The purpose of this study was to assess the EMG activity of knee muscles during destabilizing perturbations performed before, immediately after, and 24 and 48 h after eccentric exercise.

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Facilitation of quadriceps activation is impaired
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2. Delayed-Onset Muscle Soreness Alters the
Response to Postural Perturbations
NOSRATOLLAH HEDAYATPOUR1
, HAMIDOLLAH HASSANLOUEI1
, LARS ARENDT-NIELSEN1
,
UWE G. KERSTING1
, and DEBORAH FALLA1,2,3
1
Centre for Sensory–Motor Interaction, Department of Health Science and Technology, Aalborg University, Aalborg,
DENMARK; 2
Pain Clinic, Center for Anesthesiology, Emergency and Intensive Care Medicine, University Hospital
Go¨ttingen, GERMANY; 3
Department of Neurorehabilitation Engineering, Bernstein Center for Computational
Neuroscience, University Medical Center Go¨ttingen, Georg-August University, Go¨ttingen, GERMANY
ABSTRACT
HEDAYATPOUR, N., H. HASSANLOUEI, L. ARENDT-NIELSEN, U. G. KERSTING, and D. FALLA. Delayed-Onset Muscle
Soreness Alters the Response to Postural Perturbations. Med. Sci. Sports Exerc., Vol. 43, No. 6, pp. 1010–1016, 2011. Introduction:
Eccentric contractions induce muscle fiber damage that is associated with delayed-onset muscle soreness and an impaired ability of
the muscle to generate voluntary force. Pain and pathophysiological changes within the damaged muscle can delay or inhibit neuro-
muscular responses at the injured site, which is expected to have an effect on reflex activity of the muscle. Purpose: The aim of the
study was to investigate the reflex activity of knee muscles to rapid destabilizing perturbations, before, immediately after, and 24 and
48 h after eccentric exercise. Methods: Bipolar surface EMG signals were recorded from 10 healthy men with seven pairs of elec-
trodes located on the knee extensor muscles (vastus medialis, rectus femoris, and vastus lateralis) and knee flexor muscles (the
medial and lateral heads of the hamstring and the medial and lateral heads of gastrocnemius) of the right leg during rapid pertur-
bations. Results: The maximal voluntary contraction force decreased by 24% T 4.9% immediately after exercise and remained reduced
by 21.4% T 4.1% at 24 h and by 21.6% T 9.9% at 48 h after exercise with respect to baseline. During the postexercise postural
perturbations, the EMG average rectified value of the knee extensor muscles was significantly lower than baseline (P G 0.001). More-
over, the decrease in average rectified value over time during postexercise sustained contractions was greatest compared with the session
before exercise (P G 0.0001). Conclusions: Reflex activity in leg muscles elicited by rapid destabilizing perturbations is reduced after
exercise-induced muscle soreness. Key Words: REFLEX, PERTURBATION, DOMS, ECCENTRIC EXERCISE
M
aintenance of posture often requires reflex muscle
activity to return a perturbed joint to its initial
position. Reflex muscle responses to rapid per-
turbations play a significant role in the dynamic alignment
and stability of musculoskeletal structures. For example, en-
hanced knee stability has been demonstrated by reflexive
responses elicited in muscles surrounding the knee by me-
chanical stimulation of the medial ligament (21,27) and by a
rapid valgus/varus movement of the knee (4).
Pain is common after a high-intensity eccentric exercise
particularly in untrained individuals, most likely due to accu-
mulation of metabolites and/or fiber injury within the skeletal
muscle (26). The quadriceps femoris muscle is particularly
susceptible to fiber damage due to its powerful action and
frequent eccentric loading of the leg during sport and daily
activities. Delayed-onset muscle soreness (DOMS) usually
manifests 24 h after eccentric exercise and persists for pro-
longed periods of time (up to 72 h after exercise) due to
pathophysiological changes in the muscle fibers. In the in-
jured muscle, phagocyte cell infiltration results in progres-
sive necrosis of the contractile elements and inflammation
(1,26,37), which, in turn, sensitizes intramyofibril pain affer-
ents (group IV) (37). The presence of pain within the quadri-
ceps muscle may delay or inhibit neuromuscular responses at
the injured site (13,14) because input from nociceptive affer-
ents can inhibit the input of muscle spindles via presynaptic
inhibition (5,42). Thus, when individuals with quadriceps pain
are faced with demanding tasks that may challenge knee sta-
bility, the neuromuscular system may be incapable of appro-
priately activating muscles to stabilize the joint. Such altered
muscle activity around the knee may expose structures of the
knee joint to abnormal loading during exercise and may con-
tribute to sport-related injuries (24,25).
