Download Motor impairment in the human hand following eccentric exercise

Eur J Appl Physiol (2001) 84: 213±220
Ó Springer-Verlag 2001
ORIGINAL ARTICLE
Andrew B. Leger á Theodore E. Milner
Motor impairment in the human hand following eccentric exercise
Accepted: 19 September 2000
Abstract Motor impairment was induced by having
subjects perform two sets of 50 maximal contractions,
using the ®rst dorsal interosseus (FDI) muscle to abduct
the index ®nger, while the muscle was being stretched.
Tests were conducted prior to the exercise (pre-exercise)
and 24 h following the exercise (post-exercise). There
were declines of 19% in maximal abduction torque and
15% in maximal ¯exion torque at the metacarpaphalangeal joint, during isometric contraction post-exercise
compared to pre-exercise. The ability to stabilize the
metacarpophalangeal joint about the abduction/adduction axis was reduced by 14% post-exercise, and the
variability in tracking an isometric torque target
increased by 30%. There was a decrement of 7%±10%
in the median frequency of the power density spectrum
of FDI electromyogram (EMG) throughout a 60 s
maintained abduction at 50% maximal voluntary
contraction. The mean recti®ed EMG, on the other
hand, increased by 100%±175% for torque levels below
40% of maximal voluntary contraction, post-exercise.
The results were consistent with preferential injury of
type II muscle ®bres in FDI. Although non-exercised
synergist muscles appeared to be inhibited during
maximal voluntary ¯exion, there was evidence that
they compensated for injured FDI muscle ®bres during
maintained contraction at sub-maximal ¯exion torque.
Key words Muscle injury á Movement á
Stability á Finger
A. B. Leger (&)
School of Rehabilitation Therapy,
Queen's University, Kingston,
Ontario K7L 3N6, Canada
e-mail: [email protected]
Fax: +1-613-5336776
T. E. Milner
School of Kinesiology,
Simon Fraser University, Burnaby,
British Columbia V5A 1S6, Canada
Introduction
Lengthening of active muscle, often referred to as
eccentric contraction, occurs commonly in normal activities, for example, when slowly lowering an object held
in the hand. Active muscle can develop much larger
forces during lengthening, than during isometric or
shortening contraction (Katz 1939). While this is advantageous for controlling the motion of objects which
are being lowered under the in¯uence of gravity, it creates greater risk of injury to muscle or tendon than activities which involve only isometric or concentric
contraction (Newham et al. 1983; Clarkson et al. 1986;
Berry et al. 1990; Faulkner et al. 1993). A number of
studies have shown that indicators of injury, induced by
eccentric loading of muscle, occur more frequently in
large, fast-twitch (type II) muscle ®bres than in the
smaller, slow-twitch (type I) ®bres (FrideÂn et al. 1983;
Lieber and FrideÂn 1988).
While the selectivity of injury to type II muscle ®bres
has received considerable attention, investigation of
functional impairment, following eccentric exercise, has
been limited. It is clear that eccentric exercise a€ects
maximal force. Following a period of eccentric exercise,
maximal isometric force has been found to decrease by
as much as 50% (Clarkson et al. 1992) before recovering gradually over a period of days to weeks (Newham et al. 1987; Ebbeling and Clarkson 1989; Howell
et al. 1993). However, most activities of daily living
require sub-maximal muscle forces. Studies examining
the e€ects of eccentric exercise on sub-maximal muscle
forces have focused almost entirely on static tasks
(Komi and Viitasalo 1977; Newham et al. 1983; Berry
et al. 1990; Kroon and Naeije 1991). Our ®rst objective
was to obtain a clear picture of motor impairment,
following eccentric exercise, by examining dynamic
tasks.
While changes to the pattern of activation of synergist muscles have been reported following eccentric
exercise (Hasson et al. 1993), previous protocols only
214
examined synergist muscles which had all been
eccentrically exercised. No study has been designed to
exercise a single muscle and then investigate activities in
which both exercised and non-exercised muscles participate synergistically. Such situations may occur in activities such as weight training. For example, the
anterior deltoid muscle is a prime mover during the
bench press, as well as being activated eccentrically each
time the weight is lowered. The posterior deltoid muscle,
on the other hand, is relatively inactive during this manoeuvre. However, during other movements, such as
shoulder abduction, the two muscles act as synergists.
