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Jour nal of Sports Sciences
, 2000 ,
18
, 163± 172
Jour nal of Sports Sciences
ISSN 0264-041 4 print/ISSN 1466-447 X online Ó 2000 Taylor & Francis Ltd
http://www.tandf.co.uk/journals/tf/00268976 .html
Electromyographic analysis of exercise resulting in
symptoms of muscle damage
M ALACHY P. M cH UGH,
1,3
* DECLAN A.J. CO NNOLLY ,
2
ROG ER G. ESTO N
1
and
GILBERT W. G LEIM
3
1
School of Sport, H ealth and Physical Education Sciences, University of Wales, B a ngor, G wynedd , UK ,
2
D epartm ent of
Physical Education, U niversity of Ver m ont, B ur lington, Ver m ont, USA and
3
N icholas Institute of Sports M edicine and
A thletic Traum a, Lenox H ill H ospital, N ew York, USA
Accepted 1 November 1999
Surface electromyographic (EM G) signals were recorded from the hamstring muscles during six sets of sub-
maximal isokinetic (2.6 rad ´ s
- 1
) eccentric (11 m en, 9 women) or concentric (6 men, 4 women) contractions.
The EM G per unit torque increased during eccentric (P < 0.01) but not during concentric exercise. Similarly,
the median frequency increased during eccentric (P < 0.01) but not during concentric exercise. The E MG p er
unit torque was lower for submaximal eccentric than maxim um isometric contractions (P < 0.001), and lower
for submaximal concentric than maxim um isometric contractions (P < 0.01). The EMG per unit torque was
lower for eccentric than concentric contractions (P < 0.05). The m edian frequency was higher for submaximal
eccentric than maxim um isom etric contractions (P < 0.001); it was similar, however, between subm aximal
concentric and m aximum isometric contractions (P
=
0.07). Eccentric exercise resulted in signi® cant isometric
strength loss (P < 0.01), pain (P < 0.01) and m uscle tender ness (P < 0.05). The greatest strength loss was seen
1 day after eccentric exercise, while the most severe pain and muscle tenderness occurred 2 days after eccentric
exercise. A lower EMG per unit torque is consistent with the selective recruitment of a sm all number of motor
units during eccentric exercise. A higher median frequency during eccentric contractions may be explained by
selective recruitment of fast-twitch motor units. The present results are consistent with the theory that muscle
damage results from excessive stress on a small number of active ® bres during eccentric contractions.
K eyw ords: concentric, eccentric, hamstrings, isometric, median frequenc y.
Introduction
Experim ents using electrically stim ulated contractions
in animals (Lieber and Frid
Š
n, 1991; MacPherson
et al.
,
1996) or voluntary exercise in h umans (Frid
Š
n
et al.
,
1983; Frid
Š
n, 1984; Clarkson
et al.
, 1992) have con-
sistently dem onstrated that untrained skeletal muscle
is susceptible to dam age by eccentric contractions.
The speci® c neural characteristics of eccentric con-
tractions may be involved in the initiation of damage.
For exam ple, results from various muscle groups have
consistently shown m otor unit activation to be 35± 60%
less for eccentric than concentric contractions at the
sam e force (Bigland and Lippold, 1954; Komi
et al.
,
1987; Tesch
et al.
, 1990; Adam s
et al.
, 1992; Potvin,
* Address all correspondence to Malachy P. M cH ugh, N ISMAT,
Lenox H ill Hospital, 130 East 77th Street, N ew York, NY 10021 ,
USA. e-mail: mchugh@nismat.org
1997). Accordingly, m uscle dam age has been attributed
to excessive stress on the small number of active ® bres
during eccentric contractions (Arm strong
et al.
, 1983;
M oritani
et al.
, 1988). However, the extent of m otor
unit activation and the type of motor units recruited
have not been determ ined for exercise resulting in
muscle damage.
It has been suggested that there is a reversal of normal
motor unit recruitment order during eccentric con-
tractions (Enoka, 1996). N ardone and Schieppati
(1988) have dem onstrated de-recruitment of the soleus
(primarily slow -twitch) and facilitation of the gastroc-
nemius (prim arily fast-twitch) for the eccentric phase
of reciprocal submaximal concentric± eccentric con-
tractions of the plantar ¯ exors. Nardone
et al.
