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Scand J Med Sci Sports 2001: 11: 134–140
COPYRIGHT CMUNKSGAARD 2001 ¡ISSN 0905-7188
Printed in Denmark ¡All rights reserved
Characteristics of isometric and dynamic strength loss following
eccentric exercise-induced muscle damage
C. Byrne, R. G. Eston, R. H. T. Edwards
School of Sport, Health and Exercise Sciences, University of Wales, Bangor, Gwynedd, Wales, UK
Corresponding author: Dr. R. G. Eston, School of Sport, Health and Exercise Sciences, University of Wales, Bangor, Gwynedd,
Wales, UK LL57 2EN
Accepted for publication 21 August 2000
Angle-specific isometric strength and angular velocity-spe-
cific concentric strength of the knee extensors were
studied in eight subjects (5 males and 3 females) following
a bout of muscular damaging exercise. One hundred maxi-
mal voluntary eccentric contractions of the knee extensors
were performed in the prone position through a range of
motion from 40æto 140æ(0æΩfull extension) at 1.57 rad ¡
s
ª1
. Isometric peak torque was measured whilst seated at
10æand 80æknee flexion, corresponding to short and opti-
mal muscle length, respectively. Isokinetic concentric peak
torque was measured at 0.52 and 3.14 rad ¡s
ª1
. Plasma
creatine kinase (CK) activity was also measured from a
fingertip blood sample. These measures were taken before,
immediately after and on days 1, 2, 4, and 7 following the
eccentric exercise. The eccentric exercise protocol re-
Unaccustomed exercise, high intensity exercise, or an
increased training workload often result in exercise-
induced muscle damage. A major functional conse-
quence of muscle damage is an immediate and pro-
longed loss of muscle strength. Strength loss is more
pronounced and longer lasting following exercise in-
volving eccentric muscle actions as opposed to con-
centric or isometric actions (Davies & White, 1981;
Newham, Mills, Quigley, Edwards, 1983; Jones, New-
ham, Torgan, 1989; Golden & Dudley, 1992).
Strength loss following eccentric exercise has
mainly been assessed by measuring isometric force at
a single muscle length (Warren, Lowe, Karwoski,
Prior, Armstrong, 1993; Ingalls, Warren, Williams,
Ward, Armstrong, 1998) or a single joint angle (New-
ham et al., 1983; Jones et al., 1989; Clarkson, No-
saka, Braun, 1992; Cleak & Eston, 1992) before and
after exercise. However, the popping sarcomere hy-
pothesis of muscle damage suggests that the length–
tension relationship of muscle undergoes a shift to
the right, towards longer muscle lengths, following
eccentric exercise (Morgan & Allen, 1999). That is to
say, a longer muscle length is required to achieve the
same myofilament overlap after exercise compared to
134
sulted in a greater relative loss of strength (P∞0.05) at
short muscle length (76.3∫2.5% of pre-exercise values)
compared to optimal length (82.1∫2.7%). There were no
differences in the relative strength loss between isometric
strength at optimal length and isokinetic concentric
strength at 0.52 and 3.14 rad¡s
ª1
. CK activity was sig-
nificantly elevated above baseline at days 4 (P∞0.01) and
7(P∞0.01). The greater relative strength loss at short
muscle length appeared to persist throughout the seven-
day testing period and provides indirect evidence of a shift
in the angle–torque relationship towards longer muscle
lengths. The results lend partial support to the popping
sarcomere hypothesis of muscle damage, but could also be
explained by an impairment of activation at short muscle
lengths.
before exercise. Such an effect would cause the joint
angle for optimum force production to occur at a
longer muscle length and would cause the force loss
associated with eccentric exercise to be affected by
the muscle length or joint angle at which it is meas-
ured. A shift in the length–tension curve towards
longer muscle lengths would cause a significantly
greater loss of relative force to occur at short muscle
lengths compared to optimal or long muscle lengths.