Thus, the two muscles may be used synergistically after
only one of them has been eccentrically exercised. Our
second objective was to provide a more comprehensive
description of motor impairment by investigating the
e€ects of eccentric exercise of one muscle on the function
of a non-exercised synergist muscle.
To achieve the ®rst objective, we examined the e€ects
of eccentric exercise on the ability to perform activities
such as ballistic movement, postural stabilization and
force tracking. Subjects performed a period of strenuous
eccentric exercise, during which the ®rst dorsal interosseus (FDI) muscle was stretched while attempting to
abduct the metacarpophalangeal (MP) joint of the index
®nger. Because FDI contributes to both abduction and
¯exion of the MP joint, we were able to quantify motor
impairment for the two actions. This allowed us to
achieve our second objective by examining tasks
involving ®nger ¯exion. In this way, we were able to
determine whether eccentric exercise of FDI, in abduction, a€ected synergist ¯exor muscles, which did not
contribute to abduction.
Methods
Fig. 1 Diagram of apparatus used for metacarpophalangeal joint
abduction of the index ®nger
Fig. 2 Diagram of apparatus used for metacarpophalangeal joint
¯exion of the index ®nger. EMG (FDS) Electromyogram (¯exor
digitorum super®cialis muscle)
Subjects
Ten healthy male subjects, with no history of neuromuscular disease or injury involving the index ®nger, participated in this study
(age range 22±31 years). Nine of the subjects were right-handed,
the other left-handed. Each gave written informed consent to
participate prior to the experiment. None had previously participated in any studies involving eccentric exercise. The subjects were
asked not to participate in any weight-training activities, speci®cally for the upper extremities, for the duration of the study. The
experiment was approved by the University Research Ethics
Review Committee at Simon Fraser University. All experiments
were conducted in compliance with Canadian Research Council
guidelines for experimentation using human subjects.
General design
The subjects were tested on two separate occasions, prior to
eccentric exercise (pre-exercise), and 24 h later (post-exercise). To
ensure reproducible placement of electromyogram (EMG) electrodes on both days, indelible ink was used to mark the skin. To
ensure accurate positioning in the test apparatus, the subject's
forearm and hand were supported and strapped in a sti€ elastoplast
splint, moulded to the shape of the hand (Figs. 1 and 2). All tests
were performed on the left hand.
Apparatus
A torque motor (PMI U16M4) was used to generate loads, which
were computer controlled. The maximal torque that could be
produced by the motor was 5 N á m. Position and velocity were
measured by a potentiometer and tachometer, respectively. The
torque was measured by a linear strain gauge mounted on a
cylinder, coupling the motor shaft to a lever arm. The resolution of
the torque sensor was 0.004 N á m.
Recording
Surface EMG was recorded from the FDI, ¯exor digitorum
super®cialis (FDS) and extensor digitorum communis (EDC)
muscles. The FDS is an extrinsic ¯exor and EDC an extrinsic extensor of the MP joint. The electrode was initially positioned over
the centre of the muscle in alignment with the muscle ®bres, approximately midway between origin and insertion. It was then
moved incrementally in di€erent directions to obtain the largest
signal while the subject performed brisk test movements of the
index ®nger. The ®nal placement varied from subject to subject,
depending on where the largest signal was obtained. Test movements comprised isolated MP abduction, ¯exion and extension.
215
Active bipolar surface electrodes were used (Liberty Mutual MYO
111) consisting of stainless steel contacts, 4 mm in diameter, at a
®xed spacing of 13 mm. The active electrode circuit had a passband
of 45±550 Hz, gain of 2,400 and common mode rejection greater
than 90 dB. The EMG, position, velocity and torque were simultaneously recorded, digitized at 2 kHz and stored on disk for later
analysis.
Exercise protocol
The forearm and wrist were immobilized in a pronated position
by the splint. The tip of the index ®nger was clamped and secured
to the lever arm of the motor and the ®nger was splinted to
prevent MP ¯exion. The ®nger adduction-abduction axis was
aligned with the axis of rotation of the motor (Fig. 1). The subjects performed two sets of ®fty maximal eccentric contractions,
using the FDI muscle to abduct the index ®nger. During each
repetition, the subjects performed a maximal isometric contraction, then resisted maximally as a 20° displacement was imposed
by the torque motor to stretch the muscle over a period of 3 s.