(1989)
con® rmed this and also demonstrated that some m otor
units, which had been silent during both the concentric
phase and ram p isometric contractions, were activated
during the eccentric phase. T he am plitudes of the
164
M cHugh
et al.
action potentials from these units were consistent with
fast-twitch motor units. Similar results were reported by
Howell
et al.
(1995) for the ® rst dorsal interosseous
muscle.
The frequency content of electromyographic (EMG )
signals from surface electrodes can provide an indirect
measure of m otor unit recruitm ent with increasing
intensity of contractions (M oritani and M uro, 1987).
High er frequencies are thought to indicate recruit-
ment of fast-twitch m otor units. H owever, several
studies (M oritani
et al.
, 1988; N akazawa
et al.
, 1993;
Potvin, 1997) have failed to dem onstrate higher surface
EM G frequencies for eccentric contractions of the
elbow ¯ exors. The low contraction intensity (< 30% of
maximum voluntary contraction; M V C) used in these
studies (M oritani
et al.
, 1988; N akazawa
et al.
, 1993;
Potvin, 1997) was probably not suý cient to cause
muscle damage. T he EM G frequency m easures may
diþ er between eccentric and concentric contractions
in other m uscle groups and at higher contraction
intensities.
The aim of this study was to compare surface EM G
activity from the hamstring m uscle group between a
bout of eccentric exercise of suý cient intensity to result
in sym ptoms of m uscle dam age and a bout of concentric
exercise performed at the same relative intensity. We
hypothesized that the EM G per unit torque would be
lower, and the median frequency would be higher, for
eccentric than concentric contractions.
M ethods
Experimental design
Thirty participants (17 men, 13 wom en) were recruited
to the study after providing inform ed consent; the study
was app roved by the institutional review board. None of
the participants had an orthopaedic injur y or had been
involved in any weight training in the preceding months.
The participants fo rmed two groups: an experimental
group (11 m en, 9 wom en) that performed eccentric
exercise and a control group (6 m en, 4 women) that
performed concentric exercise. T he participants were
asked to refrain from other exercise and not to take
any m edication to alleviate pain during the course of
the study. The participants’ physical characteristics are
shown in Table 1.
The response of the hamstring muscle group to sub-
maximal isokinetic exercise was studied. T he ham string
muscle group was chosen because its three major
muscles (biceps fem oris long head, sem itendinosus and
semim embranosus) are biarticular. Signi® cant dif-
ferences in the frequency content of the surface EM G
signal have been demonstrated between uniarticular
and biarticular quadriceps muscles (Gerdle
et al.
, 1991).
By studying the hamstring muscles, we avoided the
potential confounding eþ ect of recruitment diþ erences
between uniarticular and biarticular muscles.
On the day before isokinetic exercise (day 0), base-
line measures of isom etric strength, pain and muscle
tenderness were performed. Immediately after the iso-
kinetic exercise, isometric strength was re-rested. Then,
1, 2 and 3 days after the isokinetic exercise, isom etric
strength, pain and m uscle tenderness were re-m easured.
The participants performed six sets of 10 subm axim al
isokinetic contractions (B iodex System 2, Shirley, N Y)
at 2.6 rad ´ s
-
1
. We chose isokinetic exercise to control
contraction speed between participants. T he partici-
pants were asked to choose w hich lim b to exercise. The
intensity of contraction was set at 60% of m axim um
isometric strength to control for relative work per-
form ed w ithin and between groups (eccentric
vs
con-
centric). Each set of 10 contractions was separated by
1 m in.
Isometric strength tests and isokinetic exercise
For the measurem ent of isometric strength, the parti-
cipants were seated in an upright position w ith the
trunk at approxim ately 1.6 rad of ¯ exion. The thigh of
the test lim b was strapped to prevent hip ¯ exion during
testing. The knee joint was aligned with the axis of
Table 1. Physical characteristics of the participants in the eccentric and
concentric groups (mean ± s)
Age (years) Heigh t (cm) Body mass (kg)
M en
Eccentric (n
=
11)
Concentric (n
=
6)
29 ± 6
33 ± 7
181 ± 7
178 ± 7
80.0 ± 9.5
90.9 ± 23.3
Wom en
Eccentric (n
=
9)
Concentric (n
=
4)
28 ± 6
31 ± 9
165 ± 16
162 ± 9
63.7 ± 12.9
56.7 ± 8.2
Electromyog raphy and muscle damage
165
rotation of the dynamometer and the leg was secured to
the dynam ometer arm at the ankle. T he knee joint was
set at 0.8 rad of ¯ exion and limb m ass was recorded.