Changes in motor control strategies induced by eccentric
exercise are reflected in features of the surface EMG (28).
Thus, the use of EMG during destabilizing perturbations that
challenge knee stability may provide greater insight into the
change in muscle activation patterns after eccentric exercise.
Address for correspondence: Deborah Falla, Ph.D., Department of Neuro-
rehabilitation Engineering, Bernstein Center for Computational Neuro-
science, University Medical Center Go¨ttingen, Georg-August University,
Von-Siebold-Str. 4, 37075 Go¨ttingen, Germany; E-mail: deborah.falla@
bccn.uni-goettingen.de.
Submitted for publication August 2010.
Accepted for publication November 2010.
0195-9131/11/4306-1010/0
MEDICINE & SCIENCE IN SPORTS & EXERCISEÒ
Copyright Ó 2011 by the American College of Sports Medicine
DOI: 10.1249/MSS.0b013e3182058628
1010
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3. This knowledge may be useful to understand the mecha-
nisms underlying the development of knee disorders (e.g.,
patella, ligament and tendon injuries) after an unaccustomed
exercise. In this study, it was hypothesized that muscle
activity elicited by rapid destabilizing perturbation would
be reduced after exercise-induced muscle soreness. There-
fore, the purpose of this study was to assess the EMG activity
of knee muscles during destabilizing perturbations performed
before, immediately after, and 24 and 48 h after eccentric
exercise.
MATERIALS AND METHODS
Experimental design and approach. This experiment
analyzed reflex activity of the knee muscles after eccentric
exercise. Reflex activity was elicited in knee muscles by rapid
destabilizing perturbations. Surface EMG signals were re-
corded from the quadriceps, hamstrings, and gastrocnemius
simultaneously during the destabilizing movements before,
immediately after, and 24 and 48 h after eccentric exercise.
Subjects. Ten healthy men (mean T SD; age = 23.2 T
3.1 yr, body mass = 75.5 T 10.4 kg, height = 1.78 T 0.06 m)
participated in the study. All subjects were right leg domi-
nant and were not involved in regular exercise of their
knee extensor muscles for at least 6 months before the ex-
periment. The study was conducted in accordance with the
Declaration of Helsinki and approved by the local ethics com-
mittee (No. 20070019). Subjects provided informed written
consent before participation in the study. The number of par-
ticipants was based on previous studies examining the effects
of eccentric exercise on muscle activity, which showed that
10 volunteers were sufficient to show a difference (14,15).
Exercise protocol. Time to task failure for a sustained
isometric contraction at 50% of maximal force and EMG
variables during both the sustained contraction and destabi-
lizing perturbations were recorded, before, immediately after,
and 24 and 48 h after exercise. Moreover, maximal voluntary
contraction (MVC) force, subjective pain intensity ratings, and
muscle circumference were measured before and after the ex-
ercise sessions. The exercise protocol was performed with a
KinCom Isokinetic Dynamometer (Chattanooga, TN) and
consisted of four bouts of 25 maximum voluntary eccentric
knee extension contractions at a speed of 60-Isj1
between 90-
and 170- of knee extension, with 3 min of rest between each
set (8,13–15). During the exercise, the subject was provided
with visual feedback of the force produced and was verbally
encouraged to generate a maximal force during the eccentric
phase, whereas during the concentric phase, the movement
was assisted by the dynamometer.
Maximal voluntary contraction. Maximal voluntary
isometric contraction force was measured using the KinCom
Dynamometer. The subject was seated on the adjustable chair
of the KinCom with the hip in 90- flexion. The chair position
was modified until the knee axis of rotation (tibiofemoral
joint) was aligned with the axis of rotation of the dyna-
mometer’s attachment arm. The subject was fixed with straps
secured across the chest and hips. The right leg was secured
to the attachment arm in 90- knee flexion with a Velcro
strap. Visual feedback of force was provided on a screen
positioned in front of the subject. The subject was asked to
perform three maximal isometric knee extensions (3–5 s in
duration) in 90- knee flexion, with 2 min of rest between
and verbal encouragement to exceed the previous force level.
