the aim of the study was to assess EMG MPF during recovery following a fatiguing contraction at multiple locations of the quadriceps femoris muscle injured by eccentric exercise.

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Effect of delayed-onset muscle soreness on
muscle recovery after a fatiguing isometric
contraction
Article in Scandinavian Journal of Medicine and Science in Sports · November 2008
DOI: 10.1111/j.1600-0838.2008.00866.x · Source: PubMed
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2. Effect of delayed-onset muscle soreness on muscle recovery after a
fatiguing isometric contraction
N. Hedayatpour1,2
, D. Falla1
, L. Arendt-Nielsen1
, D. Farina1
1
Department of Health Science and Technology, Centre for Sensory-Motor Interaction (SMI), Aalborg University, Aalborg,
Denmark, 2
Faculty of Physical Education and Sport Science, Boali Sina University of Hamadan, Hamadan, Iran
Corresponding author: Dario Farina, PhD, Department of Health Science and Technology, Center for Sensory-Motor
Interaction (SMI), Aalborg University, Fredrik Bajers Vej 7D-3, DK-9220 Aalborg, Denmark. Tel: 145 99 40 88 21, Fax:
145 98 15 40 08, E-mail: df@hst.aau.dk
Accepted for publication 11 July 2008
An increase to above-baseline levels of electromyography
(EMG) mean power spectral frequency (MPF) has been
observed previously during muscle recovery following fati-
guing contractions and has been explained by membrane
hyperpolarization due to increased activation of the Na1
–
K1
pump. It is hypothesized that this membrane mechanism
is impaired by muscle fiber damage following eccentric
exercise. Thus, the aim of the study was to investigate
surface EMG signal characteristics during recovery from
fatigue after eccentric exercise. Ten healthy subjects per-
formed sustained isometric knee extensions at 40% of the
maximal torque (MVC) until task failure before, immedi-
ately after and 24 and 48 h after eccentric exercise. Bipolar
surface EMG signals were recorded from six locations over
the quadriceps during the sustained isometric contraction
and during 3-s long contractions at 40% MVC separated by
1-min intervals for 15 min (recovery). Before the eccentric
exercise, MPF of EMG signals increased to values above
baseline during recovery from the fatiguing isometric
contraction (Po0.001), whereas immediately after and 24
and 48 h after the eccentric task, MPF was lower than
baseline during the entire recovery period (Po0.01). In
conclusion, delayed-onset muscle soreness abolished the
supranormal increase in EMG MPF following recovery
from fatigue.
Eccentric exercise is characterized by high force
generation, reduced muscle activation per tension
level (Komi et al., 1987) and low energy expenditure
(Evans et al., 1983) and is effective in inducing muscle
hypertrophy (Hather et al., 1991) compared with
other types of exercise. Including recovery periods
between eccentric training bouts is important for
subsequent muscle performance because appropriate
recovery improves muscle strength and prevents
muscle fiber damage (Pincivero et al., 1997).
Recovery is defined as the return to normal phy-
siological conditions after fatigue (Renaud, 1989).
Recovery of the excitation–contraction process at the
level of the muscle fibers depends on the ability of the
fiber membrane to counterbalance the ionic gradient
across the membrane (Lindinger & Sjogaard, 1991;
Lindinger, 1995). Re-establishment of ion gradients
is necessary to facilitate subsequent muscle contrac-
tion by stimulating calcium release from the sarco-
plasmatic reticulum.
Eccentric contractions induce disruption of the
muscle fibers and therefore may reduce the capacity
of the muscle to preserve the ionic gradients after
fatiguing contractions at the injured sites. In large
muscles, such as the quadriceps, the extent of muscle
fiber damage after eccentric exercise depends on the
morphological and architectural characteristics of
the fibers, which are different in different muscle
locations (Blazevich et al., 2006) as shown by biopsy
(Takekura et al., 2001), magnetic resonance imaging
(Bosboom et al., 2003) and electromyographic stu-
dies (Hedayatpour et al., 2008b). Eccentric exercise
contributes to membrane depolarization of the in-
jured sites during delayed-onset muscle soreness and
thus to reduced action potential amplitude (McBride
et al., 1994).
Although the effect of muscle fiber damage in-
duced by eccentric exercise is well documented
(McBride et al., 1994; Pincivero et al., 2006;
Hedayatpour et al., 2008b), there are no studies that
have investigated recovery after a fatiguing contraction
of the injured muscle following eccentric exercise. This
knowledge would provide further understanding of
impaired muscle performance after unaccustomed
tasks and may be important for rehabilitation and
exercise programs following muscle injury.