The participants were then instructed to m axim ally
contract the knee ¯ exors; visual feedback and consistent
verb al encouragement were provided to ensure m axim al
eþ ort (Baltzopoulos
et al.
, 1991). Four 5 s contractions
were performed with 10 s between eþ orts. Peak torque
and total area under the torque± time cur ve were
recorded after removing the contribution of lim b mass.
For the isokinetic exercise, the participants remained
seated. Eccentric contractions were perform ed from
1.6 to 0 rad of knee ¯ exion and concentric contractions
were performed from 0 to 1.6 rad (0 rad
=
full exten-
sion). T he dynam ometer arm was set to move through
the selected range of m otion at 2.6 rad ´ s
-
1
and the
participants contracted with suý cient intensity to reach
a visually displayed target. The area under the torque±
tim e curves was recorded for each set (the contribution
of limb m ass was removed).
EMG measu rements
During the isometric strength tests and isokinetic
exercise, E M G signals were recorded from surface
electrodes placed over the biceps fem oris, sem itend-
inosus and semimemb ranosus m uscles. T he sk in was
shaved, cleaned and ab raded before application of
10 m m diam eter Ag/AgC l electrodes on a 34
´
22 m m
adhesive gel surface. For the biceps femoris, a pair of
electrodes was placed 3 cm apart, midway along a line
between the ischial tuberosity and the ® bular head.
For the semitendinosus, electrodes were placed m idway
along a line between the ischial tuberosity and the
medial fem oral condyle. For the semimembranosus,
electrodes were p laced at the apex of the inver ted `V’
fo rmed by the distal portions of the biceps fem oris and
semitendinosus muscles. A ground electrode was placed
on the patella.
The EM G signal was recorded by telemetr y, band-
pass ® ltered from 12 to 500 H z and sampled at a rate of
1000 Hz with a common-m ode rejection ratio of 135 dB
(Noraxon, Scottsdale, AZ). For analysis of the amplitude
of the E M G activity, the raw EM G signal was full-wave
recti® ed and integrated (iEM G ). T he EMG signal
am plitude was expressed relative to torque (
m
V/N ´ m )
and was com puted by dividing iEM G by the area
under the torque± time curve for the corresp onding
tim e interval. D iþ erences in EM G am plitude between
types of contractions (isometric, isokinetic eccentric
and isokinetic concentric) were also analysed relative to
torque. The EMG per unit torque was used because the
main aim of m easuring EM G am plitude was to quantify
diþ erences in activation relative to force between types
of contractions. Changes in EM G activity (iE M G and
median frequency) during the m axim um isometric
strength tests (immediately after and 1, 2 and 3 days
after exercise) were determined as the percent change
from baseline. For these analyses, iEM G was not
expressed relative to torque, since our intent was to
determine if the ability to m axim ally activate the ham-
strings was aþ ected by sym ptoms of muscle dam age.
Fast Fourier transforms (F FT) were applied to the
raw EMG signals from which the median frequency was
com puted. A ll computations were performed using
the software supplied by the m anufacturer (Noraxon,
Scottsdale, AZ). At 2.6 rad ´s
-
1
, it should have taken 600
ms for the limb to move through 1.6 rad. However,
the acceleration and deceleration phases meant that this
motion took approximately 900 ms to complete. The
number of data points analysed in an FFT m ust be a
power of two (e.g. 16, 32, 64, . . . 512, etc.) (Kam en and
Caldwell, 1996). The length of the FFT was set at 4096
data points, which, at a sam pling rate of 1000 H z,
covered 4096 ms. We chose this length of FFT so as
to include two consecutive contractions in each calcula-
tion. Although this length of FF T entered low power
noise into the frequency calculations, it avoided the
problem of identifying when a contraction began and
ended. The duration of contraction varied during the
isokinetic contractions as the participants reacted to
the m ovement of the dynam ometer (Fig. 1). Further-
more, during the contractions m uscle length changed
rapidly. T he eþ ect of m uscle length on m edian fre-
quency (M orimoto, 1986; Potvin, 1997) necessitated
an analysis of all contractile activity to avoid analysing
activity at diþ erent lengths between contractions.
