Download Altering the Length-Tension Relationship with Eccentric Exercise

Sports Med 2007; 37 (9): 807-826
0112-1642/07/0009-0807/$44.95/0
REVIEW ARTICLE
© 2007 Adis Data Information BV. All rights reserved.
Altering the Length-Tension
Relationship with Eccentric Exercise
Implications for Performance and Injury
Matt Brughelli and John Cronin
School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Joondalup, Western
Australia, Australia
Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 807
1. Eccentric Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808
2. The Length-Tension Relationship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 808
2.1 Single Fibre (Sarcomere) Force-Length Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 809
2.2 Whole Muscle Force-Length Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810
2.3 Single Joint Torque-Angle Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810
3. Studies Reporting a Shift in Optimum Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811
3.1 Dynamometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 812
3.2 Curve Fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813
3.3 Equipment and Testing Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813
3.4 Exercise Methodology and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814
4. Mechanisms for the Shift in Optimum Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816
4.1 First and Second Shift in Optimum Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 816
4.2 Theory of Sarcomereogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817
4.3 Theory of Passive Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818
5. Implications for Athletic Performance and Injury Prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819
5.1 Muscle Injury Prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819
5.2 Eccentric Exercise and Athletic Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 820
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823
Abstract
The effects of eccentric exercise on muscle injury prevention and athletic
performance are emerging areas of interest to researchers. Of particular interest
are the adaptations that occur after a single bout, or multiple bouts of eccentric
exercise. It has been established that after certain types of eccentric exercise, the
optimum length of tension development in muscle can be shifted to longer muscle
lengths. Altering the length-tension relationship can have a profound influence on
human movements. It is thought that the length-tension relationship is influenced
by the structural makeup of muscle. However, the mechanism responsible for the
shift in optimum length is not readily agreed upon. Despite the conflict, several
808
Brughelli & Cronin
studies have reported a shift in optimum length after eccentric exercise. Unfortunately, very few of these studies have been randomised, controlled training
studies, and the duration of the shift has not yet been established. Nonetheless, this
adaptation may result in greater structural stability at longer muscle lengths and
consequently may have interesting implications for injury prevention and athletic
performance. Both contentions remain relatively unexplored and provide the
focus of this review.
1. Eccentric Exercise
Pre
Post
160
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Torque (Nm)
140
Over the last 10 years, interest in eccentric exercise and its adaptations has grown substantially.
There are a number of reviews dedicated to the
effects of eccentric exercise on muscle damage,[1-4]
delayed onset of muscle soreness (DOMS),[5,6] the
repeated bout effect,[6-8] injury prevention,[9,10] athletic performance[11,12] and rehabilitation.[13] The interest can be attributed to the unique physiological
and mechanical properties of eccentric muscle contractions. For example, skeletal muscles are capable
of producing the greatest magnitudes of force,[14]
with little metabolic effort[15,16] when contracting
eccentrically. The high force levels and/or longer
muscle lengths[17] are thought to be responsible for
the damage caused to the contractive,[3,6] connective[18,19] and cytoskeletal elements of muscle.[20-22]
Muscle damage is also associated with decreased
force capabilities,[23] inflammation[24] and impairment of the excitation-contraction coupling process.[25-27] All forms of exercise can cause muscle
damage; however, only eccentric exercise has been
found to induce severe stiffness and soreness in the
days following the eccentric exercise bout.
An interesting adaptation occurs after certain
types of eccentric exercise that affects the mechanical properties of muscle. After a single bout of
eccentric exercise, the length-tension relationship of
muscle can be altered. The optimum length of peak
tension will occur at longer lengths, thus shifting the
length-tension curve to the right (see figure 1). This
phenomenon may have interesting implications for
120
100
80
60
40
20
0
0˚
10˚
20˚
30˚
40˚ 50˚
Angle
60˚
70˚
80˚
90˚
Fig. 1. Schematic representation of a shift in the optimum lengthtension relationship. Pre = pre-exercise; Post = post-exercise.
injury and performance, the discussion of which
provides the focus of this article. First, the lengthtension relationship at a single fibre, whole muscle
and single joint levels are briefly discussed. Secondly, the literature that has reported a shift in optimum
length after eccentric exercise is critiqued. Thirdly,
the proposed mechanisms responsible for the shift
are described. Finally, the implications of this rightward shift with regards to athletic performance and
injury prevention is elaborated upon.
2. The Length-Tension Relationship
The length-tension relationship plays a very important role in the function of skeletal muscle. The
magnitude of force a muscle can generate depends
on its length, velocity and stimulation.[28,29] When
creating a length-tension curve, muscle velocity and
stimulation are held constant. Force levels are then
plotted against each muscle length, thus creating the
length-tension curve. The length-tension curve
Sports Med 2007; 37 (9)
Length-Tension Relationship with Eccentric Exercise
2.1 Single Fibre (Sarcomere)
Force-Length Curve
The isometric force-length curve is generated by
maximally stimulating a single muscle fibre
(sarcomere) over a range of lengths and measuring
force outputs. In this situation, velocity is held constant at zero and stimulation is held constant at
maximum levels. Maximum force levels are plotted
against each starting length to generate the forcelength curve. The active force levels produced and
the shape of the curve are a direct result of the
© 2007 Adis Data Information BV. All rights reserved.
Plateau
region
a
Force (%)
100 Ascending
limb
Descending
limb
2.0 2.2
Passive
force
1.7
50
0
1.27 1.7
2.0 2.2
Sarcomere length (µm)
3.6
b
80
Combined
tension
Passive
tension
Force (N)
60
40
Active
tension
20
0
0.5
0
1.0
1.5
Length (cm)
2.0
2.5
Optimum length
c
150
Torque (Nm)
gives valuable information on the lengths that can
produce the greatest or least amount of tension.
From this information, the regions of the ascending
limb, descending limb and optimum length can be
determined (see figure 2a). Optimum length is often
used to describe sarcomere length (see section 2.1),
muscle length (see section 2.2), or joint angle (see
section 2.3), and tension is often used to describe
force (see sections 2.1 and 2.2), or torque (see
section 2.3).
Note that the length-tension curves for the single
fibre (sarcomere), whole muscle and single joint all
have different shapes. At the single fibre level (see
figure 2a), only the cross-bridge interaction between
myosin and actin contributes to the shape of the
curve and the development of active force within the
sarcomere. At a whole muscle (see figure 2b) and
single joint level (see figure 2c), the curve has more
of a smooth shape because of the variations in
muscle, tendon and joint designs, and the contribution of all these factors to the length-tension curve.
There are many inconsistencies in the literature
regarding the length-tension relationship. Most of
the confusion revolves around the reporting of force
and torque interchangeably. A brief treatise of forcelength curves (see sections 2.1 and 2.2) and torqueangle curves (see section 2.3), and how they are
measured should clarify some of the conjecture in
the area.