A supranormal value of the mean power spectral
frequency (MPF) of the surface electromyography
Scand J Med Sci Sports 2008 Copyright & 2008 The Authors
Journal compilation & 2008 Blackwell MunksgaardPrinted in Singapore .All rights reserved
DOI: 10.1111/j.1600-0838.2008.00866.x
1

3. (EMG) has been reported during recovery following
fatiguing isometric contractions. This observation
has been attributed to an increased activity of the
Na1
–K1
pump (van der Hoeven & Lange, 1994;
Hedayatpour et al., 2008a). Given that eccentric
exercise is associated with increased fiber membrane
ionic permeability (McNeil & Khakee, 1992;
McBride et al., 2000), in this study it is hypothesized
that this mechanism is impaired in the presence of
muscle damage due to eccentric exercise. Therefore,
the aim of the study was to assess EMG MPF during
recovery following a fatiguing contraction at multi-
ple locations of the quadriceps femoris muscle in-
jured by eccentric exercise.
Materials and methods
Subjects
Ten healthy men [age, mean Æ standard deviation (SD),
23.3 Æ 4.2 year, body mass 72.5 Æ 12.4 kg and height
1.78 Æ 0.06 m] participated in the study. All subjects were
right leg dominant and were not involved in regular exercise of
their knee extensor muscles the year before the experiment.
The study was conducted in accordance with the Declaration
of Helsinki and approved by the local ethics committee
(N-20070019). Subjects provided informed written consent
before participation in the study.
Procedure
Time-to-task failure, skin temperature, muscle circumference
and surface EMG were recorded during fatigue and recovery
from a 40% maximal isometric voluntary contraction (MVC)
sustained until task failure under four conditions: pre-
eccentric exercise (phase 1), immediately following eccentric
exercise (phase 2), 24 h (phase 3) and 48 h (phase 4) after
eccentric exercise. In each condition, MVC was measured
before the sustained contraction and after 15 min of recovery.
The eccentric exercise was performed with a KinCom isoki-
netic dynamometer (Chattanooga, Tennessee, USA) and con-
sisted of four bouts of 25 maximal voluntary concentric/
eccentric contractions at a speed of 601/s between 901 and
1701 of knee extension, with a 3-min rest between each set. A
load equal to twice the MVC was applied during the eccentric
contractions. During the exercise, the subject was provided
with visual feedback of the torque produced and was verbally
encouraged to generate maximal force.
Maximal voluntary torque
MVC torque was measured using the KinCom dynamometer.
The subject sat comfortably on the adjustable chair of the
KinCom with his hip in 901 flexion. The chair position was
adjusted so that the axis of rotation of the knee (tibio-femoral
joint) was aligned with the axis of rotation of the dynam-
ometer’s attachment arm. The subject was fixed with straps
secured across the chest and hips. The right leg was secured in
901 knee flexion to the attachment arm with a Velcro strap.
Visual feedback of torque was provided on a screen positioned
in front of the subject. The subject was asked to perform three
MVCs of 3–5-s duration, with 2 min of rest in between and
with verbal encouragement to exceed the previous torque
level. The highest MVC value was selected as reference for
calculating the submaximal torque level. The submaximal
torque level for each day was determined from the baseline
MVC performed during that session.
Sensory assessment
Perceived pain intensity in the quadriceps was assessed 24 and
48 h after eccentric exercise using a 10-cm visual analog scale,
labeled with end points ‘‘no pain’’ (left) and ‘‘worst pain
imaginable’’ (right). Participants were required to rate the
average pain intensity corresponding to their soreness level
during daily activities (e.g. climbing stairs) since their last visit
to the laboratory (over the last 24 h). In order to characterize
the distribution of pain, subjects were also asked to document
the areas of pain on a body chart. Pain drawings were
subsequently digitized (ACECAD D90001, ACE CAD En-
terprise Co. Ltd, Taipei Hsien, Taiwan), and pain areas were
estimated in arbitrary units for comparison among days
(Hedayatpour et al., 2008b). The total mapped pain area
was the sum of all pain areas reported by the subject on the
body chart.
EMG recording and analysis
Surface EMG signals were recorded from six locations dis-
tributed over the quadriceps muscles by circular Ag-AgCl
surface electrodes (Ambu
s
Neuroline, Ambu A/S, Ballerup,
Denmark, sensor area: 28 mm2
, 2-cm interelectrode distance)
(Fig. 1). The distance from the anterior superior iliac spine
(ASIS) to the medial and lateral border of the patella were
measured to mark the medial and lateral sides of the quad-
riceps, respectively (Zipp, 1982). Six pairs of electrodes were
To ASIS
Vastus
Medialis
Vastus
Lateralis
80%20%
Patella
10%
20%
30%
Surface EMG electrode placement
Fig. 1. Schematic representation of surface EMG electrode
locations in the longitudinal and transverse direction over
the quadriceps muscle. The locations correspond to dis-
tances from the patella of 10% (distal), 20% (middle) and
30% (proximal) of the distance between the anterior super-
ior iliac spine (ASIS) and medial or lateral border of the
patella. EMG, electromyography.
Hedayatpour et al.
2

4. placed longitudinally at a distance from the patella of 10%,
20% and 30% (distal, middle and proximal site) of the
measured anatomical lengths. At each distance, electrode pairs
were placed longitudinally along the half circumference of the
thigh, at distances of 20% and 80% (lateral and medial site)
from the femur midline. Before electrode placement, the skin
was shaved and lightly abraded. The positions of the electro-
des were marked on the skin during the first session (day 1),
enabling replication of electrode location 24 and 48 h post-
exercise.