There was a 3 s break between successive repetitions and 5 min
rest between the two sets. The subjects were provided with visual
feedback of their torque levels and encouraged verbally to produce maximal e€ort.
Test procedures
Each subject was seated comfortably in a chair with the left forearm and hand splinted and supported in a comfortable position,
as described above. To avoid fatigue during the test session, the
subjects relaxed for at least 30 s after each trial and for up to 5 min
between di€erent tests. The total duration of the session was
60±75 min. For tests performed during MP ¯exion, the forearm
was orientated midway between pronation and supination with the
wrist, hand and three remaining ®ngers splinted to allow only
movement of the index ®nger in ¯exion/extension (Fig. 2). The
hand was positioned such that the axis of rotation of the MP joint
was directly over the shaft of the motor. A clamp at the end of
the lever arm was used to hold the distal phalanx of the ®nger.
The ®nger was taped to prevent any movement of the proximal
and distal interphalangeal joints.
The tests were performed in the sequence listed below for all
subjects.
MP abduction
1. Ballistic movement. A target window, which represented the
neutral angle of the index ®nger, was displayed on the computer screen. The subject was instructed to move a cursor
representing the position of the ®nger into the target window.
Once in the target window for 1 s, the cursor changed colour
and the subject abducted the ®nger as rapidly as possible,
contacting a mechanical stop placed at the end of the range of
movement. The subject was encouraged to accelerate
throughout the entire movement. The subject was provided
with feedback of his maximal velocity following each trial. Five
trials were collected.
2. Postural stability. To determine the limit of postural stability of
the MP joint, the subject was required to move the index ®nger
into a 1° target window, located at the neutral angle, while the
torque motor produced a destabilizing load. The load increased
in proportion to the distance from the centre of the target
window and was always directed away from the target. The limit
of stability was determined by systematically increasing the
proportionality constant of the load, i.e. the gain. A trial was
considered successful if the ®nger could be positioned in the
target window within 20 s of the cue to start and held there for a
least 1 s. If successful, the gain was incremented by
0.01 N á m á °)1 for the next attempt.
3. Maximal voluntary contraction (MVC). The motor was locked
at the neutral angle of the index ®nger. The subject warmed up
in preparation for maximal e€ort by performing several submaximal isometric contractions, building from about 50% to
80% of maximal e€ort. Two or three warm-up contractions
were generally performed, although the number varied according to the preference of each subject. Three trials at maximal
e€ort, each lasting 3 s, were then performed and stored for later
analysis.
4. Torque tracking. While the motor was locked at the neutral
angle of the index ®nger, the subject contracted the FDI muscle
isometrically to produce MP abduction torque. The target was
initially 0 and increased at a constant rate over a period of 5 s to
65% of the torque recorded during pre-exercise MVC. The
subject was instructed to match the increasing target torque as
closely as possible. Five trials were collected.
5. Maintained contraction. While the motor was locked at the
neutral angle of the index ®nger, the subject maintained a
constant isometric MP abduction torque for 60 s, equal to 50%
of the value recorded during pre-exercise MVC. The subject's
torque and the target torque were displayed on the oscilloscope.
Only one trial was performed.
MP ¯exion
6. MVC. The same protocol as test 3 with the subject ¯exing the
index ®nger.
7. Maintained contraction. The same protocol as test 5 with the
subject ¯exing the index ®nger.
8. Ballistic movement. The same protocol as test 1 with the subject
¯exing the index ®nger.
Analysis
Ballistic movement
Trials for each subject were analysed individually to obtain peak
velocity and time of peak velocity. Movement onset was de®ned as
the time when velocity exceeded 5° á s)1. The values for each
parameter were averaged for the ®ve trials to produce a mean value
for each subject.
Postural stability
The limit of stability was de®ned as the maximal gain for which the
subject was able to successfully complete the task. The mean recti®ed EMG (MEMG) was determined for each muscle over a
250 ms interval while the subject was stable in the target window.
Torque tracking
The ®ve trials for each subject were individually analysed to
determine the MEMG during each 1 s interval of the 5 s tracking
task. The variance about the best-®t straight line from beginning to
end of the torque tracking was also determined (expressed in
Newton metres squared). The mean parameters for the ®ve trials
were then calculated.