809
Active and
passive tension
100
50
0
0˚
20˚
40˚
Angle
60˚
80˚
Fig. 2. Schematic representations of (a) single fibre force-length
curves; (b) whole muscle force-length curve; and (c) torque-angle
curve. Active and passive tension curves as well as the ascending,
plateau and descending regions are indicated on the single fibre
and whole muscle curves.
magnitude of overlap between active and myosin
filaments (see figure 2a). Larger magnitudes of
Sports Med 2007; 37 (9)
810
force can be produced with greater numbers of
cross-bridge attachments. Altering the length of the
fibre affects the number of attached actin and myosin cross bridges[22,29] and subsequent force development.
Gordon et al.[30] was the first to use isolated,
intact single skeletal muscle fibres to generate the
force-length curve. As detailed in figure 2a, the
maximum percentage of tension is plotted against
sarcomere length. At short sarcomere lengths
(<1.7µm), the double overlap between the actin and
myosin filaments results in depressed force levels.
As the length of the sarcomere increases to the steep
portion (1.7–2.0µm), the actin filaments from one
side of the sarcomere no longer interfere with the
cross-bridge formations on the other side of the
sarcomere, and force increases. The region of the
curve where active force is first measured (1.27µm),
until maximum force levels (2.0µm) is called the
ascending limb of the force-length curve. The plateau region (2.0–2.2µm) of the curve is where maximum force levels are attained and held constant. At
this range, the greatest amount of actin and myosin
overlap occurs. Although there is greater overlap
from 2.2 to 2.0µm, no additional cross-bridges are
formed. This is due to a bare region (0.2µm in
length) at the centre of the myosin molecule that is
devoid of cross-bridges.[29] The plateau region is
also known as the ‘optimum length’ of the forcelength curve. The descending limb region of the
force-length curve occurs from the sarcomere
lengths of 2.2–3.65µm. At sarcomere lengths of
>3.65µm, active force levels drops to zero as there is
no overlap between actin and myosin. Notice in
figure 2a that there is also a passive tension curve.
As the muscle is stretched to greater lengths, passive
tension increases dramatically. The elements responsible for passive tension lie outside of the crossbridges and do no need active stimulation.
© 2007 Adis Data Information BV. All rights reserved.
Brughelli & Cronin
2.2 Whole Muscle Force-Length Curve
The force-length curve of whole muscle is generated similar to that of the single fibre. The whole
muscle is maximally stimulated over a range of
discrete lengths. Muscle velocity is again held constant at zero, i.e. isometric contraction. Maximum
isometric force is plotted against muscle length, thus
creating the whole muscle force-length curve. Maximum force at each muscle length is due to both
active and passive forces. At shorter lengths, all the
force is due to active components (cross-bridges),
and at longer lengths most of the force is due to the
passive components.[31]
The whole muscle force-length curve has a different shape to the single fibre force-length curve.
The magnitude of active force generated is not due
solely to the interaction of cross-bridges as is the
case for the single fibre force-length curve. The
different shape is due to the contractile properties
and design of whole muscles and tendons.[29] Whole
muscles are made up of a variety of fibre types and
sarcomere lengths. This variety means that the different fibres will have different optimum lengths of
force development, which smoothes the curve and
broadens the plateau region.[29,31] Architecture also
plays a role as the number of sarcomeres in series,
parallel, or angle of pinnation contribute to force
development and help to shape the force-length
curve. Tendon fibres in series with muscle fibres
also contribute to the shape of the curve. Tendon
compliance can affect force development by allowing simultaneous muscle shortening and tendon
lengthening. All of these factors lead to a smoother
force-length curve with an extended peak region.[29,32]
2.3 Single Joint Torque-Angle Curves
At the single fibre level, sarcomere force is plotted against sarcomere length, whereas at the whole
muscle level, muscle force is plotted against muscle
Sports Med 2007; 37 (9)
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15.4 (IMM), 10.5 (4d post),
quadriceps
Prasartwuth et
MI, eccentric elbow extension
LL, 60° to full elbow extension
16.7 (2h post), 14 (1d post),
al.[46]
biceps brachii
Philippou et al.[45]
HI, maximum eccentric elbow
MV, 2 sets of 25 reps = 50 total
LL, 50–170° of elbow extension
16 (1d post), 18 (2d post),
extension
reps
biceps brachii
HI = high intensity; HV = high volume; IMM = immediately post-exercise; LI = low intensity; LL = long muscle length; LV = low volume; max = maximum; MI = moderate intensity;
ML = moderate muscle length; MV = moderate volume; post = post-exercise; reps = repetitions.
LL, platform height set at the patella
ML, 80–130° of elbow extension
7.7 (IMM), 8.5 (4d post),
hamstrings
10.4 (IMM), triceps brachii
ML, 90 to <30° of knee extension
HV, 12 sets of 6 reps = 76 total
reps
MV, 3 sets of 25 reps = 75 total
reps
HV, 12 sets of 20 reps = 240 total
reps
HV, 40–160 total reps
LV, 2–3 sets of 5–8 reps
ML, 90 to <30° of knee extension
6 (IMM), 7.6 (2h post),
triceps surae
6.5 (4wk post), hamstrings
ML, toe-to-heel action
ML, 1h duration
4.4 (IMM), triceps surae
ML, toe-to-heel action
ML, 1h duration
Muscle length
ML, toe-to-heel action
Exercise volume
ML, 2h duration
Bowers et al.[42]
Pettitt et al.[44]
Brockett et al.[47]
Clark et al.[48]
Whitehead et al.[40]
Whitehead et al.[39]
Exercise intensity
LI, walking backwards on treadmill,
1.3 km/h
LI, walking backwards on treadmill,
3.5 km/h
MI, walking backwards on treadmill,
>3.5km/h with 5–10kg weight belt
MI, bodyweight exercise to failure,
Nordic hamstring with partner
MI, bodyweight exercise to failure,
Nordic hamstring
HI, max eccentric elbow contractions,
isokinetic dynamometer
MI, step-down exercise
Several studies have reported a shift in optimum
length after eccentric exercise (table I). This phenomenon has been observed in animal and human
studies. Animal studies have reported shifts in single
toad fibres,[33,34] single frog fibres,[14,35] rat fibres[36]
and individual motor units in cats.[37] Acute shifts
have also been reported in human plantar flexors,[38-40] forearms,[41] quadriceps,[42,43] triceps
brachii,[44] biceps brachii[45,46] and hamstrings.[47,48]
Muscle architecture, in terms of pinnation angle and
fibre type, does not appear to have an affect on the
shift in optimum length after eccentric exercise. The
greatest shifts have occurred in the human quadriceps[42] and biceps brachii.[45]
Study
Jones et al.[38]
3. Studies Reporting a Shift in
Optimum Length
Table I. Influence of eccentric exercise intensity, volume and muscle length on the magnitude of the shift in optimum length
length. Conversely, at the single joint level, joint
torque is plotted against joint angle. Torque is determined by the magnitude of muscle force and moment arm relative to the joint.[29,31] Both muscle
force and the length of the moment arm change
throughout a joint movement and affect torque development. They can both be graphed against joint
angle. Joint torque is the product of these two measurements. Peak torque and peak force can actually
occur at different joint angles.[29] Muscle force can
only be measured at this level if the moment arm
was determined at each discrete joint angle.