EMG signals were recorded during an isometric contrac-
tion at 40% MVC sustained until task failure and during a
period of recovery following the sustained contraction. The
recovery contractions commenced 1 min after the sustained
contraction was terminated and consisted of 3-s contractions
at 40% MVC performed every minute over a 15-min period.
Surface EMG signals were amplified (EMG amplifier, EMG-
16, LISiN - OT Bioelectronics, Torino, Italy; bandwidth 10–
500 Hz), sampled at 2048 Hz and stored after 12-bit A/D
conversion.
Task failure was defined as a drop in torque 45% MVC for
more than 5 s after strong verbal encouragement to the subject
to maintain the target torque. The EMG signals recorded
during the fatiguing contraction were divided into time inter-
vals of 10% duration of the time-to-task failure. The average
rectified value (ARV) and MPF were estimated from the EMG
signals over epochs of 1 s and were then averaged to obtain
one representative value for each 10% interval. This allowed
comparison of subjects with different time-to-task failure. The
same EMG variables were computed during each of the 3-s
contractions throughout the 15-min of recovery. MPF has
lower estimation variance with respect to the median power
frequency (Farina & Merletti, 2000) and thus was selected for
data analysis in this study. The mean frequency was computed
from intervals of 1 s, thus leading to a resolution of 1 Hz, using
the periodogram approach (Farina & Merletti, 2000).
Skin temperature was measured at the mid point of the
quadriceps using a skin thermometer (Ellab Ltd., Copenha-
gen, Denmark) from the beginning of the sustained contrac-
tion until the end of the recovery period. The mid point of the
quadriceps was defined as 50% of the distance between the
superior border of the patella and ASIS. Distal thigh circum-
ference was measured before the sustained contraction on each
day using a standard tape measure.
Statistical analysis
A three-way repeated measures ANOVA was applied to assess
changes in ARV and MPF at the beginning of the fatiguing
contraction with condition (pre- and post-eccentric exercise,
24 and 48 h) and electrode location in two directions (long-
itudinal: distal, middle and proximal site; transverse: medial
and lateral site) as dependent factors. Three-way ANOVA,
with the same dependent factors, was also applied to the
percent change of ARV and MPF at the end with respect to
the beginning of the fatiguing contraction and to the percent
difference after 15 min of recovery with respect to the begin-
ning of the fatiguing contraction. A one-way ANOVA was
applied to analyze MVC, time-to-task failure and muscle
circumference with condition as a factor. Finally, a two-way
ANOVA was used to evaluate skin temperature changes
during the fatiguing contraction and the recovery period
with condition and time as factors.
The significance level was Po0.05 for all statistical proce-
dures. Pair-wise comparisons were performed with the Stu-
dent–Newman–Keuls (SNK) post hoc test when ANOVA was
significant. Results are reported as mean and SD in the text
and tables and standard error in the figures.
Results
The MVC and the time-to-task failure depended on
the condition (Po0.001), with lower values immedi-
ately after and at 24 and 48 h post-eccentric
exercise compared with the pre-eccentric exercise
condition (Po0.001). The percent decline in MVC
with respect to the pre-eccentric exercise phase was
À 30.4 Æ 8.1% (immediately after), À 28.5 Æ 16.2%,
(24 h) and À 32.2 Æ 20.4% (48 h). There was no
significant difference in baseline MVC and time-to-
task failure recorded 24 and 48 h post-eccentric
exercise (all P40.05). In all conditions, MVC at
the end of the recovery period was not significantly
different from the MVC level measured at the begin-
ning of the fatiguing contraction (P40.05). Muscle
circumference measured at 24 and 48 h post-eccentric
exercise conditions was larger than in the pre-
eccentric exercise phase (Po0.0001) (Table 1).
In all conditions, skin temperature at the begin-
ning of the fatiguing contraction was not different
from the temperature at task failure (P40.05), but
skin temperature was higher after the 15-min recov-
ery than at the beginning of the fatiguing contraction
(Po0.05). Greater values of skin temperature
were observed immediately after the eccentric
exercise compared with all other conditions
(Po0.05) (Table 2).
Sensory assessment
No differences were observed for the total perceived
pain areas recorded 24 and 48 h post-eccentric ex-
ercise (24 h: 1.30 Æ 0.52, 48 h: 2.31 Æ 0.83, arbitrary
units) (Fig. 2). The average reported pain intensity
was 3.60 Æ 0.70 and 3.50 Æ 0.55 at 24 and 48 h post-
eccentric exercise, respectively.
