GL, and Agarwal GC.
Electromechanical delay: An experimental
artifact. J Electromyogr Kinesiol 2: 59–68,
1992.
13. Cormie P, McBride JM, and
McCaulley GO. Power-time, force-time,
and velocity-time curve analysis of the
countermovement jump: Impact of training.
J Strength Cond Res 23: 177–186, 2009.
14. Cormie P, McGuigan MR, and Newton RU.
Adaptations in athletic performance after
ballistic power versus strength training.
Med Sci Sports Exerc 42: 1582–1598,
2010.
15. Cormie P, McGuigan MR, and Newton RU.
Changes in the eccentric phase contribute
to improved stretch-shorten cycle
performance after training. Med Sci Sports
Exerc 42: 1731–1744, 2010.
16. Cormie P, McGuigan MR, and Newton RU.
Developing maximal neuromuscular power.
Sports Med 41: 17–39, 2011.
17. Costa PB, Herda TJ, Walter AA,
Valdez AM, and Cramer JT. Effects of short-
term resistance training and subsequent
detraining on the electromechanical delay.
Muscle Nerve 48: 135–136, 2013.
18. Delp SL, Loan JP, Hoy MG, Zajac FE,
Topp EL, and Rosen JM. An interactive
graphics-based model of the lower
extremity to study orthopaedic surgical
procedures. IEEE Trans Biomed Eng 37:
757–767, 1990.
19. Domire ZJ and Challis JH. Maximum height
and minimum time vertical jumping.
J Biomech 48: 2865–2870, 2015.
20. Earp JE. The influence of external loading
and speed of movement on muscle-tendon
unit behaviour and its implications for
training. In: Computing, Health and
Science. Perth, Australia: Edith Cowan
University, 2013.
21. Ebersole KT, Cramer JT, Housh TJ,
Johnson GO, Perry SR, and Bull AJ. The
effect of isometric strength training on
electromechanical delay. Med Sci Sports
Exerc 33: S296, 2001.
22. Feeney D, Stanhope SJ, Kaminski TW,
Machi A, and Jaric S. Loaded vertical
jumping: Force-velocity relationship, work,
and power. J Appl Biomech 32: 120–127,
2016.
23. Finni T, Ikegaw S, Lepola V, and Komi P. In
vivo behavior of vastus lateralis muscle
during dynamic performances. Eur J Sport
Sci 1: 1–13, 2001.
24. Finni T, Komi PV, and Lepola V. In vivo
human triceps surae and quadriceps
femoris muscle function in a squat jump
and counter movement jump. Eur J Appl
Physiol 83: 416–426, 2000.
25. Foure´ A, Nordez A, McNair P, and Cornu C.
Effects of plyometric training on both active
and passive parts of the plantarflexors
series elastic component stiffness of
muscle-tendon complex. Eur J Appl Physiol
111: 539–548, 2011.
26. Gathercole R, Sporer B, and
Stellingwerff T. Countermovement jump
performance with increased training loads
in elite female rugby athletes. Int J Sports
Med 36: 722–728, 2015.
27. Gillis GB and Biewener AA. Hindlimb
muscle function in relation to speed and
gait: In vivo patterns of strain and activation
in a hip and knee extensor of the rat (Rattus
norvegicus). J Exp Biol 204: 2717–2731,
2001.
28. Gillis GB and Biewener AA. Effects of
surface grade on proximal hindlimb muscle
strain and activation during rat locomotion.
J Appl Physiol (1985) 93: 1731–1743,
2002.
29. Gillis GB, Flynn JP, McGuigan P, and
Biewener AA. Patterns of strain and
activation in the thigh muscles of goats
across gaits during level locomotion. J Exp
Biol 208: 4599–4611, 2005.
30. Gregersen CS, Silverton NA, and
Carrier DR. External work and potential for
elastic storage at the limb joints of running
dogs. J Exp Biol 201: 3197–3210, 1998.
31. Grosset JF, Piscione J, Lambertz D, and
Perot C. Paired changes in
electromechanical delay and musculo-
tendinous stiffness after endurance or
plyometric training. Eur J Appl Physiol 105:
131–139, 2009.
32. Ha¨kkinen K and Komi PV. Changes in
neuromuscular performance in voluntary
and reflex contraction during strength
training in man. Int J Sports Med 4: 282–
288, 1983.
33. Ha¨kkinen K and Komi PV. Training-induced
changes in neuromuscular performance
under voluntary and reflex conditions. Eur J
Appl Physiol Occup Physiol 55: 147–155,
1986.
34. Hannah R, Folland JP, Smith SL, and
Minshull C. Explosive hamstrings-to-
quadriceps force ratio of males versus
females. Eur J Appl Physiol 115: 837–
847, 2015.