The shape of the single joint torque-angle curve
is unique to that of the single fibre or whole muscle
force-length curves. Joint movements typically involve multiple muscle groups crossing the joint and
producing torque, thus affecting the shape of the
curve. Other factors that affect the shape of the
curve include the constraints of the joint itself, and
range of motion. These differences lead to a different shaped curve. The torque-angle curve is smoother and flatter than the muscle-force curves. Notice
also that the peak of the curve is extended and
occurs at longer lengths (see figure 2c).
811
Shift magnitude (°)
3.9 (IMM), triceps surae
Length-Tension Relationship with Eccentric Exercise
Sports Med 2007; 37 (9)
812
Brughelli & Cronin
A pilot study by Clark et al.[48] reported a sustained shift in optimum length after 4 weeks of
eccentric exercise. This is the only training study, to
our knowledge, that has reported a sustained shift in
optimum length. Brockett et al.[47] and Bowers et
al.[42] both observed a sustained shift after two separate eccentric exercise sessions, which were separated by 8 days. The shifts lasted for 18[47] and 24[42]
days, respectively. Although many authors have argued that eccentric exercise can lead to a sustained
shift in optimum length,[9,47,49,50] there are currently
no randomised/controlled training studies that have
reported a sustained shift in optimum length with
eccentric exercise. The reader should be aware of
this major limitation in the literature.
The following sections will review the human
studies that have directly measured a shift in optimum length. Both men and women were used as
subjects in these nine studies.[38-40,42,44-48] Overall, 60
men and 40 women participated. All subjects were
between 18 and 37 years of age. With the exception
of Clark et al.,[48] who used amateur Australian
Rules football players, all of the subjects were classified as being recreationally active. The reader
should be aware of these limitations and delimitations when interpreting the results and developing
conclusions regarding shift in optimum length. The
following sections of this discussion will focus on
dynamometry, subjects, muscle groups, eccentric
exercise protocols, measurement protocols, magnitude of the shift and duration of the shift. Such an
approach will expose some of the delimitations and
limitations associated with the literature in this area.
3.1 Dynamometry
All the human studies that have directly measured a shift in optimum length after eccentric exercise measured tension at the single joint level, thus
joint torque was measured and plotted against joint
angle. Isokinetic dynamometers, such as the Bi© 2007 Adis Data Information BV. All rights reserved.
odex® III (Biodex Medical, Inc., Shirley, NY,
USA)[42,47,48] or Kin-Com (Chattecx Corp., Inc.,
Hixson, TN, USA),[44,45] were the most frequently
used to measure joint torques and angles. These
machines have an attached lever arm that is controlled by a motor. The angular velocity of the lever
arm is controlled by the machine. The subjects can
either push or pull against the lever arm to measure
torque capabilities. Isokinetic dynamometers also
typically have an isometric function that allows for
isometric torque measurements. Regardless of
whether the muscle contraction is isometric or
isokinetic, torque levels can be measured over a
range of discrete joint angles. Peak torque and joint
angle can then be plotted against each other to
generate the torque-angle curve.
There are two methods for measuring the range
of peak torque levels. The first (and more time
consuming) method involves measuring maximum
isometric contractions over a range of discrete joint
angles. The second involves measuring peak torque
during a continuous concentric contraction, which is
an easier and faster way of generating torque-angle
curves (a maximum isokinetic muscle contraction
can be completed in only a few seconds). This
second method was first utilised only recently.[47]
More studies have since utilised isokinetic dynamometers to generate in vivo torque-angle
curves.[42,44,48] In all these studies, peak torque was
measured for each contraction and plotted against
ankle angle; the curves were then used to determine
the optimum angle of peak torque.
The shift in optimum length can be measured
directly or indirectly with this equipment. Three
studies reported a decrease in isometric peak torque
at short lengths, and an increase in isometric peak
torque at longer lengths after eccentric exercise.[33,43,51] This would be considered an indirect
measure of a shift in the optimum length since the
optimum length was not directly measured.
Sports Med 2007; 37 (9)
Length-Tension Relationship with Eccentric Exercise
3.2 Curve Fitting
Direct measures of optimum length shifts can be
determined by utilising curve fitting procedures. By
fitting a curve to a range of peak torque values at
discrete joint angles, a torque-angle curve can be
generated. Curve fitting is a convenient way of
identifying the angle of peak torque. The most frequently used curve fitting procedures are the Gaussian and quadratic (fourth order) polynomial. In
order to use a Gaussian fitting curve, the data must
be normally distributed. However, the shape of the
single joint torque-angle curve is not symmetrical,
thus the data are not normally distributed. Only
datapoints >75–90% peak torque can be used to
generate the curve. Datapoints above these values
have more of a normal distribution. Thus, only the
top of the torque-angle curve is used to identify the
angle of peak torque. The Gaussian fitting curve is
the most commonly used length-tension fitting
curve in the literature for both human and animal
studies. Jones et al.[38] and Whitehead et al.[39,40]
fitted their datapoints above 75% peak torque. Several isometric contractions were measured over an
ankle range of 50–90°. Others have used Gaussian
fitting curves for datapoints >90% peak torque during isokinetic concentric contractions.[42,47]
The greatest concern with the Gaussian curve
fitting is that it does not describe changes in the
entire torque-angle curve, since only the peak is
considered. For this reason, some researchers use
quadratic polynomial curve fitting for their data
collection.[45,46] Like the Gaussian curve, the angle
of peak torque is easily identified. However, with
the polynomial fitting curve, changes can be described across the entire range of motion. Another
advantage of this procedure is that the data do not
need to be normally distributed.
© 2007 Adis Data Information BV. All rights reserved.
813
3.3 Equipment and Testing Protocol
Brockett et al.,[47] Clark et al.,[48] and Bowers et
al. utilised a Biodex® III isokinetic dynanometer
to measure knee joint angle and peak torque values
during maximum concentric contraction. They all
used a constant velocity of 60° per second during
knee extension and knee flexion. The ‘zero angle’
was set at 90° of leg flexion for Brockett et al.,[47]
thus an increase in degrees meant an increase in
length (joint angle). Conversely, Clark et al.[48] and
Bowers et al.[42] set their zero angles at full leg
extension. Either five[48] or seven[47] maximum concentric contractions were averaged and used as data
in these studies.