Table 1. Maximal torque, thigh circumference, and time-to-task failure (mean Æ SD, n 5 10 subjects) in the four conditions analyzed
Pre-exercise Immediately after 24 h post-exercise 48 h post-exercise
Maximal torque (N m) 427.2 Æ 109.1 299.2 Æ 82.3* 306.3 Æ 11.4* 294.5 Æ 134.5*
Task failure (s) 54.4 Æ 22.8 33.4 Æ 17.3* 36.9 Æ 19.6* 36.9 Æ 16.0*
Thigh circumference (cm) 40.9 Æ 3.8 41.7 Æ 3.9* 41.5 Æ 3.7* 41.6 Æ 3.8*
Values for all parameters were significantly different for the three post-eccentric exercise conditions compared with the pre-eccentric exercise condition.
*Po0.05.
Muscle recovery and DOMS
3

5. Fatiguing contraction
MPF at the beginning of the fatiguing contraction
depended on the condition (Po0.001; MPF values
were lower at the post-eccentric exercise phases com-
pared with the pre-exercise condition, SNK: Po0.05)
and electrode location in the longitudinal direction
(Po0.0001; higher values of MPF were observed for
the most distal location compared with the other two
locations; SNK: Po0.01). The percent difference of
MPF values at the beginning of the fatiguing con-
traction in the three post-eccentric exercise phases
(immediately after, 24 and 48 h post-eccentric exer-
cise) with respect to the pre-exercise phase did not
depend on the electrode location in either direction.
The percent change in MPF at the end (last epoch)
with respect to the beginning (first epoch) of the
fatiguing contraction depended on the electrode
location in the longitudinal direction [Po0.05; a
greater decrease was observed for the most distal
location compared with the most proximal location;
SNK: Po0.05; 10% (most distal): À 8.0 Æ 0.6%;
30% (most proximal): À 3.7 Æ 0.1%]. Moreover,
the percent change of MPF depended on the inter-
action between condition and electrode location in
the longitudinal direction (Po0.001; at the post-
eccentric exercise phases, the most distal location
showed a greater reduction than that in the pre-
eccentric exercise phase; SNK: Po0.05) (Figs 3 and 4).
Figure 5 shows an example of MPF values for a
representative subject.
The initial value of ARV at the beginning of the
fatiguing contraction depended on the condition
(Po0.001; initial values of ARV were lower during
the post-exercise conditions compared with the pre-
eccentric exercise phase; SNK: Po0.05) and elec-
trode location in both the longitudinal (Po0.0001;
higher initial values of ARV were observed for the
most distal location compared with the other two
locations; SNK: Po0.05) and the transverse direc-
tions (Po0.0001; higher initial values of ARV were
observed for the lateral location compared with the
medial location; SNK: Po0.001). The percent
change in the initial value of ARV in the three
post-eccentric exercise conditions (immediately after,
24 and 48 h post-eccentric exercise) with respect to
the pre-eccentric exercise phase did not depend on
the electrode location.
The percent change in ARV at the end of the
contraction with respect to the initial value depended
on the condition (Po0.001) electrode location in the
longitudinal direction (Po0.001), and on the interaction
between condition and electrode location in the long-
itudinal direction (Po0.001). The relative change was
smaller in the pre-eccentric phase with respect to the
three post-eccentric exercise conditions (Po0.001;
pre-eccentric exercise: À 7.7 Æ 7.2%; immediately post:
À 39.1 Æ 20.1%; 24h: À 49.8 Æ 18.6%; 48h: À 51.0 Æ
13.7%). Moreover, the two most distal electrode loca-
tions resulted in a higher decrease of the ARV compared
with the proximal locations [Po0.05; 10% (distal):
À 50.1 Æ 16.2%; 20%: À 45.8 Æ 19.4%; 30% (proxi-
mal): À 43.7 Æ 19.3%].
Recovery
In the pre-eccentric exercise condition, MPF was
higher after 15-min of recovery compared with the
beginning of the fatiguing contraction (Po0.001).
The percent increase in MPF at the end of the
recovery period with respect to the pre-fatigue value
depended on the electrode location in the longitudi-
nal direction (Po0.0001) and was highest at the most
distal location (11.3 Æ 7.1%) with respect to the
other two locations (middle: 4.1 Æ 7.4%; proximal:
5.2 Æ 9.3%) (Po0.05; Fig. 3).
Table 2. Skin temperature (mean Æ SD, 1C, n 5 10) in the four condi-
tions analyzed at the beginning of the fatiguing contraction, at task failure,
and after 15 min of recovery
Exercise
phase
Beginning Task
failure
Following 15-min
recovery
Pre-exercise 30.9 Æ 0.7* 31.2 Æ 0.6 31.8 Æ 0.3*
Immediately after 32.5 Æ 1.2* 32.8 Æ 1.2 32.9 Æ 1.3*
24 h post-exercise 31.4 Æ 1.2* 31.6 Æ 1.3 31.7 Æ 1.2*
48 h post-exercise 31.1 Æ 0.8* 31.5 Æ 1.0 31.7 Æ 1.1*
*Po0.05.
Fig. 2. Area of pain reported by the subjects during their
regular activities of daily living (e.g. walking, climbing stairs)
in the 24 h preceding the experimental sessions performed at
24 h (a) and 48 h (b) post-eccentric exercise. The multiple
tracings represent the areas of pain reported by each subject.
Hedayatpour et al.