The first part of this study aimed to determine how
reductions in isometric force following eccentric exer-
cise are affected by the muscle length at which force
is measured. According to the popping sarcomere hy-
pothesis (Morgan, 1990), lengthening of active
muscle does not occur by uniform lengthening of all
sarcomeres, but by a non-unifor m distribution of sar-
comere length change, with some sarcomeres rapidly
over-extending (‘popping’) beyond filament overlap
and becoming permanently over-extended. Such
over-extended sarcomeres would cause the remaining
functional sarcomeres to adopt a shorter length to
compensate. It is this mechanism which is suggested
to cause the length–tension curve to shift towards
longer muscle lengths. By measuring isometric force
Isometric and dynamic muscle strength loss
at short versus optimal muscle length before and
after eccentric exercise, we can indirectly determine
whether the length–tension curve has shifted towards
longer muscle lengths. A significantly greater loss of
relative force at short versus optimal muscle length
would indicate a rightward shift in the length–tension
curve, towards longer muscle lengths. Evidence from
studies employing this method has shown the greatest
loss of force to occur at short muscle length following
eccentric exercise of the human elbow flexors (Sax-
ton & Donnelly, 1996) and knee extensors (Child,
Saxton, Donnelly, 1998). However, it is not clear
whether the greater force loss is an acute effect only
(Child et al., 1998) or persists for several days (Sax-
ton & Donnelly, 1996) after eccentric exercise. This
study aimed to determine the magnitude and time
course of the effect over a seven-day period following
eccentric exercise.
The second part of this study aimed to compare
the effect of exercise-induced muscle damage on iso-
metric and concentric strength at slow and fast angu-
lar velocities of movement. Warren, Lowe, Armstrong
(1999) recently advocated the investigation of
strength as a function of velocity in addition to the
‘standard’ measurement of isometric strength. This
followed evidence from several studies, which have
suggested that the recovery of strength may be de-
pendent upon the type of muscle action (isometric vs.
concentric vs. eccentric) and/or the angular velocity
of movement (slow vs. fast) (Friden, Sjostrom, Ek-
blom, 1983; Golden & Dudley, 1992; Gibala, Mac-
Dougall, Tarnopolsky, Stauber, Elorriga, 1995; Es-
ton, Finney, Baker, Baltzopoulos, 1996). In addition,
research suggests that fast-twitch muscle fibres are
selectively damaged during eccentric exercise (Friden
et al., 1983; Lieber & Friden, 1988; McHugh, Con-
nolly, Eston, Gleim, 2000) and this may predispose
high angular velocity muscle actions to a slower re-
covery than low velocity or isometric actions.
Methods
Participants and design
Eight healthy participants, five males and three females (age
21.4∫3.5 yr (mean∫SD), height 1.73∫0.13 m, mass 68.1∫5.0
kg) were involved in the study. All individuals were moderately
active but had not participated in any resistance training for six
months prior to the study, and none had any musculo-skeletal
defects. Each individual gave written informed consent to par-
ticipate in the study which had previously been approved by the
School ethics committee. The muscle group studied was the
knee extensors. Using a single group design, joint angle-specific
isometric strength, angular velocity-specific concentric strength,
and plasma creatine kinase (CK) activity were measured before
and after a bout of maximal repetitive isokinetic eccentric exer-
cise. Isokinetic eccentric exercise and all strength measurements
were performed with the non-dominant limb using a Kin-Com
(500H, Chattecx, Chattanooga, TN, USA) isokinetic dyna-
mometer. Each subject was evaluated for each criterion
135
measure prior to, immediately afterwards, and on days 1, 2, 4,
and 7 following the eccentric exercise bout.
Isometric and dynamic assessment of muscle function
Participants were tested in the seated position with the lateral
femoral epicondyle aligned to the dynamometer axis of ro-
tation. The pelvis, chest, and active limb were secured with re-
straining straps to prevent extraneous movement. The pad of
the lever arm was positioned at a distal point on the tibia near
the malleoli. The dynamometer lever arm length and the verti-
cal, horizontal, and seat positions were recorded for each indi-
vidual in order to replicate the exact testing position from trial
to trial. All participants performed at least two familiarization
sessions during which they were introduced to standardized
written instructions to work as hard and fast as possible against
the resistance of the dynamometer, the isometric and isokinetic
testing protocols, and the use of visual feedback to enhance
torque output from one repetition to the next (Baltzopoulos,
Williams, Brodie, 1991).