The reliability of the two criterion m easurements,
EM G per unit torque and m edian frequenc y, was
assessed by asking the participants in the concentric
group to repeat the bout of concentric exercise 2 weeks
later. Reliability was not assessed in the eccentric group,
since a repeated bout results in reduced sym ptoms
and this eþ ect may be explained by a change in neural
control of eccentric exercise (M cHugh
et al.
, 1999).
Pain and muscle tenderness m easurem ents
Before and 1, 2 and 3 days after isokinetic exercise,
the participants were asked to report their pain. Th ey
were speci® cally asked to report a single score fo r
hamstring pain elicited with activities of daily living,
such as walking , stepping and squatting. Pain ratings
were recorded on a scale of 0
=
`no discomfort’ to 10
=
`walking with a limp’ .
M uscle tenderness was evaluated by pressing a probe
of 18 mm diameter, attached to a load cell, into the
respective m uscles at the sites identi® ed fo r electrode
placem ent. T he participants were asked to report dis-
com fort elicited by application of the probe. The signal
166
M cHugh
et al.
from the load cell was recorded during each trial and
was interrupted at the instant of discom fort. The force
at that instant was com puted and subtracted from 40 N
to give a tenderness index (Newham
et al.
, 1983).
Any discom for t elicited at forces above 40 N was not
included. The values from each muscle were sum m ed
for analysis.
Statistics
A mixed-m odel analysis of variance (AN OVA) was used
for combined within- and between-individuals analyses,
while repeat-measures ANOVA was used for analyses
performed independently on a group. G roup (eccentric
vs
concentric) was the only between-individuals factor
used in the analyses. For the analyses pertaining to
isokinetic exercise (day 0), the within-individual inde-
pendent variables were set (1± 6) and m uscle (biceps
femoris, sem im em branosus and semitendinosus). For
the analyses pertaining to m easurements made over
subsequent days, the within-individual independent
variable was time (before, im m ediately after and 1, 2
and 3 days after exercise). G reenhouse-Geisser correc-
tions were applied to signi® cant analyses of variance
that did not m eet M auchly’ s sphericity assumption;
probabilities that have been corrected are denoted b y
the subscript
G G
. A ll
post-hoc
pairwise com parisons were
performed after Bonferroni correction. The m ean
±
standard error of the m ean is displayed in Fig. 3, and the
mean
±
standard deviation is reported in the text and
tab les. Linear regression analysis was used to assess the
relative reliability of EM G per unit torque and m edian
frequency, w hile B land and A ltman’ s 95% limits of
agreement were used to assess the absolute reliability
of these m easures (Atkinson and Nevill, 1998).
Results
Torque production
Eccentric torque production was 66.4
±
18.3 N ´ m
(64.3
±
4.8% of maximum isometric strength), which
occurred at 0.6
±
0.33 rad of knee ¯ exion. Torque was
consistent across the six sets of eccentric contractions
(Table 2; set eþ ect
P
=
0.22, coeý cient of variation
=
9.9
±
7.4%). Concentric torque production was
68.0
±
25.0 N ´ m (58.3
±
2.0% of m axim um isom etric
strength), which occurred at 0.49
±
0.26 rad. Torque
was consistent across the six sets of concentric contrac-
tions (Table 2; set eþ ect
P
=
0.45, coeý cient of
variation
=
8.4
±
12.3%). The coeý cient of variation
of the area under the torque curve fo r the six sets
was 13.1
±
7.5% for the eccentric group and 10.7
±
5.8% for the concentric group.
EM G activity
Typical full-wave recti® ed EM G recordings for a repre-
sentative p articipant in the eccentric group (Fig. 1a)
and concentric g roup (Fig. 1b) are shown for m axim um
isometric and subm axim al isokinetic contractions.
The EMG frequency sp ectrums from the raw EMG
signals of selected contractions are displayed in Fig. 2a
(eccentric) and Fig. 2b (concentric).