4

6. For the three conditions following the eccentric
exercise, MPF values after the 15 min of recovery
were lower compared with the beginning of the
sustained contraction (Po0.0001). The percent de-
crease of MPF at the end of the recovery period with
respect to the pre-fatigue values depended on the
condition (Po0.0001) and location in the transverse
(Po0.05) and longitudinal direction (Po0.0001).
The percent decrease of MPF was higher immedi-
ately after the eccentric exercise compared with 24
and 48 h post-eccentric exercise (Po0.0001; immedi-
ately post: À 6.2 Æ 7.3%; 24 h: À 1.5 Æ 6.1%; 48 h:
À 2.1 Æ 8.2%) and medial locations of the quadri-
ceps showed a larger reduction of MPF compared
with the lateral locations (Po0.05; medial:
À 3.2 Æ 1.8%; lateral: À 1.8 Æ 1.2%). Moreover,
the most distal location of the medial aspect of the
quadriceps showed a greater reduction of MPF
compared with the other two medial locations
(Po0.05) (Fig. 4). Figure 5 illustrates representative
power spectra of EMG from one subject obtained
during the fatiguing and recovery contractions.
In all conditions, ARV during the entire recovery
period was significantly lower than at the beginning
of the fatiguing contraction (Po0.05).
Discussion
The results of this study demonstrate that the supra-
normal increase in EMG MPF during recovery from
fatigue is absent immediately following, 24 and 48 h
after eccentric exercise.
Muscle performance
A significant reduction in baseline maximal torque
and time-to-task failure was observed immediately
after eccentric exercise and persisted 24 and 48 h
post-eccentric exercise. These results indicate that
the eccentric exercise protocol used in this study
contributed to the reduced muscle power and physi-
cal work capacity and thus to the sensation of fatigue
and the inability to continue the sustained task
(Friden & Lieber, 1992).
Fatiguing contraction
Amplitude and MPF of the surface EMG at the
beginning of the fatiguing contraction decreased in
the post-eccentric exercise conditions, in agreement
with previous studies (Serra˜ o et al., 2003; Piitulainen
et al., 2008). However, other studies have also shown
an increase in EMG amplitude after eccentric ex-
ercise (Semmler et al., 2007). The lack of consistency
of EMG amplitude is a common problem to many
electromyographic studies (Felici et al., 1997). In this
study, a direct comparison of the absolute values of
these EMG variables across the 3 measurement days
is not possible due to differences of MVC values and
thus absolute forces expressed. However, the force
level was the same in the pre-eccentric exercise
contraction and in the contraction immediately fol-
lowing the eccentric exercise. Thus, the lower EMG
amplitude and MPF in the post-eccentric exercise
phase is probably due to fatigue from the eccentric
exercise and due to muscle damage (Newham et al.,
Pre eccentric exercise EMG mean power spectral frequency of the quadriceps
MeanPowerSpectralFrequency(Hz)
MeanPowerSpectralFrequency(Hz)
Task
Failure
Task
Failure
Distal
Middle
Proximal
50
55
60
65
70
75
80
85
90
95
100
50
55
60
65
70
75
80
85
90
95
100
LateralMedial(a) (b)
*
*
10% 100% 1 15 10% 100% 1 15
Time to task
failure
Recovery (min) Time to task
failure
Recovery (min)
Fig. 3. EMG mean power spectral frequency (MPF) (mean Æ SE, n 5 10 subjects) in 10 time intervals during the fatiguing
knee extension contraction (increments of 10% of the time-to-task failure) and in 15 intervals (separated by 1 min) during
recovery for the three electrode locations in the longitudinal direction (distal, middle and proximal) and the two locations in the
transverse direction (a: medial, b: lateral) obtained in the pre eccentric exercise condition. EMG, electromyography.
Muscle recovery and DOMS
5

7. 1983; Lieber et al., 1991). Accordingly, after eccentric
exercise a significant reduction in MPF and a greater
decrease in EMG amplitude over time were also
observed during the sustained contraction. In addi-
tion to changed membrane properties of the muscle
fibers, this result may also reflect an inability of the
injured muscle to recruit additional motor units
which is required to compensate for contractile fail-
ure (Kirsch & Rymer, 1992). The global nature of the
surface EMG analysis performed does not allow
these potential mechanisms to be differentiated.
Recovery
In accordance with previous findings, the results
showed that EMG MPF increased to values above
baseline after recovery from a fatiguing contraction,
whereas EMG amplitude decreased with respect to
baseline (van der Hoeven & Lange, 1994; Hedayat-
pour et al., 2008a). At the end of the recovery period,
maximal voluntary contraction torque recovered to
baseline values, as observed previously (van der
Hoeven & Lange, 1994). The main finding of this
study is the observation that MPF of the surface
EMG was smaller than the baseline value after
recovery from a fatiguing isometric contraction per-
formed immediately after, 24 and 48 h post-eccentric
exercise. This result indicates that the membrane
mechanisms associated with the supranormal values
of MPF, observed previously in undamaged muscles,
were impaired following high-tension eccentric ex-
ercise.