35. Hannah R, Minshull C, Smith SL, and
Folland JP. Longer electromechanical delay
impairs hamstrings explosive force versus
quadriceps. Med Sci Sports Exerc 46:
963–972, 2014.
36. Hauraix H, Nordez A, and Dorel S.
Shortening behavior of the different
components of muscle-tendon unit during
isokinetic plantar flexions. J Appl Physiol
(1985) 115: 1015–1024, 2013.
37. Herbert RD, Clarke J, Kwah LK, Diong J,
Martin J, Clarke EC, Bilston LE, and
Gandevia SC. In vivo passive mechanical
behaviour of muscle fascicles and tendons
in human gastrocnemius muscle-tendon
units. J Physiol 589: 5257–5267, 2011.
38. Herbert RD, He´roux ME, Diong J,
Bilston LE, Gandevia SC, and
Lichtwark GA. Changes in the length and
three-dimensional orientation of muscle
fascicles and aponeuroses with passive
Strength and Conditioning Journal | www.nsca-scj.com 85
Copyright ª National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
length changes in human gastrocnemius
muscles. J Physiol 593: 441–455, 2015.
39. Hirata K, Kanehisa H, Miyamoto-Mikami E,
and Miyamoto N. Evidence for intermuscle
difference in slack angle in human triceps
surae. J Biomech 48: 1210–1213, 2015.
40. Hong J, Kipp K, Johnson ST, and
Hoffman MA. Effects of 4 weeks whole
body vibration on electromechanical delay,
rate of force development, and presynaptic
inhibition. IJPHY 1: 30–40, 2010.
41. Hug F, Gallot T, Catheline S, and Nordez A.Electromechanical delay in biceps brachii
assessed by ultrafast ultrasonography.
Muscle Nerve 43: 441–443, 2011.
42. Hug F, Lacourpaille L, Maisetti O, and
Nordez A. Slack length of gastrocnemius
medialis and Achilles tendon occurs at
different ankle angles. J Biomech 46:
2534–2538, 2013.
43. Ishida K, Moritani T, and Itoh K. Changes in
voluntary and electrically induced
contractions during strength training and
detraining. Eur J Appl Physiol Occup
Physiol 60: 244–248, 1990.
44. Jidovtseff B, Quievre J, Harris NK, and
Cronin JB. Influence of jumping strategy on
kinetic and kinematic variables. J Sports
Med Phys Fitness 54: 129–138, 2014.
45. Kawakami Y, Muraoka T, Ito S, Kanehisa H,
and Fukunaga T. In vivo muscle fibre
behaviour during counter-movement
exercise in humans reveals a significant
role for tendon elasticity. J Physiol 540:
635–646, 2002.
46. Kijowksi KN, Capps CR, Goodman CL,
Erickson TM, Knorr DP, Triplett NT,
Awelewa OO, and McBride JM. Short-term
resistance and plyometric training
improves eccentric phase kinetics in
jumping. J Strength Cond Res 29: 2186–
2196, 2015.
47. Komi PV, Ishikawa M, and Jukka S. IAAF
sprint start research project: Is the 100ms
limit still valid. New Stud Athle 24: 37–47,
2009.
48. Konradsen L, Peura G, Beynnon B, and
Renstro¨m P. Ankle eversion torque
response to sudden ankle inversion torque
response in unbraced, braced, and pre-
activated situations. J Orthop Res 23:
315–321, 2005.
49. Kopper B, Csende Z, Sa´fa´r S,
Hortoba´gyi T, and Tihanyi J. Muscle
activation history at different vertical jumps
and its influence on vertical velocity.
J Electromyogr Kinesiol 23: 132–139,
2013.
50. Kopper B, Csende Z, Trzaskoma L, and
Tihanyi J. Stretch-shortening cycle
characteristics during vertical jumps
carried out with small and large range of
motion. J Electromyogr Kinesiol 24: 233–
239, 2014.
51. Ko¨sters A, Wiesinger HP, Bojsen-Møller J,
Mu¨ller E, and Seynnes OR. Influence of
loading rate on patellar tendon mechanical
properties in vivo. Clin Biomech 29: 323–
329, 2014.
52. Kubo K, Kanehisa H, Ito M, and
Fukunaga T. Effects of isometric training on
the elasticity of human tendon structures
in vivo. J Appl Physiol (1985) 91: 26–32,
2001.
53. Kubo K, Morimoto M, Komuro T, Yata H,
Tsunoda N, Kanehisa H, and Fukunaga T.
Effects of plyometric and weight training on
muscle-tendon complex and jump
performance. Med Sci Sports Exerc 39:
1801–1810, 2007.
54. Kurokawa S, Fukunaga T, and Fukashiro S.
Behavior of fascicles and tendinous
structures of human gastrocnemius during
vertical jumping. J Appl Physiol (1985) 90:
1349–1358, 2001.