Isometric assessment of muscle function
Angle–torque relationship. Selection of two joint angles for iso-
metric torque measurement, corresponding to short muscle
length and optimal muscle length, were established from the
angle–torque relationship of the non-dominant limb of the knee
extensors of four participants by testing each eight times. Full
knee extension (0æ) was entered as a reference value into the
Kin-Com visual display. This was used to set angles for iso-
metric torque measurement at 12æincrements throughout a 96æ
range of motion. Two maximal voluntary contractions (MVCs)
of 3 s duration were performed at each joint angle with a 1-
min rest period between successive attempts. The highest peak
torque elicited from the two attempts was used as the criterion
score.
Isometric peak torque at short and optimal muscle length. Par-
ticipants performed isometric MVCs of the quadriceps at 10æ
and 80æknee flexion corresponding to short and optimal muscle
length, respectively. The testing positions were obtained by en-
tering full knee extension (0æ) as a reference value into the Kin-
Com visual display. The reproducibility of this method was
checked on each testing occasion by noting the Kin-Com angle
display when the lever arm was at true 90æ(determined by spirit
level). The pre-test angle display at true 90æwas used as the
criterion. If any difference existed, the process was repeated
until the criterion was achieved. Three submaximal and one
maximal practice repetitions acted as warm-up at each testing
position. Three MVCs of 3 s duration were performed at each
joint angle with a 1-min rest period between repetitions. The
highest peak torque from the three contractions was used as the
criterion score for short and optimal muscle length, respectively.
Dynamic assessment of muscle function
Isokinetic concentric peak torque. Concentric peak torque of the
knee extensors was measured at angular velocities of 0.52 and
3.14 rad ¡s
ª1
(30 and 180 deg ¡s
ª1
). The problem of torque
overshoot or artifact becomes increasingly important when
testing isokinetic strength at high angular velocities. We were
unable to distinguish between muscular torque and torque over-
shoot at velocities above 3.14 rad ¡s
ª1
and so this angular velo-
city was the highest measured. Range of motion for the dy-
namic contractions was from 90æto 10æknee flexion. Three sub-
maximal and one maximal practice repetitions acted as warm-
up for each velocity. The highest peak torque elicited from three
MVCs was used as the criterion score for each angular velocity.
A continuous protocol was employed with a passive return to
the start angle following each MVC. One minute of rest was
Byrne et al.
Fig. 1. Angle–torque relationship of the knee extensors deter-
mined through isometric maximal voluntary contractions
(MVCs) at each joint angle. Torque values are means (∫SEM)
expressed as a percentage of maximum torque.
allowed between repetitions and each angular velocity. The or-
der of testing between isometric and dynamic torque was ran-
domized. However, the slower angular velocity was always
tested before the faster angular velocity during the dynamic
protocol as this has been shown to enhance reproducibility
(Wilhite, Cohen, Wilhite, 1992).
Creatine kinase activity. Plasma CK activity was determined
from a fingertip blood sample. A warm fingertip was cleaned
with a sterile alcohol swab and allowed to dry. Capillary punc-
ture was made with an autoclix lancette and a sample of whole
fresh blood (32 ml) was pipetted from a capillary tube onto the
test strip and analysed for CK activity via a colorometric assay
procedure (Reflotron, Boehringer Mannheim, Lewes, UK). This
system uses a plasma separation principle which is incorporated
in the reagent carrier on the test strip.
Isokinetic eccentric exercise protocol
Each subject performed a bout of 100 isokinetic eccentric
MVCs at an angular velocity of 1.57 rad ¡s
ª1
(90 deg ¡s
ª1
)
using the Kin-Com dynamometer. The eccentric actions were
performed as 10 sets of 10 repetitions with 10 s rest between
repetitions and 1 min between sets. Participants exercised in the
prone position through a range of motion from 40æto 140æ
(0æΩfull extension) of knee flexion. Each eccentric action was
followed by a passive return to the start angle. The testing posi-
tion and range of motion were selected to exercise the knee
extensors at long muscle length. Eccentric exercise performed
at long muscle lengths results in greater functional impairment
and evidence of muscle damage than eccentric exercise per-
formed at short muscle lengths (Newham, Jones, Ghosh, Aur-
ora, 1988; Child et al., 1998).