Post eccentric exercise EMG mean power spectral frequency of the quadriceps
MeanPowerSpectralFrequency(Hz)
Task
Failure
Task
Failure
Task
Failure
Task
Failure
Distal
Middle
Proximal
MeanPowerSpectralFrequency(Hz)
MeanPowerSpectralFrequency(Hz)MeanPowerSpectralFrequency(Hz)
MeanPowerSpectralFrequency(Hz)MeanPowerSpectralFrequency(Hz)
50
55
60
65
70
75
80
85
90
95
100
50
55
60
65
70
75
80
85
90
95
100
50
55
60
65
70
75
80
85
90
95
100
50
55
60
65
70
75
80
85
90
95
100
50
55
60
65
70
75
80
85
90
95
100
50
55
60
65
70
75
80
85
90
95
100
Immediately after 24h post eccentric 48h post eccentric
Medial
(a)
(d) (e) (f)
(b) (c)
Lateral
*
*
*
*
*
*
10% 100% 1 15
Time to task
failure
Recovery (min) Time to task
failure
Recovery (min)
10% 100% 1 15
Time to task
failure
Recovery (min)
10% 100% 1 15
Time to task
failure
Recovery (min)
10% 100% 1 15
Time to task
failure
Recovery (min)
10% 100% 1 15
Time to task
failure
Recovery (min)
Task
Failure
Task
Failure
10% 100% 1 15
Fig. 4. EMG mean power spectral frequency (MPF) (mean Æ SE, n 5 10 subjects) in 10 time intervals during the fatiguing
knee extension contraction (increments of 10% of the time to task failure) and in 15 intervals (separated by 1 min) during
recovery immediately after (a and d), 24 h (b and e), and 48 h post-exercise (c and f), for the medial (a–c) and lateral (d–f)
electrode locations in the transverse direction. In each graph MPF is reported for the three electrode locations in the
longitudinal direction (distal, middle, and proximal). EMG, electromyography.
Hedayatpour et al.
6

8. The long-lasting supranormal value of MPF has
been attributed to various factors, such as muscle
fiber swelling and increased muscle temperature
(Lundvall et al., 1972; Stewart et al., 2003). However,
in this study an increase in skin temperature and
muscle circumference in the post-eccentric exercise
conditions was not associated with an increase in
MPF above baseline values. The increase in MPF to
supranormal values has also been attributed to
membrane hyperpolarization due to the activation
of Na1
–K1
ATPase, which helps to preserve the
ionic gradients across the cell membrane after fati-
guing contractions (van der Hoeven & Lange, 1994;
Rongen et al., 2002). Given that an increase in fiber
membrane ionic permeability can occur after ec-
centric exercise (McNeil & Khakee, 1992; McBride
et al., 2000), the lack of supranormal values of
MPF following recovery in the post-exercise con-
ditions may indicate membrane depolarization due
to uptake of ions and large-molecular-weight mar-
kers (McNeil & Khakee, 1992; McBride et al., 1994,
2000) and the inability of Na1
–K1
ATPase to
maintain the ionic gradients for K1
and Na1
essential for cell excitability. Although, other fac-
tors such as a change in motor unit recruitment or
discharge rate following nociceptive input (Gande-
via, 2001), cannot be excluded from the present
results.
0 50 150 250 3500 50 150 250 350 0 50 150 250 350
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
Fatiguing Contraction
First 10% Epoch
Power/frequency(dB/Hz)Power/frequency(dB/Hz)Power/frequency(dB/Hz)Power/frequency(dB/Hz)
Frequency (Hz) Frequency (Hz)
Fatiguing Contraction
Last 10% Epoch
First Recovery
Contraction
Last Recovery
Contraction
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
(a)
(b)
(c)
(d)
–10
0
10
20
30
40
–10
0
10
20
30
40
–10
0
10
20
30
40
EMG power spectra from one subject during the fatiguing contraction and recovery
0 50 150 250 350
0 50 150 250 3500 50 150 250 3500 50 150 250 3500 50 150 250 350
0 50 150 250 350 0 50 150 250 350 0 50 150 250 350 0 50 150 250 350
0 50 150 250 350 0 50 150 250 350 0 50 150 250 350 0 50 150 250 350
Frequency (Hz) Frequency (Hz)
Fig. 5. EMG power spectra from one subject obtained at the beginning (first 10% interval) and the end (last 10% interval) of
the fatiguing contraction and recovery contractions during pre eccentric (a), immediately after (b), 24 h (c) and 48 h post-
exercise (d) for the medial-distal location of quadriceps muscle. The black circle indicates the MPF for each spectrum. EMG,
electromyography; MPF, mean power spectral frequency.
Muscle recovery and DOMS
7

9. This finding may partly explain the effectiveness of
eccentric contractions in inducing muscle hypertro-
phy (Hather et al., 1991) via Na1
influx and stimu-
lating protein synthesis (Goldspink et al., 1992).
