The effect of muscle length on motor unit recruitment during
isometric plantar flexion in humans
P.M. Kennedy 1 and A.G. Cresswell 2
1
School of Human Kinetics,
The University of British Columbia,
Vancouver, Canada
2
Department of Neuroscience,
Karolinska Institute
Stockholm, Sweden
To be published in: Experimental Brain Research
Keywords: single motor unit, triceps surae, gastrocnemius, electromyography, intramuscular
Abstract
The triceps surae muscle group consisting of the mono-articular soleus (SOL) and bi-articular
gastrocnemius (GAS) muscles primarily generates plantar flexor torque. Since the GAS
muscle crosses the knee joint, flexion of the knee reduces the length of this muscle, thus
limiting its contribution to torque output. However it is not clearly understood how the
central nervous system activates muscles that are at inefficient or non-optimal force
producing lengths. Therefore, the present study was designed to determine the effect of
muscle length on motor unit recruitment in the medial GAS muscle. Single motor unit
activity was recorded from the medial GAS muscle while electromyography (EMG) was
recorded from the SOL muscle in nine male subjects. With the ankle angle held constant at
90º, the knee angle was changed from 180º to 90º corresponding to a long and short GAS
muscle length respectively. Levels of voluntary plantar flexor torque were produced at a rate
of 2 Nm · s -1 until motor unit activity was detected. A total of 229 motor units were recorded
of which 121 and 108 were obtained at the long and short muscle lengths, respectively. At the
short length, onset of motor unit activity occurred at significantly higher levels of plantar
flexor torque and SOL EMG. Motor unit onset occurred at 2.97 ± 7.78 Nm and 32.14 ±
10.25 Nm, corresponding to 0.045 ± 0.075 mV and 0.231 ± 0.129 mV of SOL EMG in the
long and short position respectively. No individual GAS motor unit could be recorded at
both muscle lengths. Motor units in the shortened GAS muscle, may be influenced by
peripheral afferents capable of reducing the excitability of the motoneurone pool. This may
also reflect a specific inhibition of motor units having shortened, non-optimal, fascicle
lengths and are thereby incapable of contributing to plantar flexor torque.
1
Introduction
The force that a muscle can produce is dependent not only on the neural commands generated
by the central nervous system (CNS) but also on its length. For a given muscle action, the
length of the muscle can vary dramatically, and based upon its length-tension relationship
may have to generate force at a length that may be less than optimal (Rack and Walmsley
1969; Herzog 2000). Studies investigating the effect of muscle length on force production
have focused on both the discharge properties of motor units and the level of electrical
activity of the muscle. As there is a decrease in the duration of both the contraction time and
half relaxation time of the twitch at shorter muscle lengths, a higher motor unit discharge
would be required to produce similar levels of force in the shortened position (BiglandRitchie et al. 1992). This has been evidenced by Vander Linden et al (1991) and Christova et
al. (1998) who showed an increase in motor unit firing frequency at reduced muscle lengths
in the tibialis anterior and biceps brachii, respectively. The changes in the surface recordings
of muscle activity with progressive shortening of muscle length seem to be less clear with
both increases (Heckathorne et al. 1981; Lunnen et al. 1981) as well as decrease (Fugl-Meyer
1979; Sale et al. 1982; Cresswell et al. 1995; Pinniger et al. 2000) in the level of EMG being
reported. These differences possibly reflect the many strategies that the CNS may use to
develop an efficient control of force when an active muscle is varying its length. Moreover,
this is further complicated when activation of a synergistic muscle group, whose individual
components are operating at differing lengths, must be controlled to provide a required joint
torque.
At the ankle, the functional group primarily responsible for plantar flexor torque production
is the triceps surae, which consists of the bi-articular GAS and mono-articular SOL muscles.