Statistical analysis
The strength data were analysed using a series of two-factor
analyses of variance. Isometric data were analysed by a two-
factor (6¿2; Measurement Time¿Angle) fully repeated meas-
ures analysis of variance (RM ANOVA). Isometric and dy-
namic strength were compared using a two-factor (6¿3; Meas-
urement Time¿Contraction Mode) fully RM ANOVA. Plasma
CK activity was analysed using a single factor fully RM ANO-
VA. The assumption of sphericity was tested by the Mauchly
test of sphericity. Any violations of this assumption were cor-
136
rected by using the Greenhouse-Geisser adjustment to raise the
critical value of F, as indicated by (GG). Statistical significance
was set at the 0.05 alpha level. Where appropriate, Tukey’s HSD
post-hoc tests were used to indicate where significant differences
lay.
Results
Angle–torque relationship
Peak isometric torque was generated in the region of
75–85æknee flexion. This is in agreement with pre-
vious results using the same muscle group (Newham,
McCarthy, Turner, 1991). The two angles selected for
torque measurement prior to and following the eccen-
tric exercise bout were 10æand 80æ, corresponding to
short and optimal muscle length, respectively. Iso-
metric peak torque at 10æwas approximately 30% of
isometric peak torque at 80æ(Fig. 1).
Isometric peak torque at 10æand 80æ
Absolute values for isometric strength were 386∫96
N and 1279∫165 N for short and optimal muscle
lengths, respectively. Isometric peak torque changed
significantly over time (F
5,35
Ω27.2, P⬍0.001) and be-
tween angle (F
1,7
Ω7.0, P⬍0.05). The interaction of
time by angle on isometric peak torque approached
significance (F
5,35
Ω2.2, PΩ0.073). The main effect
for angle indicated that following the eccentric exer-
cise protocol, the relative decline in isometric strength
was significantly greater at short versus optimal
muscle length. Isometric torque at short muscle
length was reduced to a mean of 76.3∫2.5% of pre-
exercise values compared to 82.1∫2.7% at optimal
muscle length. As there was no significant interaction
of time¿angle, we did not conduct post-hoc tests to
Fig. 2. Changes in isometric peak torque at short and optimal
muscle length across time following 100 eccentric MVCs of the
knee extensors. Torque values are means (∫SEM) expressed as
a percentage of the pre-eccentric exercise torque. *Significantly
different (P⬍0.05) from pre-exercise.
Isometric and dynamic muscle strength loss
Fig. 3. Changes in isometric peak torque at optimal muscle
length (80æ) and concentric peak torque at 0.52 rad ¡s
ª1
(slow)
and 3.14 rad ¡s
ª1
(fast) across time following 100 eccentric
MVCs of the knee extensors. Torque values are means (∫SEM)
expressed as a percentage of the pre-eccentric exercise torque.
*Significantly different (P⬍0.05) from pre-exercise.
determine differences between means at the various
time points following eccentric exercise. However, the
cell means demonstrate a clear trend showing that
isometric torque at short muscle length was reduced
to a greater extent throughout the seven-day testing
period (Fig. 2 and Table 1).
Isometric versus isokinetic peak torque
Absolute values for isometric and dynamic strength
were 1279∫165 N, 397∫62 Nm, and 224∫49 Nm for
isometric 80æ, concentric 0.52 rad ¡s
ª1
, and concen-
tric 3.14 rad ¡s
ª1
, respectively. There was a highly sig-
nificant main effect for time (F
5,35
Ω28.9, P⬍0.001)
on peak torque, but no statistical difference between
the peak torque for each contraction mode (F
2,14
Ω
2.1, P⬎0.05). The time by contraction mode interac-
tion on peak torque was also non-significant (F
10,70
Ω
1.6, P⬎0.05). Isometric MVC torque at optimal
muscle length and isokinetic MVC torque at 0.52 and
3.14 rad ¡s
ª1
were therefore affected to a similar ex-
Table 1. Changes in isometric peak torque across time at short and optimal muscle length and concentric peak torque at 0.52 rad¡s
ª1
and 3.14 rad
¡s
ª1
following 100 eccentric MVCs of the knee extensors. Torque values are means (∫SEM) expressed as a percentage of the pre-eccentric exercise
torque
Pre Post Day 1 Day 2 Day 4 Day 7 Mean
Isometric (10æ) 100 65.4∫3.5 72.1∫5.2 68.5∫3.6 72.1∫3.9 79.9∫3.1 76.3∫2.4*
Isometric (80æ) 100 70.3∫2.8 72.9∫3.3 78.1∫4.7 81.9∫5.1 89.6∫2.7 82.1∫2.7
Concentric (0.52 rad ¡s
ª1
) 100 76.8∫2.3 83.6∫3.8 82.5∫2.8 85.2∫3.8 91.7∫2.5 86.6∫2.0
Concentric (3.14 raD ¡s
ª1
) 100 70.3∫2.4 72.9∫3.5 84.4∫3.3 82.2∫2.6 90.4∫2.7 84.3∫1.5
*Significantly different (
P
⬍0.05) from isometric (80æ).