Effect of location
Alterations of the normal trend of MPF values
during recovery depended on the location of the
recording electrodes, which may be due to a differ-
ential effect of eccentric exercise on different regions
of the quadriceps. After eccentric exercise, the distal
locations of the quadriceps showed the greatest
reduction in MPF during the recovery phase with
respect to the baseline value. In this region of the
quadriceps, the fibers are obliquely attached to the
patella (Peeler et al., 2005), with larger physiological
cross-sectional areas, and therefore have a greater
capacity to produce tension. Moreover, in this region
of the quadriceps, the muscle fibers counteract the
lateral pull of the patella during extension (Sakai et
al., 2000). A high-intensity contraction of the quad-
riceps during eccentric exercise may produce a strong
lateral pulling force and may therefore expose the
fibers of this region to further damage and stretch
throughout the range of knee extension. Finally, the
frequency of exposure to stretch as a result of the
change in fiber pennation angle (Herbert & Gande-
via, 1995) may also expose specific muscle fibers to
further injury.
Methodological considerations
Two adjacent recording electrode systems may have
detected common signal components in this study,
due to EMG crosstalk. For this reason, it is not
possible to associate electrode locations to specific
muscle compartments (e.g. medial locations to the
vastus medialis). This problem is common to many
physiological measurements with limited spatial
selectivity and impedes precise localization of the
sources. The aim of the study was to detect differ-
ences among portions of the quadriceps that were
large enough to be identified with the limited spatial
resolution that the available techniques provided.
Despite the limited selectivity of the recordings, it
was possible to identify different behaviors in differ-
ent regions of the quadriceps.
Perspectives
The supranormal increase in MPF during recovery
has been explained previously by a change in fiber
membrane mechanisms. Therefore, although changes
in motor unit control strategies cannot be excluded,
the absence of MPF increase during the recovery
from a fatiguing contraction following eccentric
exercise probably indicates impaired function of the
muscle fiber membrane. This may be an adaptive
mechanism that contributes to muscle hypertrophy
since changes in membrane ionic permeability act as
prerequisite for this physiological process (Gold-
spink et al., 1992). However, an insufficient ability
of membrane mechanisms to preserve the ionic
gradient may expose the muscle fibers to further
damage (Armstrong et al., 1991).
Key words: EMG, mean power frequency, recovery,
eccentric contraction, DOMS.
Acknowledgements
This study was supported by Boali Sina University of Hama-
dan, Iran (Nosratollah Hedayatpour), and the Danish Tech-
nical Research Council [project ‘‘Centre for Neuroengineering
(CEN)’’, contract no 26-04-0100] (Dario Farina). Deborah
Falla is supported by the National Health and Medical
Research Council of Australia (ID 351678).
References
Armstrong RB, Warren GL, Warren JA.
Mechanisms of exercise-induced
muscle fibre injury. Sports Med 1991:
12: 184–207.
Blazevich AJ, Gill ND, Zhou S. Intra- and
intramuscular variation in human
quadriceps femoris architecture
assessed in vivo. J Anat 2006: 209:
289–310.
Bosboom EM, Bouten CV, Oomens CW,
Baaijens FP, Nicolay K. Quantifying
pressure sore-related muscle damage
using high-resolution MRI. J Appl
Physiol 2003: 95: 2235–2240.
Evans WJ, Patton JF, Fisher EC,
Knottier HG. Muscle metabolism
during high intensity eccentric exercise.
In: Knuttgen HG, Vogel JA,
Poortmans J, eds. Biochemistry of
exercise. Champaign, IL: Human
Kinetics, 1983: 225–228.
Farina D, Merletti R. Comparison of
algorithms for estimation of EMG
variables during voluntary isometric
contractions. J Electromyogr Kinesiol
2000: 10: 337–349.
Felici F, Colace L, Sbriccoli P. Surface
EMG modifications after eccentric
exercise. J Electromyogr Kinesiol 1997:
7: 193–202.
Friden J, Lieber RL. Structural and
mechanical basis of exercise-induced
muscle injury. Med Sci Sports Exerc
1992: 24: 521–530.
Gandevia SC. Spinal and supraspinal
factors in human muscle fatigue.
Physiol Rev 2001: 81: 1725–1789.
Goldspink G, Scutt A, Loughna PT, Wells
DJ, Jaenicke T, Gerlach GF. Gene
expression in skeletal muscle in response
to stretch and force generation. Am J
Physiol Regulatory Integrative Comp
Physiol 1992: 262: 356–363.
Hather BM, Tesch PA, Buchanan P,
Dudley GA. Influence of eccentric
actions on skeletal muscle adaptations
to resistance training. Acta Physiol
Scand 1991: 143: 177–185.
Hedayatpour et al.
8

10. Hedayatpour N, Arendt-Nielsen L,
Farina D. Non-uniform
electromyographic activity during
fatigue and recovery of the vastus
medialis and lateralis muscles. J
Electromyogr Kinesiol 2008a: 18:
390–396.
Hedayatpour N, Falla D, Arendt-Nielsen
L, Farina D. Sensory and
electromyographic mapping during
delayed-onset muscle soreness. Med Sci
Sports Exerc 2008b: 40: 326–334.