[42]
Pettitt et al.[44] and Philippou et al.[45] used a KinCom isokinetic dynamometer to measure torque and
corresponding joint angles. Philippou et al.[45] had
their subjects perform maximum isometric contractions of the elbow flexors (biceps brachii). The
contractions were measured over a range of five
joint angles (50, 70, 90, 140 and 160°) in random
order. The subjects performed two maximum voluntary isometric contractions of 3-second durations at
each joint angle. The better of the two trials was
used for the data collection. The subjects in the
study of Pettitt et al.[44] performed three maximum
concentric contractions of elbow extension (triceps
brachii). The repetition with the highest torque value
was used to determine optimum length.
Prasartwuth et al.[46] had their subjects perform
maximum isometric contractions on an isometric
myograph, which measured peak torque and joint
angle. The average of two maximum isometric contractions (biceps brachii) was measured at each discrete joint angle. Joint angle measurements started
at 60° and progressed in 10° increments to 150°. In
addition, during each contraction, paired-pulse stimulation was used to evoke a superimposed twitch. It
was reported that when voluntary force is reduced,
paired-pulse stimulation is an appropriate stimulus
Sports Med 2007; 37 (9)
814
Brughelli & Cronin
for estimations of voluntary activation. Peak torque
was plotted against each discrete joint angle.
Whitehead et al.[39,40] and Jones et al.[38] used a
custom-made adjustable chair with a steel frame to
measure torque and joint angle. The steel frame
supported a footplate that was attached to a rotating
axis. Torque was measured with four strain gauges
attached to an axle supporting the footplate. Torque
was measured isometrically from 90 to 50° in 5°[38]
or 10°[39,40] increments. Ankle angle was defined as
the angle between the footplate and the shins. All
three studies determined the torque-angle curve by
using double pulse stimulation of the tibial nerve,
using 1ms pulses 20ms apart. It was reported that a
torque-angle curve constructed this way was easier
to control and similar in shape and optimum length
to those taken from maximum voluntary contractions.[35]
3.4 Exercise Methodology and Results
Up until this point there has been no discussion
on the magnitude of the shift in optimum length after
eccentric exercise. Table I lists the nine studies in
order from the smallest to largest shifts in optimum
length. There are three main factors that influence
the magnitude of the shift:
1. eccentric exercise intensity;[40,44,45]
2. eccentric exercise volume;[42,46,47]
3. length of the muscle during eccentric exercise.[42,45,46]
Jones et al.[38] and Whitehead et al.[39,40] used
similar eccentric exercise protocols and thus reported similar shift magnitudes (3.9, 4.4 and 6°). Their
subjects walked backwards on an inclined moving
treadmill, which was meant to eccentrically strain
the triceps surae. However, the exercise protocols
were not exactly the same, and thus may explain the
differences in their shifts. For the experimental legs,
the subjects were instructed to step backwards with
a toe-to-heel action. This technique ensured that the
triceps surae was contracted eccentrically. The sub© 2007 Adis Data Information BV. All rights reserved.
jects performed the toe-to-heel action in one leg
(experimental), while the other leg (control) landed
with the toe and heel simultaneously. When the toe
and heel land together, eccentric contraction is limited. All three studies inclined their treadmills to 13°.
Jones et al.[38] used a very low intensity protocol (1.3
km/h stepping rate), which was carried out for a long
duration (2 hours). Whitehead et al.[39] had their
subjects step at a higher rate (3.5 km/h), which
increased the intensity of the eccentric contractions.
Whitehead et al.[40] had their subjects step at a
slightly higher stepping rate (>3.5 km/h). However,
the main difference between the studies was that
Whitehead et al.[40] had their subjects carry an additional 5–10kg weight belt. The additional load and
step rate increased the intensity of their eccentric
exercise protocol, which may explain why they had
a greater magnitude of shift in optimum length.
Brockett et al.[47] and Clark et al.[48] used the
same exercise (Nordic hamstring) and reported similar shifts (7.7 and 6.3°). The Nordic hamstring
exercise involves a subject kneeling on the ground
(or a board), and while maintaining a constant and
open hip angle, the subject slowly lowers their body
towards a prone position. In this exercise, the hamstrings are isolated and control the lowering body
while contracting eccentrically. Brockett et al.[47]
used a custom-made 2m long wooden board with
upholstered areas for the chest and knees. The ankles of the subjects were stabilised to the board with
ankle straps. Clark et al.[48] used the same exercise,
except the subjects were placed on the ground and
had a partner. The subjects placed a towel below
their knees and had a partner apply pressure to their
heels to ensure that their feet stayed in contact with
the ground throughout the movement. The Nordic
hamstring exercise can be classified as a submaximal exercise intensity, but Brockett et al.[47] used a
very high exercise volume (12 sets of 6 repetitions).
This volume was meant to acutely damage the hamstrings, and then measurements would be taken to
Sports Med 2007; 37 (9)
Length-Tension Relationship with Eccentric Exercise
determine how long the shift in optimum length
would last. The subjects in the study by Clark et
al.[48] performed 2–3 sets of 5–8 repetitions. This
protocol was repeated 1–3 times per week for 4
weeks. This protocol started with a low volume that
increased over a 4-week period. The objective was
not to cause acute damage, but to slowly increase the
intensity and volume of eccentric exercise. From
these two studies, it appears that a high-volume
eccentric protocol can cause an acute shift in optimum length. If volume is increased overtime, acute
muscle damage can be avoided, while a shift in
optimum length may occur over a 4-week period.
Important technical notes were to slowly lower the
body while maintaining contraction of the hamstrings for as long as possible.
Pettitt et al.[44] and Philippou et al.[45] utilised an
isokinetic dynamometer (Kin-Com) to perform
maximum eccentric contractions for their exercise
protocols. The elbow flexors (biceps brachii)[45] and
the elbow extensors (triceps brachii)[44] were used to
perform maximum eccentric contractions. The nondominant arms were utilised in both protocols. In the
study of Pettitt et al.,[44] the subjects performed three
sets of 25 maximum eccentric repetitions (50–170°)
at a velocity of 60° per second. This study investigated the effects of eccentric exercise performed at
short muscle lengths (0–80°) and long muscle
lengths (80–130°). The short-length group did not
show a shift in optimum length after eccentric exercise. The long-length group did show a significant
shift in optimum length (10.4°). The eccentric exercise protocol used a much higher intensity than the
previous studies. The eccentric contractions were
performed with maximum voluntary effort. This
could explain why the shift was higher than the
previous studies. However, Philippou et al.[45] used a
very similar exercise protocol to Pettitt et al.,[44] but
reported a much greater shift in optimum length
(16.1° vs 10.4°). In fact, the subjects performed 25
fewer repetitions (two sets of 25 repetitions). The
© 2007 Adis Data Information BV. All rights reserved.