137
Fig. 4. Changes in plasma CK activity across time following 100
eccentric MVCs of the knee extensors. CK values are means
(∫SEM). *Significantly different (P⬍0.05) from pre-exercise.
tent following eccentric exercise-induced muscle dam-
age (Fig. 3 and Table 1).
Creatine kinase activity
Plasma CK activity changed significantly over time
(F
(GG)5,35
Ω18.3, P⬍0.001). Tukey’s post-hoc tests in-
dicated that CK activity was significantly higher than
baseline on days 4 (P⬍0.01) and 7 (P⬍0.01) after the
eccentric exercise (Fig. 4).
Discussion
The eccentric exercise protocol resulted in an immedi-
ate and prolonged reduction in muscle strength and
an increase in circulating levels of the myofibre pro-
tein creatine kinase (CK). Both of these measures are
commonly used as indicators of exercise-induced
muscle damage (Warren et al., 1999). Immediate post-
exercise values (expressed as a percentage of pre-exer-
cise values) ranged from 65.4% to 76.8% for all
methods of measurement and by day 7 these values
ranged from 79.9% to 91.7%. Creatine kinase activity
Byrne et al.
was significantly elevated above baseline at days 4
and 7 post-exercise, with the highest values being re-
corded at day 7 post-exercise. The delayed response in
CK activity is typical following high-force eccentric
exercise and has been shown to peak between 4 and
7 days post-exercise (Friden et al., 1983; Jones, Ne-
wham, Round, Tolfree, 1986; Clarkson et al., 1992).
It is possible that CK activity reached a peak during
this time in our study. Using a very similar eccentric
exercise protocol as the one used in this study, Child
et al. (1998) reported peak CK activity to occur at 5
days post-exercise. In our study plasma CK activity
had a poor temporal relationship with the functional
measures of muscle strength. Immediately and 24 h
post-eccentric exercise when muscle strength was
affected to the greatest extent, CK activity was not
significantly elevated above baseline. By day 7 when
muscle strength was returning to pre-exercise values,
CK activity was at its highest. For this reason, War-
ren et al. (1999) questioned the usefulness of myofibre
proteins as a criterion for measuring muscle damage.
Our results suggest that reductions in isometric
strength following eccentric exercise are dependent on
the muscle length at which they are measured. Iso-
metric strength was reduced to a significantly greater
extent at short versus optimal muscle length
(76.3∫2.5% vs. 82.1∫2.7%). Previous research using
the elbow flexors (Saxton & Donnelly, 1996) and the
knee extensors (Child et al., 1998) also observed a
length dependence of strength loss following eccentric
exercise. Saxton and Donnelly (1996) reported a
greater relative loss of strength at short versus long
muscle length which persisted for 4 days, whereas
Child et al. (1998) only observed this effect immedi-
ately post-exercise. Due to the non-significant interac-
tion of time by angle in our study we were not able
to determine the time course of the greater relative
strength loss at short muscle length. However, a clear
trend was evident, suggesting that the greater strength
loss persisted throughout the seven-day testing
period.
Greater strength loss at short versus optimal or
long muscle length indirectly supports the hypothesis
that the length–tension curve shifts to the right, to-
wards longer muscle lengths following eccentric exer-
cise. Indeed, shifts in the optimum of the length–ten-
sion curve have been reported following eccentric ex-
ercise of the human ankle extensors (Jones, Allen,
Talbot, Morgan, Proske, 1997; Whitehead, Allen,
Morgan, Proske, 1998) and whole toad sartorious
muscle (Wood, Morgan, Proske, 1993; Talbot & Mor-
gan, 1998). Morgan & Allen (1999) suggest that over-
stretched sarcomeres failing to produce active tension
and a reduction in sarcomere length of the remaining
functional sarcomeres are the factors responsible for
changes in the length–tension curve. Over-stretched
sarcomeres would mean a reduction in the number of
138
cross-bridges available for force generation and this
could explain the strength loss associated with exer-
cise-induced muscle damage. A reduction in sarco-
mere length would account for the greatest loss of
strength at short muscle lengths.