The highest MVC value was used as a reference for the defi-
nition of the submaximal force level. The submaximal forces
were relative to the MVC measured on the same day of the
test. Thigh circumference was measured using a tape measure
around the distal portion of the thigh at 10% of the distance
between superior border of the patella and anterior superior
iliac spine.
Pain assessment. A 10-cm visual analog scale, labeled
with end points on the left (no pain) and right (worst pain
imaginable), was used to assess the perceived pain intensity
24 and 48 h after exercise. The subjects were asked to rate
the average pain intensity in the quadriceps during their regu-
lar activities of daily living (e.g., climbing stairs) since their
last visit to the laboratory (during the past 24 h).
EMG. Seven pairs of circular Ag–AgCl surface electrodes
(Ambu Neuroline, Ambu A/S, Ballerup, Denmark; conductive
area = 28 mm2
) were placed in bipolar configuration (inter-
electrode distance = 2 cm) over the quadriceps, hamstring,
and gastrocnemius muscles. Electrodes were placed over the
quadriceps at 10% of the distance between medial border
(vastus medialis; VM), superior border (rectus femoris; RF),
and lateral border (vastus lateralis; VL) of the patella and
anterior superior iliac spine. The position of electrodes on
hamstring muscle was determined by palpation of the most
distal portion of the medial (biceps femoris; BF) and lateral
(semitendinosus; ST and semimembranosus; SM) belly of the
hamstrings during light isometric contractions with the subject
in a prone position. For the gastrocnemius muscle electrode
pairs were placed on the lateral and medial head of gastroc-
nemius (LG and MG, respectively) at one-fourth the distance
from the popliteal fossa to the insertion of the Achilles
tendon. Before electrode placement, the skin was shaved and
lightly abraded at the selected locations. Surface EMG sig-
nals were amplified (EMG amplifier (EMG-128; LISiN-OT
Bioelettronica, Turin, Italy) with a bandwidth of 10–500 Hz),
sampled at 2048 Hz, and stored after 12-bit A/D conversion.
EMG during sustained contractions. Surface EMG
signals were recorded from the VM, VL, and RF during an
isometric knee extension contraction at 50% MVC, which
was sustained until task failure. The sustained contraction
was performed on the KinCom Dynamometer with the
subject in the same position as in the maximal voluntary
contractions, i.e., with the knee and hip in 90- of flexion.
Task failure was defined as a drop in force greater than 5%
MVC for more than 5 s after strong verbal encouragement
to the subject to maintain the target force.
The average rectified value (ARV) was estimated from
the EMG signals for epochs of 1 s. The values obtained
from 1-s-long epochs in intervals of 10% of the time to task
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4. failure were averaged to obtain one representative value for
each 10% interval. This allows for comparison of data with
different times to task failure. To compare changes across
testing sessions, the percent change in ARV over time was
calculated by subtracting the final value from the initial
value of ARV and dividing by the initial value.
EMG during postural perturbations. Subjects stood
with their feet shoulder width apart and their right limb on a
movable platform. A positioning actuator (41) translated the
platform 6 cm frontally (forward and backward direction)
for 150 ms. The subject’s left foot was positioned on the
ground, and an oscilloscope was positioned in front of the
subject to monitor weight bearing. To restrict hip joint mo-
tion, a strap was secured around the pelvis and bolted firmly
to the wall. The subject stood comfortably with equal weight
on each limb using the visual feedback. Four 3-s trials were
collected in which the plate was triggered to move at a ran-
dom interval within the 3 s. Subjects were unaware of when
the plate would be triggered to move. Surface EMG was
recorded from the quadriceps, hamstring, and gastrocnemius
of the right limb.
To assess the amplitude of muscle reflex activity, the ARV
of individual muscles was calculated over a fixed window,
which was 175 ms after the onset of plate movement. This
time window reflects the muscle response that occurs after
the monosynaptic stretch reflex, which is thought to be medi-
ated subconsciously, with afferent commands being sent to the
cerebellum and brainstem (40). The ARV obtained from the
175-ms epochs in four trials were averaged to obtain a repre-
sentative value. A reference electrode was placed around the
right ankle. The positions of the electrodes were marked on
the skin during the first session (day 1) so that the locations
could be replicated 24 and 48 h after exercise.