Despite being classified as synergists, the relationship between these two muscles is
constantly changing depending upon the task. For example, during standing, SOL is most
always active, while minimal activation of the GAS (Hodgson 1983; Duysens et al. 1991) is
observed. It is with an increase in movement velocity that a corresponding increase in the
activity of the GAS is observed (Hutchison et al. 1989, Duysens et al. 1991). This
dissociation has been further illustrated during rapid lengthening actions where the SOL can
be rendered silent while there is a preferential increase in GAS activity (Nardone et al. 1989).
The differential activation patterns observed in these two muscles may be explained by the
composition of the respective motoneurone pools, with the SOL predominantly composed of
slow twitch motoneurones and GAS having a higher number of fast twitch motoneurones.
The variation in force generating capabilities between the SOL and GAS muscles may,
however, be influenced more by the differences in their architectural design than their fibre
type distributions (Kawakami et al. 1998; Herzog 2000). The GAS muscle crosses both the
knee and ankle joints and therefore, for a given ankle angle, its contribution to torque output
is considerably altered by the position of the knee (Herzog 2000) with greater torque
production occurring when the muscle is lengthened via extension of the knee and or
dorsiflexion of the ankle (Cresswell et al. 1995; Kawakami et al., 1998). On the other hand,
the SOL muscle, with its length unchanged, would thereby be required to generate a greater
or lesser percentage of the overall torque depending on the degree of knee flexion. Despite
the increased demand of the SOL muscle to plantar flexor torque with a reduction in knee
angle, there may still be a minor torque contribution from the GAS as it has been shown that
even in the most flexed knee angles there is still EMG activity from GAS (Cresswell et al.
1995).
2
In an earlier study (Cresswell et al. 1995) we found that the surface EMG amplitude from
both heads of the GAS muscle during voluntary plantar flexor efforts with the knee flexed to
shorten the GAS was significantly less than that produced with the knee extended. However,
it was unclear if the EMG reduction was due to electrode-muscle configuration changes, or
due to inhibition of GAS motor units induced by the reduction in their fibre lengths.
The question therefore remains as to be how the CNS activates a muscle when many of its
fibres are at a non-optimal force producing lengths. The purpose of the present study was
therefore to determine the effect of muscle length on motor unit recruitment for the medial
gastrocnemius muscle (MG) during isometric plantar flexion. The onset of single motor unit
activity with respect to plantar flexor torque and SOL EMG activity was compared with the
knee extended at 180° and flexed at 90° corresponding to long and short lengths of the MG
muscle.
Materials and methods
Subjects
The following experiment included nine healthy male participants between 25 and 42 years
of age. Their mean (±SD) for age, height and body mass were 32.0 (6.0) years, 179.7 (4.5)
cm and 78.3 (3.9) kg, respectively. All subjects regularly participated in physical activity and
none of these individuals had any known neurological or motor disorders prior to testing.
The experimental protocol was explained and the subjects gave their informed consent to
participate in the investigation. The local institutional ethics committee approved the
following experimental procedures.
Experimental Design
Subjects lay in a prone position on a padded bed. Their left (n = 6) or right (n = 3) foot was
tightly secured to a non-compliant footplate at a constant angle of approximately 90º
(measured as the internal angle between the shank and the foot) to ensure isometric
conditions. Force was measured from a load-cell (Bofors KRG-4, Nobel Electronic, Sweden,
unloaded frequency range 0-2.6 kHz, maximum force 2 kN) that was placed at the distal end
of the footplate. The axes of the ankle and footplate were aligned as close as possible. The
perpendicular distance between the load-cell and axis of the footplate was used to convert
force to ankle plantar flexor torque.
From this initial prone position, knee angle (measured as the included angle between the
thigh and shank) was manipulated by raising the trunk above the support surface (see Fig. 1).