Herbert RD, Gandevia SC. Changes in
pennation with joint angle and muscle
torque: in vivo measurements in human
brachialis muscle. J Physiol 1995: 484:
523–532.
Kirsch RF, Rymer WZ. Neural
compensation for fatigue-induced
changes in muscle stiffness during
perturbations of elbow angle in human.
J Neurophysiol 1992: 68: 449–470.
Komi PV, Kaneko M, Aura O. EMG
activity of leg extensor muscles with
special reference to mechanical efficiency
in concentric and eccentric exercise. Int
J Sports Med 1987: 8: 22–29.
Lieber RL, Woodburn TM, Friden J.
Muscle damage induced by eccentric
contractions of 25% strain. J Appl
Physiol 1991: 70: 2498–2507.
Lindinger MI. Potassium regulation
during exercise and recovery in
humans: implications for skeletal and
cardiac muscle. J Mol Cell Cardiol
1995: 27: 1011–1022.
Lindinger MI, Sjogaard G. Potassium
regulation during exercise and recovery.
Sports Med 1991: 11: 382–401.
Lundvall J, Mellander S, Westling H,
White T. Fluid transfer between blood
and tissues during exercise. Acta
Physiol Scand 1972: 85: 258–269.
McBride TA, Gorin FA, Carlsen RC.
Membrane depolarization following
high resistance eccentric exercise in rat
tibialis anterior. J FASEB 1994: 8:
A307.
McBride TA, Stockert BW, Gorin FA,
Carlsen RC. Stretch-activated ion
channels contribute to membrane
depolarization after eccentric contrac-
tions. J Appl Physiol 2000: 88:
91–101.
McNeil PL, Khakee R. Disruptions of
muscle fiber plasma membranes. Role
in exercise-induced damage. Am J
Pathol 1992: 140: 1097–1109.
Newham DJ, Mills KR, Quigley BM,
Edwards RH. Pain and fatigue after
concentric and eccentric muscle
contractions. Clin Sci (London) 1983:
64: 55–62.
Peeler J, Cooper J, Porter MM, Thliveris
JA, Anderson JE. Structural
parameters of the vastus medialis
muscle. Clin Anat 2005: 18: 281–289.
Piitulainen H, Komi P, Linnamo V, Avela
J. Sarcolemmal excitability as
investigated with M-waves after eccentric
exercise in human. J Electromyogr
Kinesiol 2008: 18: 672–681.
Pincivero DM, Gandhi V, Timmons MK,
Coelho AJ. Quadriceps femoris
electromyogram during concentric,
isometric and eccentric phases of
fatiguing dynamic knee extensions. J
Biomech 2006: 39: 246–254.
Pincivero DM, Lephart SM, Karunakara
RG. Effects of rest interval on
isokinetic strength and functional
performance after short term high
intensity training. Br J Sports Med
1997: 31: 229–234.
Renaud JM. The effect of lactate on intra-
cellular pH and force recovery of fatigued
sartorius muscles of the frog, Rana
pipiens. J Physiol 1989: 416: 31–47.
Rongen GA, van Dijk JP, van Ginneken
EE, Stegeman DF, Smits P, Zwarts
MJ. Repeated ischaemic isometric
exercise increases muscle fibre
conduction velocity in humans:
involvement of Na1
–K1
ATPase. J
Physiol 2002: 540: 1071–1078.
Sakai N, Luo ZP, Rand JA, An KN. The
influence of weakness in the vastus
medialis oblique muscle on the
patellofemoral joint: an in vitro
biomechanical study. Clin Biomech
(Bristol, Avon) 2000: 15: 335–339.
Semmler JG, Tucker KJ, Allen TJ, Proske
U. Eccentric exercise increases EMG
amplitude and force fluctuations
during submaximal contractions of
elbow flexor muscles. J Appl Physiol
2007: 103: 979–989.
Serra˜ o FV, Foerster B, Spada S, Morales
MM, Monteiro-Pedro V, Tannu´ s A,
Salvini TF. Functional changes of
human quadriceps muscle injured by
eccentric exercise. Braz J Med Biol Res
2003: 36: 781–786.
Stewart D, Macaluso A, De Vito G. The
effect of an active warm-up on surface
EMG and muscle performance in
healthy humans. Eur J Appl Physiol
2003: 89: 509–513.
Takekura H, Fujinami N, Nishizawa T,
Ogasawara H, Kasuga N. Eccentric
exercise-induced morphological
changes in the membrane systems
involved in excitation-contraction
coupling in rat skeletal muscle. J
Physiol 2001: 533: 571–583.
van der Hoeven JH, Lange F.
Supranormal muscle fiber
conduction velocity during inter-
mittent isometric exercise in human
muscle. J Appl Physiol 1994: 77:
802–806.
Zipp P. Recommendations for the
standardization of lead positions in
surface electromyography. Eur J Appl
Physiol 1982: 50: 41–45.
Muscle recovery and DOMS
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