Maintained contraction
During the 60 s contraction, the median frequency (MDF) of the
EMG power density spectrum for each muscle was computed over
5 s intervals, using a computer program written by one of the
authors. A 5 s interval was divided into nine segments of
1,024 points each, which overlapped by half their length. The data
was windowed using a Hanning window prior to computing the
216
fast Fourier transform. The power density spectrum, obtained by
averaging the nine resulting spectra, reduced the variance of the
spectrum estimation by a factor of about 7.5 compared to using a
single segment. The slope of MDF plotted against time was then
determined. The MEMG was computed for the ®rst and last 5 s
intervals.
The e€ects of eccentric exercise were evaluated by comparing
parameter values pre- and post-exercise for each task using paired
Student's t-tests.
Results
There was no signi®cant di€erence in the maximal
velocity of ballistic MP abduction (P > 0.6) or latency
from movement onset to peak velocity (P > 0.5) postexercise [mean 487 (SD 98)° á s)1; mean 76 (SD 8) ms]
from pre-exercise [mean 513 (SD 123)° á s)1; mean 74
(SD 8) ms]. The maximal velocity of ballistic ®nger
¯exion showed a small, but statistically signi®cant
decrement (P < 0.05) post-exercise [mean 992 (SD
176)° á s)1] compared to pre-exercise [mean 1,040
(SD 142)° á s)1], but there was no signi®cant di€erence
in latency to peak (P > 0.3) post-exercise [mean 74
(SD 7) ms] compared to pre-exercise [mean 71 (SD
6) ms].
Postural stability decreased following eccentric
exercise. The mean limit of stability, which was
0.118 (SD 0.017) N á m á °)1 pre-exercise, was reduced
to 0.101 (SD 0.019) N á m á °)1 post-exercise (P <
0.001). The MEMG during the period of stabilization
was not signi®cantly di€erent for the FDI (P > 0.7),
FDS (P > 0.8) or EDC (P > 0.5) muscles, pre-exercise
[FDI mean 25.0 (SD 15.7) lV; FDS mean 26.8
(SD 20.3) lV; EDC mean 26.8 (SD 20.3) lV], compared to post-exercise [FDI mean 27.2 (SD 18.2) lV;
FDS mean 28.8 (SD 19.8) lV; EDC mean 34.0
(SD 30.4) lV].
Maximal torque during MVC in abduction was reduced from a mean of 2.15 (SD 0.35) N á m pre-exercise
to 1.74 (SD 0.28) N á m post-exercise, a decline of 19%
(P < 0.001). This was accompanied by a signi®cant
decrease in FDI MEMG from a mean of 75.2
(SD 35.7) lV pre-exercise to 61.7 (SD 22.5) lV postexercise (P < 0.05). Maximal torque during MVC
in ¯exion also decreased, from a mean of 4.84
(SD 1.35) N á m to 4.11 (SD 1.24) N á m, a decline of
15% (P < 0.001). However, there was no signi®cant
change in MEMG of FDI (P > 0.7), FDS (P > 0.9) or
EDC (P > 0.8) pre-exercise [FDI mean 57.9 (SD
39.8) lV; FDS mean 61.3 (SD 50.3) lV; EDC mean
14.3 (SD 34.1) lV] compared to post-exercise [FDI
mean 53.1 (SD 29.6) lV; FDS mean 58.7 (SD 52.6) lV;
EDC mean 11.4 (SD 19.2) lV]. The reduction in maximal voluntary torque after 24 h is unlikely to have been
due to prolonged e€ects of fatigue, i.e. diminished force
generating capacity of healthy muscle ®bres. This was
tested by means of repeated maximal isometric abduction of the ®nger, which reduced the maximal voluntary
torque by approximately 30% immediately after exercise, but showed no prolonged e€ect. Maximal volun-
tary isometric torque was normal when measured 24 h
later (unpublished observations).
The torque tracking performance pre- and postexercise is illustrated in Fig. 3. Variability in tracking a
target torque was greater post-exercise [mean 27.4 (SD
16.5) N á m2] than pre-exercise [mean 19.2 (SD 12.6)
N á m2; P < 0.05]. This was accompanied by signi®cantly higher FDI MEMG for the ®rst 4 s of the
tracking (P < 0.001, P < 0.001, P < 0.01 and P <
0.05, respectively), as illustrated in Fig. 4.