55. Kurokawa S, Fukunaga T, Nagano A, and
Fukashiro S. Interaction between fascicles
and tendinous structures during counter
movement jumping investigated in vivo.
J Appl Physiol (1985) 95: 2306–2314,
2003.
56. Kyro¨la¨nen H, Avela J, McBride JM,
Koskinen S, Andersen JL, Sipila S,
Takala TE, and Komi PV. Effects of power
training on muscle structure and
neuromuscular performance. Scand J Med
Sci Sports 15: 58–64, 2005.
57. Kyro¨la¨nen H, Komi PV, and Kim DH.
Effects of power training on
neuromuscular performance and
mechanical efficiency. Scand J Med Sci
Sports 1: 78–87, 1991.
58. Lacourpaille L, Hug F, and Nordez A.
Influence of passive muscle tension on
electromechanical delay in humans. PLoS
One 8: e53159, 2013.
59. Leontijevic B, Pazin N, Bozic PR, Kukolj M,
Ugarkovic D, and Jaric S. Effects of loading
on maximum vertical jumps: Selective
effects of weight and inertia.
J Electromyogr Kinesiol 22: 286–293,
2012.
60. Linford CW, Hopkins JT, Schulthies SS,
Freland B, Draper DO, and Hunter I. Effects
of neuromuscular training on the reaction
time and electromechanical delay of the
peroneus longus muscle. Arch Phys Med
Rehabil 87: 395–401, 2006.
61. Manal K and Buchanan TS. Subject-
specific estimates of tendon slack length: A
numerical method. J Appl Biomech 20:
195–203, 2004.
62. Markovic G and Jaric S. Positive and
negative loading and mechanical output in
maximum vertical jumping. Med Sci Sports
Exerc 39: 1757–1764, 2007.
63. Markovic G, Vuk S, and Jaric S. Effects of
jump training with negative versus positive
loading on jumping mechanics. Int J Sports
Med 32: 365–372, 2011.
64. Markovic S, Mirkov DM, Knezevic OM, and
Jaric S. Jump training with different loads:
Effects on jumping performance and power
output. Eur J Appl Physiol 113: 2511–2521,
2013.
65. Marshall BM and Moran KA.
Biomechanical factors associated with
jump height: A comparison of cross-
sectional and pre-to-posttraining change
findings. J Strength Cond Res 29:
3292–3299, 2015.
66. Milner TE, Cloutier C, Leger AB, and
Franklin DW. Inability to activate muscles
maximally during cocontraction and the
effect on joint stiffness. Exp Brain Res 107:
293–305, 1995.
67. Morin JB, Edouard P, and Samozino P.
Technical ability of force application as
a determinant factor of sprint performance.
Med Sci Sports Exerc 43: 1680–1688,
2011.
68. Muraoka T, Muramatsu T, Fukunaga T,
and Kanehisa H. Influence of tendon
slack on electromechanical delay in the
human medial gastrocnemius in vivo.
J Appl Physiol (1985) 96: 540–544,
2004.
69. Neptune RR and Kautz SA. Muscle
activation and deactivation dynamics: The
governing properties in fast cyclical human
movement performance? Exerc Sport Sci
Rev 29: 76–80, 2001.
70. Nordez A, Gallot T, Catheline S, Guevel A,
Cornu C, and Hug F. Electromechanical
delay revisited using very high frame rate
ultrasound. J Appl Physiol (1985) 106:
1970–1975, 2009.
71. Roach NT, Venkadesan M, Rainbow MJ,
and Lieberman DE. Elastic energy storage
in the shoulder and the evolution of high-
speed throwing in Homo. Nature 498:
483–486, 2013.
72. Roberts TJ and Konow N. How tendons
buffer energy dissipation by muscle. Exerc
Sport Sci Rev 41: 186–193, 2013.
73. Salles AS, Baltzopoulos V, and Rittweger J.
Differential effects of countermovement
magnitude and volitional effort on vertical
jumping. Eur J Appl Physiol 111: 441–
448, 2011.
Influence of Muscle Slack on Sport Performance
VOLUME 38 | NUMBER 5 | OCTOBER 201686
Copyright ª National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
74. Sano K, Nicol C, Akiyama M, Kunimasa Y,
Oda T, Ito A, Locatelli E, Komi PV, and
Ishikawa M. Can measures of muscle-
tendon interaction improve our
understanding of the superiority of Kenyan
endurance runners? Eur J Appl Physiol
115: 849–859, 2015.
75. Sarre G and Lepers R. Cycling exercise
and the determination of electromechanical
delay. J Electromyogr Kinesiol 17: 617–
621, 2007.
76. Sasaki K, Sasaki T, and Ishii N.
Acceleration and force reveal different
mechanisms of electromechanical delay.