Statistical analysis. A one-way repeated-measures
ANOVA was applied to analyze MVC, time to task failure, and
thigh circumference across testing days. Two-way repeated-
measures ANOVA was used to assess the percent change
of ARV across the sustained contraction at 50% MVC (per-
cent change from the first to the last epoch), with day and
muscle as dependent factors. To assess changes in the am-
plitude of reflex muscle activity during the perturbations, a
three-way ANOVA was applied to the change of ARV value
from baseline (day 1) to the ARV value after exercise
(immediately after and 24 and 48 h after), with muscle and
perturbation direction (backward and forward) as dependent
factors. Pairwise comparisons were performed with the
Student–Newman–Keuls post hoc test when ANOVA was
significant. The significance level was set at P G 0.05 for all
statistical procedures. Results are reported as mean and SD in
the text and SE in the figures.
RESULTS
Functional properties and pain assessment.
Significant reductions in maximum voluntary force (F3 = 9.1,
P G 0.0001) and time to task failure (F3 = 9.5, P G 0.0001)
were observed immediately after and 24 and 48 h after exer-
cise compared to baseline (Table 1). MVC and time to task
failure were not significantly different between the three post-
exercise sessions (immediately after and 24 and 48 h after,
P 9 0.05). Thigh circumference measured immediately after
and 24 and 48 h after exercise was larger than that in the mea-
sure before exercise (P G 0.001; Table 1). The average
reported pain intensity was 5.8 T 0.9 and 6.2 T 0.8 at 24 and
48 h after exercise, respectively.
Sustained contractions. The ARV of the EMG de-
creased during the sustained isometric contraction. The per-
cent decrease of ARV over time (in the final epoch with
respect to the initial epoch) depended on the session (F3 = 118,
P G 0.0001) and the interaction between session and muscle
(F6 = 3.3, P G 0.05), with a greater reduction identified for
the postexercise sessions (immediately after and 24 and 48 h
TABLE 1. Mean T SD (n = 10) for maximal voluntary contraction (MVC), time-to-task failure, and thigh circumference.
Baseline Immediately after 24 h after 48 h after
MVC (NIm) 674.1 T 133.6 489 T 107.6* 503.8 T 101.7* 486.1 T 56.2*
Task failure (s) 92.3 T 23.7 46.1 T 17.3* 57.9 T 15.2* 68.5 T 25.1*
Thigh circumference (cm) 41.7 T 2.9 42.4 T 3.7* 42.1 T 3.5* 42.2 T 3.9*
Values for all parameters were significantly different immediately after and 24 and 48 h after eccentric exercise with respect baseline.
* P G 0.05.
FIGURE 1—Percent decrease in the ARV of the EMG (mean T SE,
n = 10 subjects) over time for the vastus medialis (VM), rectus femoris
(RF), and vastus lateralis (VL) muscle during sustained contractions
performed at 50% MVC, recorded before the eccentric exercise (base-
line), immediately after, and 24 and 48 h after the eccentric exercise;
*P G 0.05.
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5. after) with respect to the session before exercise, and a larger
reduction identified for the VM muscle at the postexercise
sessions compared with RF and VL muscles (Fig. 1).
Rapid destabilizing perturbation. Figure 2 presents
representative surface EMG data detected from the distal
portion of the right vastus medialis muscle of one subject
during the destabilizing perturbations at baseline, immedi-
ately after, and 24 and 48 h after exercise. During the rapid
destabilizing perturbations, ARV depended on the pertur-
bation direction (F1 = 4.8, P G 0.05), session (F3 = 5.8, P G
0.001), muscle (F2 = 19.1, P G 0.0001), and the interaction
between session and muscle (F3 = 3.7, P G 0.05). The ARV
value (average over all muscles) was larger for the back-
ward perturbations compared with the forward direction
(P G 0.05). ARV values were smaller in the postexercise
sessions (immediately after and 24 and 48 h after) with re-
spect to the preexercise session (P G 0.001), and among the
knee extensor muscles, VL and VM had greater values of
ARV compared with the RF muscle (P G 0.001). Moreover,
VL and VM showed a higher decrease in ARV during the
postexercise sessions for the backward perturbation (imme-
diately after and 24 and 48 h after) with respect to the pre-
exercise session (P G 0.05) than the RF muscle (Fig. 3). For
the knee flexor muscles (MH, LH, MG, and LG), ARV
values for the postexercise sessions (immediately after and
24 and 48 h after) were not significantly different from
baseline (F3 = 2.2, P 9 0.05).