There was a strong linear correlation of MDF related
to time for FDI EMG during maintained abduction at
50% MVC, as has been previously reported in muscle
fatigue tests (Kranz et al. 1985). The mean correlation
coecient for all subjects was 0.96 (SD 0.03) pre-exercise and 0.95 (SD 0.05) post-exercise. In all cases, the
slope was signi®cantly less than 0, indicating a gradual
decrease in MDF throughout the contraction (Fig. 5).
However, there was no signi®cant di€erence between
slopes, pre- and post-exercise. This suggests that the
FDI muscle did not fatigue at a faster rate post-exercise
than pre-exercise. Post-exercise, MDF was lower
Fig. 3 Metacarpophalangeal joint abduction torque and electromyogram of ®rst dorsal interosseus muscle (FDI EMG) while tracking
the same ramp increase in target torque, pre-exercise (A, B) compared
to post-exercise (C, D). The increase in torque is less smooth postexercise, particularly at higher torque levels. Traces are single trials
for the same subject
217
Fig. 4 Mean recti®ed electromyogram (MEMG), calculated for 1 s
intervals while tracking a ramp increase in metacarpophalangeal joint
abduction torque. MEMG is expressed as a percentage of maximal
voluntary contraction (%MVC) electromyogram (EMG) on the day
of recording. Values are means and standard deviations for ten
subjects. *Statistically signi®cant di€erences pre- and post-exercise.
FDI First dorsal interosseus muscle
throughout the contraction. Initial MEMG was signi®cantly greater than pre-exercise, but there was no
signi®cant di€erence in ®nal MEMG. Parameter values
are listed in Table 1.
In the case of maintained ¯exion at 50% MVC,
there was again a strong linear correlation between
MDF and time for FDI EMG. The mean correlation
coecient for all subjects was 0.95 (SD 0.04) pre-exercise and 0.96 (SD 0.03) post-exercise. In all cases,
the slope was signi®cantly less than 0. However, there
was no signi®cant di€erence between pre-exercise and
Fig. 5 Median frequency of power density spectrum of ®rst dorsal
interosseus muscle electromyogram (FDI EMG) calculated at 5 s
intervals during maintained abduction of metacarpophalangeal joint
at 50% of pre-exercise maximal voluntary contraction. Values are
means and standard deviations for nine subjects. Statistical tests were
conducted only on initial and ®nal values. *Statistically signi®cant
di€erences pre- and post-exercise
post-exercise values of initial MDF, ®nal MDF or
slope. Nor were there any signi®cant di€erences in
initial and ®nal MEMG post-exercise compared to
pre-exercise (Table 1). Since FDS is a synergist of FDI
in ¯exion, we compared FDS EMG pre- and postexercise. The linear correlation between MDF and
time was weaker for FDS than FDI. Pre-exercise, the
slope was signi®cantly di€erent from 0 for only ®ve
out of the ten subjects, although post-exercise it was
signi®cantly less than 0 for seven subjects. The mean
correlation coecient for these subjects was
0.84 (SD 0.10) pre-exercise and 0.92 (SD 0.06) postexercise. There were no signi®cant di€erences between
pre- and post-exercise values of mean initial MDF,
®nal MDF, slope or initial MEMG for FDS. However, mean ®nal MEMG was greater post-exercise. We
also compared EDC MEMG pre- and post-exercise,
but found no signi®cant di€erence in either initial or
®nal mean values.
Discussion
At 24 h following a period of strenuous eccentric exercise, involving index ®nger abduction, both maximal
voluntary abduction torque and maximal voluntary
¯exion torque at the MP joint were signi®cantly lower
than pre-exercise. There were also impairments in postural stability and the ability to regulate muscle force
precisely. As a whole, the impairments were consistent
with preferential injury of type II muscle ®bres in the
FDI and suggest that functional impairment was not
limited to the exercised muscle.
One sign of impaired control was a de®cit in the
ability to stabilize the MP joint against an externally
imposed mechanical instability. Mechanical stability
has usually been achieved by cocontraction of antagonistic muscles to increase joint sti€ness (Hogan 1984;
DeSerres and Milner 1991). The ®nding that maximal
joint stability was reduced post-exercise suggests that
the decrement in maximal joint torque was also accompanied by a decrement in maximal joint sti€ness.
The ability to activate the exercised muscle did not
appear to be compromised since FDI EMG was
comparable, pre- and post-exercise. The results of a
parallel study, in which we found that short range
sti€ness decreased 24 h following eccentric exercise
(Leger and Milner 2000), suggest that the de®cit in
sti€ness was due to preferential injury of type II
muscle ®bres.