Med Sci Sports Exerc 43: 1200–1206,
2011.
77. Schiaffino S and Reggiani C. Fiber types in
mammalian skeletal muscles. Physiol Rev
91: 1447–1531, 2011.
78. Siegel D. Fractionated reaction time and
the rate of force development. Q J Exp
Psychol A 40: 545–560, 1988.
79. Stock MS, Olinghouse KD, Mota JA,
Drusch AS, and Thompson BJ. Muscle
group specific changes in the
electromechanical delay following short-
term resistance training. J Sci Med Sport
19: 761–765, 2015.
80. Szpala A, Rutkowska-Kucharska A, and
Drapala J. Electromechanical delay of
abdominal muscles is modified by low back
pain prevention exercise. Acta Bioeng
Biomech 16: 95–102, 2014.
81. Tauchi K, Kubo Y, Ohyama Byun K, and
Takamatsu K. A mechanism for power
output of the upperlimbs during overhead
throw with stretch-shortening cycle. Int J
Sport Health Sci 3: 286–295, 2005.
82. Tillin NA, Pain MTG, and Folland JP. Short-
term training for explosive strength causes
neural and mechanical adaptations. Exp
Physiol 97: 630–641, 2012.
83. Van Cutsem M and Duchateau J.
Preceding muscle activity influences motor
unit discharge and rate of torque
development during ballistic contractions
in humans. J Physiol 562: 635–644, 2005.
84. Van Diee¨n JH, Thissen CEAM, van de
Ven AJGM, and Toussaint HM. The electro-
mechanical delay of the erector spinae
muscle: Influence of rate of force
development, fatigue and electrode
location. Eur J Appl Physiol Occup Physiol
63: 216–222, 1991.
85. van Ingen Schenau GJ. From rotation to
translation: Constraints on multi-joint
movements and the unique action of bi-
articular muscles. Hum Mov Sci 8: 301–
337, 1989.
86. Van Ingen Schenau GJ, Bobbert MF, and
de Haan A. Mechanics and energetics of
the stretch-shortening cycle: A stimulating
discussion. J Appl Biomech 13: 484–496,
1997.
87. Vint PF, McLean SP, and Harron GM.
Electromechanical delay in isometric
actions initiated from nonresting levels.
Med Sci Sports Exerc 33: 978–983, 2001.
88. Vuk S, Gregov C, and Markovi�c G.
Relationship between knee extensor
muscle strength and movement
performance: The effect of load and body
size. Kinesiology 47: 27–32, 2015.
89. Vuk S, Markovic G, and Jaric S. External
loading and maximum dynamic output in
vertical jumping: The role of training history.
Hum Mov Sci 31: 139–151, 2012.
90. Wallerstein LF, Tricoli V, Barroso R,
Rodacki ALF, Russo L, Aihara AY, da
Rocha Correa Fernandes A, de Mello MT,
and Ugrinowitsch C. Effects of strength
and power training on neuromuscular
variables in older adults. J Aging Phys Act
20: 171–185, 2012.
91. Waugh CM, Korff T, Fath F, and
Blazevich AJ. Effects of resistance training
on tendon mechanical properties and rapid
force production in prepubertal children.
J Appl Physiol (1985) 117: 257–266,
2014.
92. Weyand PG, Sandell RF, Prime DN, and
Bundle MW. The biological limits to
running speed are imposed from the
ground up. J Appl Physiol (1985) 108:
950–961, 2010.
93. Weyand PG, Sternlight DB, Bellizzi MJ, and
Wright S. Faster top running speeds are
achieved with greater ground forces not
more rapid leg movements. J Appl Physiol
(1985) 89: 1991–1999, 2000.
94. Williams CD, Salcedo MK, Irving TC,
Regnier M, and Daniel TL. The length-
tension curve in muscle depends on lattice
spacing. Proc Biol Sci 280: 20130697,
2013.
95. Wu YK, Lien YH, Lin KH, Shih TT,
Wang TG, and Wang HK. Relationships
between three potentiation effects of
plyometric training and performance.
Scand J Med Sci Sports 20: e80–e86,
2010.
96. Zajac FE. Muscle and tendon: Properties,
models, scaling, and application to
biomechanics and motor control. Crit Rev
Biomed Eng 17: 359–411, 1989.
97. Zatsiorsky VM. Biomechanics of
strength and strength training. In:
Strength and Power in Sport. Komi PV,
ed. Oxford, UK: Blackwell Science Ltd,
2003. pp. 439–487.
98. Zhou S, McKenna MJ, Lawson DL,
Morrison WE, and Fairweather I. Effects of
fatigue and sprint training on
electromechanical delay of knee extensor
muscles. Eur J Appl Physiol Occup Physiol
72: 410–416, 1996.
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