The EM G per unit torque increased from 3.3
±
2.1
m
V/N ´m (set 1) to 4.3
±
2.7
m
V/N ´ m (set 6) during
eccentric exercise (
P
< 0.01
GG
), but did not change
during concentric exercise (
P
=
0.34; Fig. 3). The EM G
per unit torque was lower for subm aximal eccentric
than maximum isometric contractions (
P
< 0.001),
with similar eþ ects between m uscles (Fig. 3). The
EM G per unit torque for concentric contractions was
also lower than for isometric contractions (
P
< 0.01).
Although EM G per unit torque was lower for eccentric
than concentric exercise (
P
< 0.05), the diþ erence
decreased from set 1 to set 6 (group
´
set
P
< 0.05
G G
;
Fig. 3).
M edian frequency increased from set 1 to set 6
during eccentric (
P
< 0.001) but not during concentric
exercise (
P
=
0.59; Table 3). The median frequency
was higher for subm aximal eccentric than m axim um
isometric contractions (
P
< 0.001). In contrast, it
was sim ilar for submaximal concentric and m axim um
isometric contractions (
P
=
0.07). T he m edian fre-
quency was higher for eccentric than concentric exercise
(
P
< 0.01), w ith the diþ erence increasing from set 1
to set 6 (group
´
set
P
< 0.001; Table 3). The EMG
per unit torque and median frequency during m axi-
mum isom etric contractions before and im m ediately
after isokinetic exercise were not diþ erent between
groups.
Table 2. Knee ¯ exion torque during six sets of eccentric and
concentric contractions (mean ± s)
a
Average torque (N ´ m)
Eccentric group
(n
=
20)
Concentric g roup
(n
=
10)
Set 1
Set 2
Set 3
Set 4
Set 5
Set 6
52.1 ± 13.2
48.7 ± 15.1
48.5 ± 14.4
48.7 ± 14.8
48.3 ± 13.6
47.3 ± 14.1
48.1 ± 17.2
50.8 ± 17.8
50.7 ± 18.7
50.6 ± 19.2
50.5 ± 19.5
46.7 ± 24.0
a
The coeý cient of variation of m ean torque between sets was
9.9 ± 7.4% for eccentric contractions and 8.4 ± 12.3% for concentric
contractions.
Electromyog raphy and muscle damage
167
Fig. 1. (A) Full-wave recti® ed EM G signals for one participant during four maximum isometric contractions, followed by six
sets of 10 eccentr ic isokinetic contractions at 60% of isom etric strength (sets 1 and 6 are shown). Note the increase in magnitude of
the EMG signal from set 1 to set 6. (B) Full-wave recti® ed EM G signals for one participant during four maxim um isom etric
contractions, followed by six sets of 10 concent ric isokinetic contractions at 60% of isometric strength (sets 1 and 6 are shown).
Note there is no increase in magnitude of the EMG signal from set 1 to set 6.
Reliability of EM G measurements
The EMG per unit torque averaged over the six sets
of concentric contractions was 5.8
±
2.3
m
V/N ´ m . For
the repeated bout of concentric contractions 2 weeks
later, E M G per unit torque was 5.4
±
2.3
m
V/N ´ m .
The mean diþ erence between the two bouts was 0.34
m
V/N ´ m , with 95% lim its of agreement of 0.21
m
V/
N ´ m . The EMG per unit torque was well correlated
between bouts (
r
=
0.90,
P
< 0.01), with a slope close to
168
M cHugh
et al.
Fig. 2. (A) Power spectrum of the raw EM G signal for one participant during selected maximum isometric contractions and
submaximal eccentr ic isokinetic contractions. The y-axes are voltage squared (V
2
). N ote how the spectrum is wider for submaximal
eccentric than m aximum isometric contractions. Additionally, the spectrum broadens to the right from set 1 to set 6. (B) Power
spectrum of the raw EMG signal for one participant during selected maximum isometric contractions and submaximal concent ric
isokinetic contractions. Note how the spectrum is similar for the subm aximal concentric and m aximum isometric contractions.
MF
=
median frequency.
one (0.91
±
0.51) and an intercept close to zero (0.18
±
3.1). A correlation coeý cient of 1.0 with a slope of one
and an intercept of zero represents perfect ag reem ent.
M edian frequency averaged across the three muscles
and over the six sets of concentric contractions was
61.9
±
9.4 Hz. For the repeated bout of concentric
Electromyog raphy and muscle damage
169
Fig. 3. The EMG per unit torque for maxim um isometric contractions and submaximal eccentric contractions (left) and
concentric contractions (right). See text for statistics.