Greater MEMG at the onset of torque tracking postexercise, indicated increased activity of low threshold
type I motor units. The elevated level of activity of type
I motor units was probably maintained throughout the
tracking task, accounting for the increased MEMG.
This is supported by the overall shift to lower MDF
during maintained isometric contraction at 50% MVC.
This shift in MDF could not have been due to a change
in the recording con®guration or electrode location
218
Table 1 Median frequency (MDF) and mean recti®ed electromyogram (MEMG) during maintained contraction at 50% maximal
voluntary contraction. FDI First dorsal interosseus, FDS ¯exum digitorum super®cialis, EDC extensor digitorum communis muscles
Task
Abduction
Abduction
Abduction
Abduction
Abduction
Flexion
Flexion
Flexion
Flexion
Flexion
Flexion
Flexion
Flexion
Flexion
Flexion
Flexion
Flexion
a
Muscle
FDI
FDI
FDI
FDI
FDI
FDI
FDS
FDI
FDS
FDI
FDS
FDI
FDS
EDC
FDI
FDS
EDC
Parameter
Initial MDF
Final MDF
MDF Slope
Initial MEMG
Final MEMG
Initial MDF
Initial MDF
Final MDF
Final MDF
MDF Slope
MDF Slope
Initial MEMG
Initial MEMG
Initial MEMG
Final MEMG
Final MEMG
Final MEMG
Units
Hz
Hz
Hz á s)1
lV
lV
Hz
Hz
Hz
Hz
Hz á s)1
Hz á s)1
lV
lV
lV
lV
lV
lV
Pre-exercise
Signi®cancea
Post-exercise
Mean
SD
Mean
SD
126
84
)0.83
43.2
35.8
132
180
98
168
)0.65
)0.25
29.3
15.9
4.3
35.3
25.2
5.5
22
18
0.36
19.1
18.8
30
27
29
24
0.39
0.14
21.9
17.1
2.5
23.4
15.9
3.3
113
75
)0.71
56.4
29.7
127
176
89
160
)0.73
)0.31
32.4
15.6
4.7
35.8
35.8
8.8
28
13
0.34
27.6
9.8
22
30
32
40
0.37
0.48
17.6
7.6
3.7
26.7
21.8
6.8
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
P
<
<
>
<
>
>
>
>
>
>
>
>
>
>
>
<
>
0.05
0.05
0.4
0.05
0.3
0.6
0.7
0.5
0.5
0.6
0.7
0.7
0.95
0.7
0.95
0.05
0.1
Signi®cance levels are for Student's t-tests of paired comparisons
since these parameters were the same pre- and post-exercise. Nor could it have been due to broadening of the
motor unit action potentials since we found that the
compound action potential (M-wave) did not increase in
duration (Leger et al. 1996). Therefore, it was most
likely indicative of greater activity of type I motor units
since type I muscle ®bres have lower conduction velocities than type II muscle ®bres. The lower conduction
velocities have been shown to result in broader action
potentials, reducing the MDF of the EMG power density spectrum (De Luca 1984). Felici et al. (1997), investigating MDF of the biceps muscle during
maintained contraction at 50% and 80% MVC, also
found a decrease in MDF for several days following
eccentric exercise.
A reduction in the MDF of the surface EMG power
density spectrum normally occurs during a maintained
fatiguing contraction, which is probably also due to
broader action potentials, resulting from decreased
muscle ®bre conduction velocity (De Luca 1984). It has
been reported that mean frequency drops at a faster rate
during maintained contraction, following eccentric exercise, due to increased fatigability of the muscle (Kroon
and Naeije 1991). However, we found that MDF of FDI
EMG declined at the same rate, during a 60 s maintained contraction at 50% MVC, pre- and post-exercise,
indicating that fatigability was the same. This again
suggests that fatigue resistant type I motor units make a
greater contribution to the total muscle force postexercise, which is more in agreement with the ®ndings
of Balnave and Thompson (1993) for the quadriceps
muscles.
In the post-exercise tracking test, torque was more
variable than pre-exercise. This impairment was most
evident at the higher torque levels where type II motor
units would have been recruited. Although several factors could have been responsible, including decreased
perception of muscle force or tremor (Saxton et al.