DISCUSSION
This study demonstrated lower EMG activity of the
DOMS-affected quadriceps muscle during rapid destabiliz-
ing perturbations immediately after and 24 and 48 h after
eccentric exercise. Moreover, a greater reduction of muscle
FIGURE 2—Example of surface EMG signals detected from the distal portion of the right vastus medialis muscle of one subject during destabilizing
perturbations before the eccentric exercise (A), immediately after (B), and 24 (C) and 48 h after the eccentric exercise (D).
FIGURE 3—ARV of EMG (mean T SE, n = 10 subjects) obtained from
175-ms epochs after the onset of plate movement (averaged for back-
ward and forward directions) for the vastus medialis (VM), rectus
femoris (RF), and vastus lateralis (VL) muscle, recorded before the
eccentric exercise (baseline), immediately after, and 24 and 48 h after
eccentric exercise; *P G 0.05.
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6. activity was observed during sustained contractions of the
quadriceps muscle for the postexercise sessions with respect
to the preexercise session. The results suggest that eccentric
exercise contributes to a reduced ability of the quadriceps to
stabilize the knee joint during a destabilizing event, indi-
cating that care should be taken to prevent knee injury when
training programs include a large number of heavily loaded
eccentric contractions.
Muscle performance. Maximal voluntary force and
time to task failure in the DOMS-affected quadriceps muscle
were significantly decreased immediately after eccentric
exercise and persisted at 24 and 48 h after exercise, which is
in agreement with previous studies on the same muscle
(13,14). A significant reduction in maximal isometric force
and time to task failure observed after exercise indirectly
suggests that the eccentric exercise used in this study con-
tributed to muscle damage and thus to the reduced muscle
power and physical work capacity. A force deficit after an
eccentric task can be explained by a failure in signal trans-
mission from higher motor centers to the muscle fiber (30)
and/or failure in signal conduction at the muscle fiber mem-
brane (15), most likely because of fatigue, fiber disruption,
and pain (23,25). After high-intensity eccentric exercise, an
increase in extracellular potassium, accumulation of metab-
olites (e.g., acid lactate, inorganic phosphate), and/or fiber
membrane disruption would decrease muscle fiber excit-
ability (15) and sensitize intramyofibril group III and IV
afferents (18), contributing to decreased maximal force
generation capacity. The presence of pain within the injured
muscle can also inhibit motor neurons at spinal and supra-
spinal levels, which, in turn, results in reduced motor unit
recruitment and discharge rate (18) and, consequently, re-
duced maximal isometric force. The subjects reported sore-
ness in the quadriceps muscle 24 and 48 h after exercise,
which may be related to damage of the contractile elements
and connective tissue (26).
Sustained contractions. A greater decrease in EMG
amplitude over time was also observed during the postex-
ercise sustained contraction with respect to the preexercise
session. Previous studies have also demonstrated a greater
EMG amplitude rate of reduction over time during sustained
contraction after eccentric exercise (13,28). A larger reduc-
tion of ARV after eccentric exercise may be related to the
inhibitory effect mediated by nociception both at the corti-
cal and spinal levels (23), which, in turn, reduces motor unit
discharge rate (11) and, consequently, results in a decreased
drive to the muscle fibers (13). The decreased ARV can be
also explained by a failure in motor unit recruitment required
to compensate for contractile failure caused by fatigue (22).
Postural perturbations. After eccentric exercise, pain
manifested and the EMG activity of knee extensor muscles
(affected by DOMS) decreased during the potentially des-
tabilizing perturbations, with respect to baseline. However,
in the knee flexor muscles (not affected by DOMS), EMG
activity was not significantly different from baseline. This
result indicates that the level of coactivation between the
knee extensors and flexors is altered after eccentric exer-
cise, which likely contributes to reduced knee joint stiffness
during rapid destabilizing movements. Altered coactivation
could result in joint instability or laxity, imposing an ab-
normal load on the structures of the knee, thus leaving them
more susceptible to injury.
The current study is the first study in which muscle activa-
tion is measured during perturbations after eccentric exercise–
induced DOMS. Previously, it has been demonstrated that
higher background muscle activity yields increased reflexive
joint stiffness, potentially providing greater joint stability (35).
Therefore, lower background muscle activity observed in the
DOMS-affected quadriceps muscle after eccentric exercise
could result in reduced reflexive joint stiffness through de-
creased muscle stiffness. The reduced reflex muscle activa-
tion immediately after exercise could be explained by
fatigue-induced changes within the muscle. The strenuous
fatiguing exercise used in this study could have altered
muscle fiber properties (19,20). Fatigue alters the behavior
of intrafusal chain and static bag fibers, which, in turn,
results in a reduction in Ia afferent inflow from mus-
cle spindles. Fatigue could also change force-feedback medi-
ated by the Golgi tendon organ and thus contribute to
the inhibition of spinal motor neurons (3). Moreover, myo-
fibrillar ATPase activity is reduced after fatigue because of
metabolic accumulation, which would likely reduce the de-
tachment rate of cross bridges and muscle contraction.