There are several consequences of the remaining
functional sarcomeres becoming shorter at a fixed
muscle length. The activation curve of muscle shifts
to higher calcium levels at short sarcomere lengths
(Endo, 1973), very high stimulation rates are required
to achieve maximum force at short sarcomere lengths
(Rack & Westbury, 1969), and the force frequency re-
lationship is shifted to the right at short muscle (sar-
comere) lengths (Edwards, Gibson, Gregson, 1989a;
Sacco, McIntyre, Jones, 1994). The impact of these
factors on muscle function will be dependent on the
muscle length at which force is measured. For ex-
ample, strength will be reduced at all muscle lengths
but the greatest loss of strength will occur at short
muscle lengths. Low frequency fatigue (LFF), a selec-
tive loss of force at low stimulation frequencies and
common feature of damaged muscle, will be evident
at all lengths but again will be greater at short muscle
lengths (Edwards, Gibson, Gregson, 1989b). A dis-
turbance in excitation–contraction coupling so that
less calcium is released per action potential is often
cited as the mechanism underlying strength loss and
LFF (Edwards, Hill, Jones, Merton, 1977; Jones,
1981; Newham et al., 1983; Warren et al., 1993; In-
galls et al., 1998). The combination of reduced cal-
cium release and a redistribution of sarcomere
lengths could certainly account for the length-de-
pendent effect of strength loss associated with eccen-
tric exercise-induced muscle damage.
Our results suggest that isometric strength at opti-
mal muscle length and concentric strength at slow
and fast angular velocities were affected to a similar
extent in terms of magnitude and rate of recovery
following exercise-induced muscle damage (isometric
82.1π2.7% vs. slow 86.6π2.0% vs. 84.3π1.5%).
There have been reports suggesting that strength loss
and recovery may be dependent on the type and/or
angular velocity of muscle action (Friden et al., 1983;
Golden & Dudley, 1992; Gibala et al., 1995; Eston
et al., 1996). Friden et al. (1983) reported a slower
restoration of concentric strength at 5.23 rad ¡s
ª1
compared to either 3.14, 1.57 rad ¡s
ª1
or isometric
strength, and Golden & Dudley (1992) reported a
slower recovery of concentric strength at 3.14 versus
1.05 rad ¡s
ª1
. For eccentric strength, Eston et al.
(1996) reported a slower restoration of strength at
2.83 versus 0.52 rad ¡s
ª1
. Evidence suggesting that
fast twitch muscle fibres are selectively damaged dur-
ing eccentric exercise (Friden et al., 1983; Jones et al.,
1986; Lieber & Friden, 1988; McHugh et al., 2000)
may account for the slower strength recovery at the
higher angular velocities in these studies.
Isometric and dynamic muscle strength loss
A potential mediating factor when assessing dy-
namic strength at different angular velocities is the
joint angle (muscle length) at which strength is meas-
ured. The joint angle at peak torque is dependent on
the angular velocity of movement, occurring later in
the range of movement as the angular velocity in-
creases (Thorstensson, Grimby, Karlsson, 1976). For
the knee extensors this means that peak torque oc-
curs at a shorter muscle length the higher the angular
velocity. Thus, analysis of peak torque data irrespec-
tive of angular position may lead to erroneous con-
clusions about muscle function due to the length-de-
pendent nature of strength loss following eccentric
exercise. Future studies could make a more meaning-
ful comparison of dynamic muscle function across
angular velocities by measuring peak torque at a pre-
determined joint angle, thereby controlling for muscle
length.
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139
Perspective
Our findings support previous indirect (Saxton &
Donnelly, 1996; Child et al., 1998) and direct (Jones
et al., 1997; Whitehead et al., 1998) evidence of a shift
in the angle–torque relationship towards longer
muscle lengths following eccentric exercise. Isometric
and dynamic strength appeared to be affected to a
similar extent in terms of magnitude and rate of re-
covery following eccentric exercise. Future studies in-
vestigating dynamic muscle function in a more eco-
logical manner may give a better insight into the ef-
fects of exercise-induced muscle damage on
functional human performance.
Key words: eccentric exercise; damage; strength; iso-
metric; isokinetic; muscle length.
Byrne et al.
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