Table 3. M edian frequency for maximum isometric contractions, submaximal eccentric contractions and submaxim al
concentric contractions (mean ± s)
Maximum isometric Subm aximal isokinetic contractions Maxim um isom etric
Pre (Hz) Set 1 (Hz) Set 6 (Hz) Post (H z)
Eccentric (n
=
20) BF
SM
ST
65.5 ± 15.5
59.7 ± 8.9
60.1 ± 8.5
72.1 ± 16.0
73.8 ± 13.0
68.4 ± 9.7
81.2 ± 15.0
80.7 ± 12.8
70.9 ± 8.2
70.1 ± 15.3
65.4 ± 10.1
66.4 ± 7.5
Concentric (n
=
10) BF
SM
ST
65.5 ± 9.0
62.0 ± 11.4
62.8 ± 14.5
64.8 ± 7.8
62.6 ± 12.3
59.7 ± 10.5
62.4 ± 11.3
63.2 ± 17.0
58.9 ± 11.3
67.1 ± 9.4
72.3 ± 15.3
69.7 ± 16.0
Abbreviations
: BF
=
biceps fem oris, SM
=
semimembranosus, ST
=
semitendinosus.
Table 4. Strength loss, pain and muscle tenderness after eccentric and concentric exercise (mean ± s)
a
Isom etric strength
(% of baseline)
Pain (0± 10
arbitrary units)
Muscle tenderness
(tenderness index)
Eccentric Concentric Eccentric C oncentric Eccentric Concentric
Baseline
Post
Day 1
Day 2
Day 3
100
105 ± 3
89 ± 3*
92 ± 4
93 ± 4
100
106 ± 3
107 ± 6
103 ± 5
102 ± 4
0
-
2.2 ± 0.6* *
3.5 ± 0.6* *
2.6 ± 0.7* *
0
-
0.1 ± 0.1
0.3 ± 0.2
0.1 ± 0.1
0
-
1.9 ± 1.7
6.4 ± 1.7*
4.7 ± 2.3
0
-
0
0
0
a
Baseline isometric torque was 104 ± 31 N ´ m in the eccentric group and 117 ± 31 N ´m in the concentric group. For m uscle tenderness, higher
numbers represent more tenderness (see M ethods for com putation of the tenderness index). Pain and tenderness were not assessed im mediately
post-exercise. Signi® cantly diþ erent from baseline: *
P
< 0.05, * *
P
< 0.01.
contractions 2 weeks later, the m edian frequency was
59.4
±
11.5 H z. T he m ean diþ erence between the two
bouts was 2.6 Hz, with 95% lim its of agreement of 14.4
H z. The correlation of m edian frequency between bouts
was
r
=
0.77 (
P
< 0.01), with a slope close to one (0.95
±
0.88) and an intercept close to zero (0.36
±
54.7).
Sym ptom s of muscle dam age
Isom etric strength was decreased on the days after
eccentric exercise (
P
< 0.001
G G
; Table 4), with no
change following concentric exercise (
P
=
0.45). The
participants experienced signi® cant pain after eccentric
170
M cHugh
et al.
exercise (
P
< 0.001
G G
; Table 4) but no pain after con-
centric exercise (
P
=
0.16). T hey also experienced sig-
ni® cant muscle tenderness after eccentric exercise
(
P
< 0.05
G G
; Table 4), but no tenderness was experi-
enced after concentric exercise.
The iEM G values during isom etric testing im m edi-
ately after and 1, 2 and 3 days after isokinetic exercise
were sim ilar to those recorded pre-exercise (eccentric
group,
P
=
0.72; concentric group,
P
=
0.58). Median
frequency was increased during the isom etric tests
im m ediately after isokinetic exercise in both groups
(eccentric,
P
< 0.01; concentric,
P
< 0.05), but was not
diþ erent from baseline on days 1, 2 or 3 post-exercise in
either group.