The observed change in reflex muscle activation after exer-
cise may be related to disturbance in the F-motoneuron system
involved in regulating muscle stiffness. The F-motoneuron
system controls muscle stiffness through sensory information
from muscles, ligaments, the joint capsule, and skin. This
afferent input has a strong effect on the F-motoneuron system
that provides continuous preparatory adjustments to muscle
stiffness (17,36). In the injured muscle, articular and peri-
articular afferent input is likely to be altered as a result of
muscle inflammation and/or increased tension in the joint
capsule (6,33,37), and this may result in a reduced capacity
of the F-motoneuron system to control muscle activity.
There are several different mechanisms by which patho-
physiological changes within the skeletal muscle may con-
tribute to decreased reflex muscle activity. For example,
decreased reflex muscle activity after eccentric exercise can
be explained by alterations in nociceptive sensitization of the
painful muscle. Altered nociceptor sensitization associated
with tissue injury can influence primary afferents of muscle
spindles in the superficial layers of the dorsal horn of the
spinal cord after eccentric exercise (42). Input from nocicep-
tive afferents inhibits the input of muscle spindles by pre-
synaptic inhibition (5) and, consequently, leads to decreased
motor unit discharge rate (11) and muscle activity (13). It has
been reported that pain by itself may lead to instability of a
joint (7,10) by inhibition of the motor system excitability
both at the cortical and the spinal levels (23). Reflex inhibi-
tion has also been demonstrated in the presence of pain in-
duced by infusion of fluid into knee extensor muscles during
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7. isometric (9) and isokinetic contractions (2). In addition,
when the quadriceps is injured, there is an arthrogenic re-
flex inhibition that contributes to a decreased ability of this
muscle to achieve full voluntary activation (39). Pain in-
duced by experimental infusion has also been reported to
reduce the H reflex (the monosynaptic reflex, motor re-
sponse to electrical stimulation of spindle afferent fibers) in
the quadriceps muscle at rest (38) and against a background
of muscle activity (16). The decreased H reflex has been at-
tributed to the reduced excitation level of the quadriceps’
anterior horn cells (34).
The reduction in reflex muscle activity after an eccentric
exercise may also be due to the tonic descending inhibition
process. There is a supraspinal loop by which noxious stim-
uli dynamically activate the descending inhibition of multi-
receptive dorsal horn cells, which is tonic in nature (12).
There is evidence that, after muscle injury, pain and inflam-
mation (26,37) increase the amount of tonic descending inhi-
bition (33), resulting in decreased afferent input from regions
of the inflamed knee (6) and, consequently, reduced muscle
activity. The increased tonic descending inhibition mediated
by nociceptive afferents has been considered to contribute to
protection of the injured tissue from further insult (29).
The vastus lateralis and vastus medialis muscles showed
a greater decrease in muscle activity during the postexercise
destabilizing perturbation. This may be related to a more
extensive damage of these regions, most likely due to high
force production in these areas to stabilize the patella during
eccentric exercise (31).
Limitations. The amplitude of the EMG can be influ-
enced by several physiological (e.g., motor unit synchroniza-
tion, muscle fiber conduction velocity) and nonphysiological
(e.g., electrode orientation) factors. Although care was taken
to place the electrodes in the same locations when the sub-
jects returned to the laboratory 24 and 48 h after exercise,
some factors may vary from session to session, which can
increase the variability of the results.
CONCLUSIONS
Maximal eccentric knee extension exercise resulted in re-
duced activity of the quadriceps muscle in response to rapid
destabilizing perturbations, most probably due to muscle fiber
disruption and pain. This finding suggests that eccentric ex-
ercise can impair reflex activity in the quadriceps and may
compromise knee stability, therefore leaving structures of the
knee more vulnerable to injury.
Funding was not received for this work from the National Institutes
of Health, Wellcome Trust, Howard Hughes Medical Institute, or
others.
The authors declare no conflict of interest.
The authors thank Prof. Dario Farina for his useful comments
on the text.
The results of the present study do not constitute endorsement
by the American College of Sports Medicine.
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