815
subjects in the study by Philippou et al.[45] performed maximum eccentric contractions of the elbow flexors to 170° elbow extension. Pettitt et al.,[44]
on the other hand, had their subjects perform maximum eccentric contractions of the triceps to 130°
elbow flexion. It should be noted that the range of
motion of elbow flexion is much less than elbow
extension. The design of the elbow joint will not
allow the triceps brachii to be fully stretched. Thus,
the differences in shifts may be due to the muscle
length operating range during eccentric contraction.
From these two studies, it appears that very little
damage occurs at short lengths (0–80° elbow extension),[44] where damage is observed at longer lengths
(80–130° elbow extension),[44] and damage is the
greatest at the longest lengths (50–170° elbow flexion).[45]
Bowers et al.[42] reported a 15.4° shift after a stepdown exercise that eccentrically contracted the
quadriceps. The subjects stepped up to a platform
(height set at the patella) with their left leg (control),
followed by their right leg (experimental). Once
both feet were placed on top of the platform, the
subjects slowly stepped down with their left leg
leading. The subjects were instructed to control the
movement with their right leg, which remained on
the platform during the step down. This allowed the
quadriceps of the right leg to contract eccentrically.
The subjects performed 12 sets of 20 step-downs for
a total of 240 steps. This study used a very high
volume of 240 total eccentric contractions. Also, the
exercise was designed to have the eccentric contractions occur at long muscle lengths. The subjects
stepped down from a platform while their experimental leg contracted eccentrically to control the
movement.
Prasartwuth et al.[46] also reported a large shift in
optimum length (16.7°) after eccentrically training
the elbow flexors (biceps brachii). A pulley wheel
was designed for the eccentric exercise protocol.
The axis of rotation of the pulley wheel was aligned
Sports Med 2007; 37 (9)
816
Brughelli & Cronin
with the subjects’ elbow joints. The subjects lowered a weight attached to the wheel from 60° to full
elbow extension. Initially, five sets of ten repetitions
were performed. The sets were continued until maximum voluntary torque fell by 40%. The total
amount of repetitions varied from 50 to 160. The
eccentric contractions were performed from 60° to
full elbow extension. The combination of high volume and long muscle lengths in this and the previous
study probably contributed to the large shifts in
optimum length (15.4[42] and 16.7°[46]).
From these nine studies, a few tentative conclusions can be made:
•
high-intensity eccentric exercise results in
greater shifts in optimum length;
•
muscles that are eccentrically contracted at longer lengths result in greater shifts in optimum
length;
•
high-volume eccentric exercise results in greater
shifts in optimum length;
•
the combinations of high-intensity/long muscle
length or high-volume/long muscle lengths produced the greatest shifts in optimum length;
•
muscle architecture does not seem to affect the
shift in optimum length;
•
eccentric exercises that involve long muscle
lengths and either high intensity or high volume
should be developed to produce the greatest
shifts in optimum length;
•
it may be possible to produce a sustained shift in
optimum length after 4 weeks of eccentric exercise;
•
muscle damage may not need to be induced for
this adaptation (shift in optimum length) to occur.
Study investigators should keep these factors in
mind when developing eccentric exercise protocols.
© 2007 Adis Data Information BV. All rights reserved.
4. Mechanisms for the Shift in
Optimum Length
Several mechanisms have been proposed for the
rightward shift after eccentric exercise, which include a partial transformation of active contractive
elements into passive elastic elements;[14] damage to
the myotendinous attachments;[41] and damage to the
calcium handling structures.[27,52] It has further been
proposed that two different shifts occur after eccentric exercise. The first shift is acute and appears after
the muscle has been damaged, and the second shift
is due to an adaptation that occurs over a period of
time (10 days to 8 weeks).[2,8,42] The mechanisms
proposed for this second shift include an increase in
sarcomeres in series (sarcomereogenesis)[53] and an
increase in passive tension at longer muscle
lengths.[12]
4.1 First and Second Shift in Optimum Length
Structural muscle damage has been observed immediately after eccentric exercise. Electron microscopic examinations have shown Z-line streaming
(sarcomeres out of register with each other), regions
of overstretched half sarcomeres and t-tubule damage.[4,54,55] The disrupted sarcomeres are thought to
be responsible for the first acute shift in optimum
length. The descending limb of the length-tension
curve is thought to be a region of instability, where
force levels decrease. It has been proposed by Morgan[53] that during active muscle lengthening when
the myofilaments are stretched onto the descending
limb, some of the weaker sarcomeres will be stretched more than others. These sarcomeres will become
progressively weaker until there is no overlap between myofilaments. When the eccentric contractions are repeated, more sarcomeres will be overextended from weakest to strongest. At the end of each
contraction, as the muscle relaxed, a number of
sarcomeres may not reintegrate, thus becoming disrupted. The disrupted sarcomeres will be scattered at
Sports Med 2007; 37 (9)
Length-Tension Relationship with Eccentric Exercise
random along muscle fibres, which increases series
compliance. The acute shift in optimum length after
eccentric exercise is thought to result from an increase in series compliance, due to disrupted
sarcomeres.[2,53]
It has also been proposed that a second shift
occurs after eccentric exercise. This delayed shift is
argued to be a result of either a mechanical[13,56] or
cellular[10,53] adaptation in the muscle. Despite the
arguments, there is a growing interest in the additional stability at longer muscle lengths due to the
shift in optimum length. One of the proposed mechanisms for this second shift is an addition of
sarcomeres in series (sarcomereogenesis) after eccentric exercise.[2,10,53] Many believe that a more
compliant muscle will avoid the unstable regions
(descending limb) of the length-tension curve during further eccentric exercise.[2,50] The other proposed mechanism for the second shift is an increase
in passive tension at longer muscle lengths. This
adaptation is thought to occur after a period of
eccentric training (6–8 weeks). The belief is that
eccentric exercise leads to greater passive stiffness,
or spring-like qualities.[12,56] LaStayo et al.[13] argued
that an increase in stiffness at longer lengths will
increase force production before failure. Stability at
longer muscle lengths may prevent active muscle
strain injuries and possibly enhance athletic performance.
4.2 Theory of Sarcomereogenesis
Sarcomereogenesis is the addition of sarcomeres
in series within a muscle fibre and is thought to have
an important role in maintaining the relationship
between sarcomere length and joint angle. The number of sarcomeres in series is thought to be an
adaptable property of muscle. After sarcomereogenesis has occurred, sarcomere length will be
shorter for a given muscle length.[53] This adaptation
is thought to keep the myofilaments off the descending limb of the length-tension curve during future
© 2007 Adis Data Information BV. All rights reserved.
817
eccentric contractions. Thus, the muscle will maintain stability at longer muscle lengths.
It has been argued that the number of sarcomeres
in series is highly plastic and can be altered with
training.[32,57,58] Herzog et al.[59] found that lengthtension properties of the rectus femoris were different between runners and cyclists. With running, the
rectus femoris experiences a stretch-shortening cycle, where high force levels were required at long
lengths during both eccentric and concentric contractions. Cycling requires high forces at shorter
muscle lengths during concentric contractions. Consequently, it would be expected that the runners
have more sarcomeres in series than the cyclists.