Discussion
EM G activity
The EM G per unit torque fo r subm aximal eccentric
contractions was 41± 51% (sets 1± 6) of m axim um iso-
metric contractions. In contrast, EM G per unit torque
for submaxim al concentric contractions was 90± 80%
(sets 1± 6) of m axim um contractions. In a direct com-
parison of groups, EM G per unit torque for eccentric
contractions was initially (set 1) 53% of concentric,
increasing to 75% by set 6. These initial between-group
values are very similar to publish ed within-group com -
parisons for the plantar ¯ exors (45± 60%; Bigland and
Lippold, 1954), knee extensors (35± 50%; Kom i
et al.
,
1987; Tesch
et al.
, 1990) and elbow ¯ exors (44± 58%;
Adams
et al.
, 1992; Potvin, 1997).
Despite reduced motor unit activation, the median
frequency was 15% higher for subm axim al eccentric
than m axim um isom etric contractions, whereas it was
sim ilar between concentric and isometric contractions.
The m edian frequency for eccentric contractions was
15± 26% higher than for concentric contractions,
although it was similar between groups during m axi-
mum isometric contractions. These results contrast
with previous studies that found no diþ erence (M oritani
et al.
, 1988; Potvin, 1997) or a lower m edian frequency
(Nakazaw a
et al.
, 1993) during eccentric than during
concentric contractions of the elbow ¯ exors. It is
possible that the ham strings behave diþ erently from the
elbow ¯ exors during eccentric exercise. Alternatively,
the con¯ icting ® ndings might be exp lained b y dif-
ferences in contraction intensity (
£
30% M VC
vs
60% in
the present study). With low -intensity contractions, it
is diý cult to distinguish selective recruitment patterns
using surface EM G analysis. Firing rates probably aþ ect
the frequency content more at low contraction inten-
sities than at high intensities (Fuglsang-F rederiksen and
Rù nager, 1988; H
„
gg , 1992; K am en and Caldwell,
1996). H igh- and low-threshold motor units have
sim ilar ® ring rates at a g iven contraction intensity
(DeLuca
et al.
, 1982; Nardone
et al.
, 1989; Erim
et al.
,
1996). At higher contraction intensities, ® ring rate
should have less of an impact on frequency analysis
(Solom onow
et al.
, 1990).
An increase in m edian frequency with increasing con-
traction intensity is thought to indicate recruitm ent of
higher threshold m otor units rather than an increase in
® ring frequency of previously active units (Solom onow
et al.
, 1990; H
„
gg, 1992). In the present study, com -
paratively higher initial m edian frequency and lower
EM G per unit torque for submaxim al eccentric contrac-
tions is consistent with selective recruitm ent of a sm all
num ber of fast-tw itch m otor units. W ith repeated
eccentric contractions, concom itant increases in EMG
per unit torque and m edian frequenc y m ay indicate
additional recruitm ent of fast-tw itch m otor units.
H owever, biological and technical factors related
to surface EM G m easurem ents may provide alternative
explanations for the observed median frequency eþ ects.
For example, eccentric exercise is known to increase
muscle tem perature more than concentric exercise
(Nadel
et al.
, 1972). Such an eþ ect could explain the
increase in m edian frequency during eccentric exercise,
but would not explain the diþ erences in m edian
frequency between types of contractions at baseline.
The baseline diþ erence in median frequency between
contraction types m ay be explained by m otor unit
synchronization. D uring voluntary contractions, m otor
unit action potentials tend to synchronize with repeated
contractions (H
„
gg, 1992). Since fewer m otor units are
activated during eccentric contractions, the recruitm ent
of units may becom e increasingly random , causing a
wider frequency spectrum.
Technical factors that can aþ ect the m edian frequenc y
include electrode orientation, electrode spacing, move-
ment ar tefact and m uscle length. T hat the participants
sat on the electrodes during ham string contractions
may have changed the orientation of the electrodes to
the motor point of the respective muscles. For exam ple,
median frequency would increase if the electrodes
moved closer together or becam e oriented m ore in line
with the m uscle ® bres (K amen and C aldwell, 1996).