1995), injury to type II muscle ®bres is likely to have
contributed signi®cantly. Orderly recruitment of new
motor units, according to the size principle, tends to
produce increments in force, which are a ®xed percentage of the force at the time of recruitment. Recruitment
of an injured motor unit would produce a smaller increment in force than normal, thereby reducing the force
at which each higher threshold motor unit was recruited.
Uninjured higher threshold motor units would produce
their usual increment in force at recruitment, but
superimposed on a lower force than normal. Thus, the
percentage increment in force provided by an uninjured
motor unit would be greater than normal after eccentric
exercise, resulting in more uneven development of
muscle force.
Surprisingly, the decrease in maximal voluntary
torque did not result in a signi®cant decrease in peak
velocity during ballistic MP abduction. This is in contrast to the study of Miles et al. (1993), investigating
control of elbow ¯exion. They found that peak velocity
decreased and latency from movement onset to peak
velocity increased, 24 h after eccentric exercise. The
failure to ®nd a signi®cant decrease in peak velocity
might have been a consequence of the novelty of the
task. Rapid ®nger abduction is not a commonplace activity, especially when performed by the non-dominant
hand. Any decrement in velocity post-exercise may have
been o€set by an increment in performance, due to
practice. Such an interpretation is consistent with the
®nding that the peak velocity of MP ¯exion did decrease
signi®cantly post-exercise. Ballistic ®nger ¯exion is more
familiar, associated with activities such as ®nger tapping
219
and typing. Consequently, it would be less a€ected by
practice.
The decline in maximal voluntary ¯exion torque
was greater than expected from the estimated decline
in FDI ¯exion torque. Thomas et al. (1986) have
shown that motor units in FDI are recruited in the
same order for MP ¯exion and abduction. Therefore,
any decrement in abduction torque would be expected
to a€ect ¯exion torque. Based upon experiments employing muscle block with lidocaine by Ketchum et al.
(1978), the estimated contribution of FDI to total MP
¯exion torque would be approximately 39%. Zijdewind and Kernell (1994), on the other hand,
concluded from anatomical and biomechanical measurements, that the FDI muscle moment potential was
only 15% of the total ¯exion torque. Assuming that
FDI is the only muscle contributing to abduction of
the index ®nger and using the estimate of Ketchum
et al. (1978) the 19% decrement in maximal abduction
torque which we observed, should have produced a
decrement in maximal ¯exion torque of 7.4%. Using
the value suggested by Zijdewind and Kernell (1994),
the expected decline would have been even lower.
Thus, the decrement in MP ¯exion torque was at least
twice that predicted by the impaired force production
of FDI. This suggests that the e€ect of eccentric exercise of the FDI muscle was to inhibit the extrinsic
synergist muscles during MP ¯exion. It is unlikely that
the extrinsic ®nger muscles could have been injured by
the eccentric exercise since they produce adductor
moments at the MP joint. Thus, the eccentric exercise
in abduction would have caused them to shorten
rather than lengthen. Since EDC EMG during maximal isometric ¯exion did not increase post-exercise, it
is unlikely that the reduction in maximal ¯exion torque could have been due to increased contraction of
antagonist muscles.
During MP ¯exion at 50% MVC there was greater
FDS activity late in the maintained contraction, following eccentric exercise. In addition, more subjects
showed a signi®cant correlation between MDF and time
post-exercise, which suggests that FDS muscle ®bres
were being activated for a suciently long time to show
greater evidence of fatigue. It would, therefore, appear
that increased activity of the extrinsic ®nger ¯exor
muscles was used to compensate for the FDI force de®cit
during MP ¯exion.
The results of this study indicate that a period of
eccentric exercise, which is suciently strenuous, can
produce de®cits in muscle mechanics that a€ect the
ability to stabilize a joint and the ability to control
muscle force smoothly. They also suggest that nonexercised synergist muscles can compensate for de®cits
in joint torque at sub-maximal activation levels, but that
these muscles exhibit inhibition during maximal activation. Because strenuous eccentric exercise has been
shown to result in damage to muscle ®bres (FrideÂn et al.
1983; Lieber and FrideÂn 1988) leading to pain (Newham
et al. 1983), the e€ects of eccentric exercise on motor
control may also serve as a model for impaired control
following musculoskeletal injury.
Acknowledgement This work was supported by the Natural Sciences and Engineering Research Council of Canada.
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