The study reported that runners produce peak torque
at long lengths, and cyclists produce peak torque at
short muscle lengths.[59]
Direct evidence for sarcomereogenesis has been
observed in several animal and human studies. Most
of the studies reported that sarcomereogensis occurred after a static stimulus (passive static stretching or shortening). However, there are a few studies
that have shown an increase in serial sarcomere
number after eccentric exercise. Lynn and Morgan[60] were the first to report direct evidence of an
increase in the number of sarcomeres in series after
eccentric exercise. Rats were trained to run uphill
(concentric group) or downhill (eccentric group) on
a treadmill for 5 days. Running downhill caused the
muscles to contract eccentrically as they controlled
the movement, while running uphill was predominantly a concentric exercise. The number of
sarcomeres in series (vastus intermedius) increased
in the downhill group. Since then, four more studies
have reported an increase in serial sarcomere number after eccentric exercise.[61-64] Butterfield et al.[63]
also reported an increase in sarcomeres in series
(vastus intermedius) after trained rats ran downhill
for 10 days. More surprisingly, the group of rats that
ran uphill decreased the number of sarcomeres in
series in the vastus intermedius muscle.
Sports Med 2007; 37 (9)
818
Brughelli & Cronin
Furthermore, Whitehead et al.[39] found that if
eccentric exercise is followed by previous concentric exercise, muscle damage and the shift in optimum length was greater than if eccentric exercise
was performed alone. The authors concluded that
concentric-based training programmes produce
changes in the muscles that make them more prone
to injury caused by eccentric exercise. Athletes such
as triathletes and marathon runners, who regularly
perform both concentric and eccentric muscle contractions for a prolonged period of time may benefit
from these findings. Programmes should be developed to help these athletes enhance their performance, while preventing muscle damage induced by
eccentric exercise.[39]
4.3 Theory of Passive Tension
Another proposed mechanism for the second
shift in optimum length after a period of eccentric
exercise (6–8 weeks) is a greater contribution of the
passive elements at longer muscle lengths.[13,56]
More passive tension at longer muscle lengths can
also shift the length-tension curve to the right. Eccentric exercise can cause disruption to the muscles
passive components. Cytoskeletal proteins, such as
desmin and titin, play a significant role in the structure and function of the sarcomere.[29] After eccentric exercise, disruption and degradation occurs to
the desmin and titin proteins.[22,29] Titin content has
been reduced by up to 30% in human subjects (vastus lateralis) after a single bout of eccentric exercise.[65] A protective adaptation has been suggested
to occur that strengthens the cytokeletal proteins,
and prevents them from being damaged in the future. Barash et al.[66] reported an increase in desmin
content 7 days after eccentric exercise-induced muscle damage in rats. Severe or continuous muscle
strain is thought to be a stimulus for adaptations in
titin expression.[67,68] Spiers et al.[68] reported that
sarcomere strain amplitude and titin isoform size
were correlated in fish (carp). Fibre types are sepa© 2007 Adis Data Information BV. All rights reserved.
rated anatomically in fish, making them a good
model for study. Sarcomeres that are exposed to
greater strains may express titins with larger elastic
segments. Differences in titin expression have also
been reported in humans. Weight lifters, power lifters and sprinters expressed a greater percentage of
titin-1 isoforms than non-athletes.[69] The subjects in
the athletic groups would be expected to experience
strain more often and to a greater degree.
The titin filament has a major role in the development of passive tension. However, the role of the
titin in muscular performance during explosive activities is unknown. It is possible that the titin is
capable of storing and releasing elastic energy.[13,70]
McBride et al.[69] reasoned that a more elastic muscle (due to altered titin isoforms) would lead to
greater power production, and proposed that explosive training can alter titin expression. In this study,
subjects performed explosive squat-jump training
for 8 weeks with either 30% or 80% of their maximum back squat. There was no change in titin
isoform expression after the training period. However, the training stimulus did not induce a great
amount of muscle strain, which would be needed for
a mechanical adaptation to occur.
Despite the claims that eccentric training can
alter titin expression, currently there are no training
studies that have examined the effects of eccentric
training on titin expression. There are, however,
studies that have observed increases in passive stiffness at longer muscle lengths after eccentric exercise. Increases in passive stiffness have been reported in animal[56,71] and human training studies[71,72]
after eccentric exercise. Labeit et al.[73] reported that
passive force enhancement was caused by a stiffer
molecular titin. It was also reported that the titin
becomes more sensitive to calcium at longer
lengths.[73,74] Since the titin filament is responsible
for the majority of passive tension and stiffness at
longer muscle lengths,[29,73] this could be an indirect
indicator that an adaptation has occurred. Even
Sports Med 2007; 37 (9)
Length-Tension Relationship with Eccentric Exercise
though the torque-angle curve is taken when a muscle is active and contracting maximally, both passive and active elements contribute to the curve. If
eccentric exercise can cause an increase in passive
tension at longer lengths, the length-tension curve
should be shifted to the right. This adaptation to the
titin filament could be the mechanism for the second
shift in optimum length.
It should be noted that this increase in passive
tension after a training period with multiple eccentric training sessions, should not be confused with
the well documented increase in passive tension that
occurs immediately and up to 10 days after a single
bout of eccentric exercise. The increase in passive
tension immediately after eccentric exercise is due
to disruption of the excitation-coupling process. The
increase in passive tension that occurs after an eccentric-based training programme of 6–8 weeks is
thought to be caused by an increase in the contribution of the passive elements.
5. Implications for Athletic Performance
and Injury Prevention
5.1 Muscle Injury Prevention
There is a growing interest in the effects of
eccentric exercise on muscle strain injuries, specifically for the hamstrings.[9,50] The design of the biarticulate hamstring muscles places them at a high
risk of injury. During the late swing phase in running, the simultaneous actions of hip extension and
knee flexion actively stretch the hamstrings to
greater lengths. Because of the prevalence of hamstring injuries in sport, finding methods of decreasing injury rates is a high priority. Hamstring strain
injuries alone account for 6–16% of all injuries in
Australian Rules football,[75] soccer,[76] basketball,[77] cricket[78] and rugby union.[79,80] In addition,
the risk of re-injuring the hamstrings is even higher
(12–31%).[76,81] Previous injury is considered one of
the greatest risk factors for re-injury. It has been
© 2007 Adis Data Information BV. All rights reserved.
819
proposed that hamstring injuries most often occur
when they are being actively lengthened,[82,83] or
when they need to switch from an eccentric contraction to a concentric contraction after being actively
lengthened.[84] The long head of the biceps femoris
is the most often injured hamstring muscle.[83] Unlike the semitendinosus and semimembranous, the
biceps femoris is actively lengthened throughout the
latter half of the swing phase during sprinting.[83]
This makes the biceps femoris more susceptible to
muscle strain injuries.