However, it is diý cult to explain why such a change in
electrode orientation would have occurred with eccen-
tric contractions but not with concentric contractions,
given that all other conditions were the sam e. M ove-
ment ar tefact and ambient noise during the contraction
and relaxation p hases of isokinetic movements m ay h ave
had an eþ ect on the EM G measurem ents. H owever,
sim ilar noise would be expected during eccentric and
concentric contractions. M inimal baseline noise was
evident, despite the high m ovem ent speed and having
the participants sit on the electrodes (Fig. 1). It is
Electromyog raphy and muscle damage
171
im portant to note that Fig. 1 displays full-wave recti® ca-
tions of the raw EM G signal without any smoothing
fu nctions. Repeated m easures in the concentric g roup
indicated good reliability for the m easurement of
EM G per unit torque, but the m easurement of median
frequency was less reliable. However, the m easurement
error for m edian frequency clearly did not obscure the
marked diþ erences between types of contractions.
It is possible that the observed diþ erence in m edian
frequency between contraction types was a result of
sam pling EM G activity at diþ erent m uscle lengths
(M orimoto, 1986). However, m edian frequency was
calculated from the EM G activity for the whole con-
traction, through the full range of motion, for both
eccentric and concentric contractions. Assuming a
similar electrom echanical delay between eccentric and
concentric contractions, diþ erences in angle of peak
torque represent diþ erences in angle at peak EMG
activity. The angle of peak torque was similar between
eccentric and concentric contractions (0.60
±
0.33
vs
0.49
±
0.26 rad), while isom etric contractions were per-
fo rmed at 0.8 rad. Based on results for the elbow ¯ exors,
diþ erences in range of m otion of greater than 0.35 rad
may be required to identify signi® cant diþ erences in
frequency m easures (Potvin, 1997). It is unlikely that the
observed m edian frequency eþ ects in the present study
were caused by diþ erences in muscle length.
Sym ptom s of muscle dam age
Strength loss (peak day 1), pain (peak day 2) and muscle
tenderness (peak day 2) were clear symptom s of m uscle
dam age after eccentric exercise. Conversely, concentric
exercise did not aþ ect strength, pain or m uscle tender-
ness. In general, the symptoms of muscle dam age were
less than expected, possibly because of the relatively low
eccentric contraction intensity (60% isometric M VC).
H owever, Teague and Schwane (1995) reported more
severe symptom s after one set of 10 eccentric isotonic
contractions of the elbow ¯ exors at 60% of isometric
strength.
Despite a signi® cant reduction in strength, EM G
activity (iEM G and median frequency) did not change
during the isometric strength tests on days 1, 2 or 3,
indicating that normal motor unit activation was m ain-
tained despite symptom s of m uscle dam age. T his is in
line with the ® ndings of Saxton and Donnelly (1996),
who dem onstrated that strength loss after eccentric
exercise was unaþ ected by superimposing supra-
maximal stimulation during m axim um voluntary
isom etric contractions. However, the results contrast
with those of Day
et al.
(1998), who demonstrated
a reduced m edian frequency during isometric knee
extension contractions on the days after eccentric
quadriceps exercise.
Several studies have indicated that fast-twitch ® bres
are m ore susceptible to damage than slow -twitch
® bres during eccentric exercise (Frid
Š
n
et al.
, 1983;
Frid
Š
n, 1984; L ieber and Frid
Š
n, 1991; M acPherson
et al.
, 1996). H owever, others have suggested that
slow-twitch ® bres are m ore susceptible (Armstrong
et al.
, 1983; M air
et al.
, 1992). It is possible that the
greater dam age to fast-twitch ® bres is caused by prefer-
ential recruitment of fast-twitch motor units. H ow-
ever, for this to be con® rmed, we would require
docum entation of selective damage to fast-twitch ® bres.
Conclusions
In the present study, surface EM G analysis indicated
diþ erences in m otor unit activity between eccentric
and concentric contractions. Eccentric contractions
were characterized by a lower EM G per unit torque
and a higher median frequency than isom etric and
concentric contractions. Our results are consistent w ith
reduced m otor unit activation and possibly selective
recruitment of fast-twitch m otor units for eccentric
exercise. Furtherm ore, increasing EMG per unit torque
and m edian frequency during eccentric exercise indi-
cated additional recruitment of fast-twitch m otor
units as eccentric exercise continued. Eccentric exercise
resulted in m oderate symptoms of m uscle dam age that
were not seen after concentric exercise. T he results are
consistent with the hypothesis that m uscle damage
is due to excessive stress on a sm all number of active
® bres during eccentric exercise. T he po ssib ility that
selective recruitm ent of fast-twitch m otor units results
in greater dam age to fast-twitch ® bres warrants fu rther
examination.
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