Many believe that athletes who produce peak
torque at shorter muscle lengths are more likely to
get injured.[9,10,47,50] A shorter optimum length
would mean that more of the muscles operating
range would be on the descending limb of the
length-tension curve. Brockett et al.[50] explored this
idea by measuring optimum lengths in athletes who
have had previous hamstring injuries in one leg
(experimental) versus their uninjured leg (control).
The mean optimum angle was 12.7° shorter for the
injured leg, although strength values (hamstring/
quadriceps ratio) were similar. Based on these findings, the authors suggested that optimum length may
be a greater risk factor for muscle strain injuries than
strength ratios. The conclusions were that a shorter
optimum length would place an athlete at greater
risk for muscle strain injuries.
Another interesting adaptation that occurs after a
single bout of eccentric exercise is called the ‘repeated bout effect’. If a second bout of eccentric
exercise is performed that is similar to the first,
muscle damage is significantly reduced.[6,18,85] This
adaptation can last for several weeks, and possibly
up to 6 months.[85] The repeated bout effect can
occur even if there is slight muscle damage.[18] The
rightward shift in the length-tension curve is considered one of the possible mechanisms for the repeated bout effect.[7] Several authors believe that during
eccentric exercise, damage occurs on the descending
limb of the length-tension curve.[42,50,53] The deSports Med 2007; 37 (9)
820
Brughelli & Cronin
scending limb is the region on the curve that is
beyond the optimum length of myofibril overlap.
Some have suggested that muscle strain injuries
occur on the descending limb of the length-tension
curve.[9,50] By shifting the optimum length to greater
lengths, the descending limb may not be reached
during subsequent eccentric activities, i.e. sprinting,
kicking and jumping.
Although there is a growing interest in eccentric
exercise for preventing muscle strain injuries, very
few studies have actually assessed injury rates after
a period of eccentric exercise. There currently are
only three published studies[49,86,87] and one unpublished study[9] that have reported a reduction in
hamstring injuries with eccentric exercise (table II).
It should be noted, however, that >700 athletes were
monitored during these four studies, and they all
reported significant drops in hamstring injury rates.
5.2 Eccentric Exercise and
Athletic Performance
Despite the recent interest in eccentric exercise
and the length-tension relationship, there is a gap in
the literature on the effects of eccentric exercise on
athletic performance. Intuitively, one would think
that if the mechanical properties of muscle are
changing, then the functional properties would also
change. Also, since human movement typically
takes place when the muscles are on or near their
optimum lengths,[29,57] performance may be altered
if the optimum length is shifted. However, this is not
the case for all muscles. Some muscles operate
solely on the descending or ascending limbs of the
length-tension curve.[29]
It has been known for some time that eccentric
contractions can produce the greatest amounts of
force, recruit fast twitch fibres with little effort, and
place greater strain on the muscle to induce favourable adaptations.[14,15,29,31] Many have suggested that
more research should be performed with eccentricbased training programmes.[13,29,32] However, re© 2007 Adis Data Information BV. All rights reserved.
search of this nature has proved problematic because
multiple spotters are needed for heavy eccentric
lifts; specialised and expensive equipment is often
required; there is a risk of DOMS; and there is a risk
of injury. These limitations, unfortunately, led practitioners and scientists to suggest that eccentric exercise should only be performed by advanced athletes.
However, given the resurgent interest in eccentrics,
exercises are being developed for subjects over a
range of abilities. The challenge for clinicians and
strength and conditioning practitioners is to be more
inventive in their prescription of eccentric exercise.
Much of the literature addressing injury and athletic performance uses the Nordic hamstring curl or
similar derivations. Such exercises have limitations.
First, it is an open-chain/bilateral exercise. It is very
likely that the stronger leg (longer optimum length)
will be strained more than the weaker leg (shorter
optimum length). Both Brockett et al.[47] and Clark
et al.[48] reported differences in optimum length between legs. After 4 weeks of training with the Nordic hamstring exercise, Clark et al.[48] reported that
the imbalance became larger between the legs. It
was thought that the leg with a longer optimum
length would receive more strain and continue to
adapt to a longer optimum length, while the other
leg would not adapt as much. Secondly, this exercise
is a single joint exercise. The hamstrings are a biarticulate muscle group, with both hip extension and
knee flexion functions. Injuries most frequently occur when the hamstrings are being actively stretched
by simultaneous hip flexion and knee extension
(during running or kicking). Multi-joint exercises
would probably be more effective at improving performance and preventing muscle strain injuries in
the lower body. Thirdly, the subjects are unable to
support their own bodyweight at around 30° of knee
extension. As the subjects lower their bodyweight,
gravity becomes a factor, and is a major factor at
30°. If the effects of gravity could be manipulated, it
would be possible to go beyond 30°, since this is
Sports Med 2007; 37 (9)
© 2007 Adis Data Information BV. All rights reserved.
Pilot study.
b
Eccentric exercise groups
(11 clubs participated)
union players 2002–4
English Premier rugby
(n = 220)
Rules football players
Amateur Australian
SSN group (n = 400)
SSS group (n = 288)
S group (n = 296)
Control group (n = 106)
NH group (n = 114)
straight leg deadlifts, knee curls, NH
football players
(n = NR)
Additional eccentric exercises:
Australian Rules
Control group (n = 15)
Yo group (n = 15), Yo curl
Results
SSN group: 0.39 hamstring injuries
per 1000h
6–7 reps (weekly basis)
per 1000h
Additional NH exercise 2–3 sets of
SSS group: 0.59 hamstring injuries
(weekly basis)
1000h
S group: 1.1 hamstring injuries per
sustained a hamstring injury
Control group: 13.2% of the players
hamstring injury
NH group: 4% of players sustained a
hamstring injuries
2003 experimental season: 2
hamstring injuries
2002 experimental season: 5
injuries
2001 control season: 16 hamstring
hamstring injury
Control group: 10 of 15 received a
hamstring injury
Yo group: 3 of 15 received a
Additional hamstring stretching
(weekly basis)
Concentric and eccentric exercise
12wk, regular training
exercise (12 sets of 6 reps)
12wk, 5 sessions of additional NH
Pre-season training (not detailed)
10wk, regular training
Yo training
10wk, 4 sets of 8 reps of additional
Exercise protocol
hamstring.
NH = Nordic hamstring; NR = not reported; reps = repetitions; S = strengthening; SSN = stretching, strengthening and NH; SSS = static stretching and strengthening; Yo = Yoyo
Preliminary results.
a
Brooks et al.[87]
Gabbe et al.[86]b
Proske et al.[9]a
Competitive soccer
Askling et al.[49]
players (n = 30)
Subjects
Study
Table II. Preventing hamstring strain injuries in sport
Length-Tension Relationship with Eccentric Exercise
821
Sports Med 2007; 37 (9)
© 2007 Adis Data Information BV. All rights reserved.
Physically active men (n = 9)
and women (n = 22). Each
served as own control
Amateur Australian Rules
football players (n = 9). No
control group
Physically active men (n = 34)
High-school basketball players
(n = NR)
Competitive soccer players
(n = 23)
Benn et al.[72]
Clark et al.[48]
Colliander and
Tesch[88]
LaStayo et al.[13]a
Mjolsnes et al.[89]
10wk, 2–3 times/wk
10wk
2–3 sets of 6–12
reps (NH)
10wk
2–3 sets of 5–10
reps (LC)
NH exercise
LC group
LC exercise
Regular training
Control group
NH group
8wk, 3 times/wk for
30 min
4–5 sets of 12 max
reps
Ecc/Con group, Con/Ecc isokinetic
dynamometer
HE group
12wk, 3 times/wk
Con group, Con only isokinetic
dynamometer
2–3 sets of 6–8
reps, 1–3 times/wk
SLS experimental leg
NH group (n = 9)
10wk, 2–3 times/wk
10wk, regular
training
Control group (n = 15)
SL control leg
10wk, 4 sets of 8
reps (Yo)
protocol
Exercise
Yo group (n = 15)
Eccentric exercise groups
↔
LC ↔
NH: 11% ↑ in H/Q ratio
Hop Hz
H/Q ratio
↔
HE: 12% ↑ (no SD reported)
Hop Hz
VJ
HE: 8% ↑ (no SD reported)
Ecc/Con group: 7.9% ↑ (ES: 0.92)
Con group ↔
NH: 6.6% ↑ (no SD reported)
SLS: 8.6% ↑ (ES = 0.25)
SL: 6.6% ↑ (ES = 0.2)
Control ↔
Yo: 2.4% ↓ in sprint times (ES = 0.8)
Results
VJ
VJ
VJ
VJ
VJ
VJ
30m sprint
30m sprint
type
Con = concentric; Ecc = eccentric; ES = effect size; HE = high-force eccentric ergometer; H/Q = hamstring-quadriceps; Hz = frequency; LC = leg curl; max = maximum; NH =
Nordic hamstring; NR = not reported; reps = repetitions; SD = standard deviation; SL = single-leg squat; SLS = single-leg squat with extra stretch load; VJ = vertical jump; Yo =
Yoyo hamstring; ↑ indicates increase; ↓ indicates decrease; ↔ indicates no change.
Preliminary results.
Competitive soccer players
(n = 30)
Askling et al.[49]
a
Subjects
Study
Table III. Enhancing athletic performance with eccentric exercise
822
Brughelli & Cronin
Sports Med 2007; 37 (9)
Length-Tension Relationship with Eccentric Exercise
around the same joint angle that most people produce peak torque during isokinetic testing. Fourthly,
after a few weeks of this exercise, subjects are able
to lower their bodyweight beyond 30° (personal
observations). If an imbalance in optimum length is
occurring between legs it would not be appropriate
to overload this exercise, which is the normal procedure for training athletes, until this imbalance has
been corrected.
Given the development of exercises and equipment that safely overload the eccentric contractile
ability of muscle, greater interest in the shift in
optimum length after eccentric exercise and its effects on athletic performance may become of increasing interest. How the shift in optimum length
affects functional and athletic performance, i.e. stiffness/compliance, storage and release of elastic energy, fast stretch-shortening cycle performance, slow
stretch-shortening cycle performance, kinetics
(force, work, power, rate of force development, and
kinematics (acceleration, peak velocity) need to be
investigated. It would appear that the shift causes
active stiffness to decrease at shorter muscle lengths
(due to sarcomereogenesis)[2,10,53] and passive stiffness to increase at longer muscle lengths (due to titin
elasticity).[11,12,56] A more compliant muscle-tension
unit is capable of storing more elastic energy, while
a stiffer muscle-tension unit is capable of producing
a faster rate of power output. If the muscle is more
compliant at the beginning of the stretch, it would be
possible to store more elastic energy. Also, if stiffness increases at the end of the stretch, more energy
could be released at a higher rate. Thus, performance of the stretch-shortening cycle would be greatly enhanced. It would be possible that each adaptation would have a role in enhancing athletic performance.
The training studies that have assessed athletic
performance after eccentric exercise have found that
performance does improve. Eccentric exercise has
been shown to improve sprint times,[49] jumping
© 2007 Adis Data Information BV. All rights reserved.
823
ability,[12,48,72] optimum hopping frequency[12] and
overall strength (table III). However, of these studies, two were unpublished,[11,12] one was a pilot
study[48] and one used a combination of eccentric/
concentric exercise versus concentric only.[88] Given
these limitations, there would seem a need for a
great deal more research in this area.
6. Conclusions
There has been a re-emergence of interest in
eccentric exercise over the last decade. The ability to
shift the optimum length with eccentric exercise
could have several implications for performance and
injury prevention. Altering the mechanical properties of muscle could have profound effects on athletic potential. Also, by allowing the muscle to operate
and maintain stability at longer lengths could decrease injury rates. Several studies have reported
either an increase in athletic performance or a decrease in injury rates with eccentric exercise. Despite the recent interest in eccentric exercise, there
are a few limitations in the literature. Currently,
there are no randomised, controlled training studies
that have reported a shift in optimum length after
eccentric exercise. Only one training study (pilot)
reported a shift after 4 weeks. There is also a lack of
studies on athletic performance after eccentric exercise. Most studies in the past only performed heavy
eccentric exercise or plyometrics and examined the
effects on performance. There is only one type of
exercise that is currently being used to prevent injuries and enhance performance, i.e. the Nordic hamstring exercise or Yoyo hamstring curl. This type of
exercise has a few inherent flaws. Being a bilateral
and open-chain exercise, the hamstrings may not be
worked equally between legs. Thus, imbalances
may occur over time. New exercises need to be
developed that do not have these limitations.
From the literature and a functional standpoint,
lower-body eccentric exercises that are designed to
increase the optimum length, prevent muscle injuSports Med 2007; 37 (9)
824
Brughelli & Cronin
ries and enhance athletic performance should include as many of the following recommendations as
possible: multi-joint movements; high/moderate eccentric force activities; muscles operating at long
lengths; closed-chain exercises; high/moderate velocity; easy to implement exercises; cost-effective
exercises; high volume (if an acute shift is desirable); and unilateral exercises. Exercise design
should take a multi-joint and multi-movement approach. This will allow greater prevention against
active strain injuries to a larger number of muscles,
and possibly improve performance.
Acknowledgements
The authors received no funding for the preparation of this
article and have no conflicts of interest directly relevant to its
contents.
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Correspondence: Matt Brughelli, School of Exercise,
Biomedical and Health Sciences, Edith Cowan University,
100 Joondalup Drive, Joondalup, WA 6027, Australia.
E-mail: [email protected]
Sports Med 2007; 37 (9)