Flexion of the hips altered the knee angle (from 180° to 90°) while the trunk and shank
remained horizontal. The subjects placed their arms on the table to provide support in this
elevated position. Manipulating the body in this manner shortened the length of the GAS
muscle while the length of the SOL muscle remained unchanged. Levels of voluntary plantar
flexor torque were produced with the aid of visual feedback. A torque ramp of 2 Nm · s-1 was
displayed as a beam on an oscilloscope and a second beam, corresponding to the voluntary
plantar flexor torque, were displayed simultaneously. For each trial, the subjects were
instructed to align the two beams as precisely as possibly. The motor task was to gradually
increase the amount of plantar flexor torque at a rate of 2 Nm · s-1 until motor unit activity
could be isolated at a signal-to-noise ratio of at least 2:1. At this point, the ramp was held at
that force level, and the subject instructed to maintain the force output for an additional 10-15
seconds, thus maintaining motor unit firing activity. This task was performed for both the
long and short lengths of the MG muscle.
3
Fig 1. An illustration of the experimental set-up including subject positions with the knees
extended and flexed. The subject’s foot was tightly secured to a footplate to minimise ankle
movement. A. Concentric needle electrode inserted into MG (EMG recording above). B.
Fine-wire intramuscular electrodes inserted into SOL (EMG recording above). C.
Oscilloscope (2 Nm · s-1 ramp and force feedback) to provide the subject with visual
feedback. D. Force Transducer located at the distal end of the footplate.
Single Motor Unit and EMG Recordings
The skin at all electrode sites was shaved and cleaned with 95% ethanol prior to electrode
insertion. Single motor unit activity was recorded from MG muscle using a sterile concentric
needle electrode (0.46 mm diameter with a 0.07 mm2 recording area, Medelec, UK) inserted
percutaneously and randomly repositioned by the experimenter after each trial. Intramuscular
EMG was recorded from the lateral aspect of the SOL muscle using fine-wire electrodes
(0.075 mm diameter, twisted stainless steel, Teflon coated, Leico Ind., USA) constructed in a
‘double-hook’ manner. A sterile hypodermic needle (0.8 x 80 mm) was used to insert the
wires, and prior to this the skin was anaesthetized by superficial injection of 0.5 - 1 ml
prilocaine (Citanest, 5 mg·ml -1). The potential sensitive area was the uninsulated end of each
wire, 2 mm in length, with an inter-electrode distance of approximately 3-4 mm. A surface
reference electrode (self-adhesive Ag/AgCl electrode, Medicotest A/S, Denmark) was placed
on the lumbar spine (L3) of each subject.
4
Signal Processing
SOL intramuscular EMG was amplified x1000 and band-pass filtered between 10 and 800 Hz
(NL 824 and NL125; Neurolog, UK). Torque signals were low-pass filtered at 20Hz and
analogue-to-digital (A/D) converted with the SOL EMG signals (16 bit) at a sample rate of 1
kHz (Spike2 and Power 1401, Cambridge Electronics Design, UK). Motor unit activity from
MG was amplified x1000, band-pass filtered between 0.3 and 5 kHz and similarly A/D
converted at a sampling rate of 15 kHz. For each trial, SOL intramuscular EMG root mean
square (rms) was calculated over the entire period using 200-ms bins. To establish the
overall torque and SOL EMG rms level that corresponded to the onset threshold of MG
motor unit activity, the peak of the first measurable action potential (exceeding 2:1 signal-tonoise ratio) was used to calculate the appropriate values. Single motor unit activity
discrimination was performed off-line using the Spike2 spike template matching software
with an 80% confidence interval to ensure the analysis of a single motor unit potentials. False
positive and/or negative classifications due to spurious frequency changes or action potential
collisions were not observed.
Statistics
Means (±SD) were calculated for all variables. A one-way analysis of variance with repeated
measures was used to analyse the torque and SOL EMG rms levels. An independent t-test
was used to evaluate differences between mean firing frequencies and amplitudes for MG
motor units in the long and short positions. Differences between the means were considered
statistically significant at a level of P≤0.05. All statistical analyses were performed using the
Statistica software package (StatSoft, USA).
Results
After several practice trials, subjects were able to consistently match and follow the visual
ramp, thereby increasing the rate of plantar flexor torque development at approximately 2 Nm
· s-1, and maintain a constant level of torque once a MG motor unit began to discharge.
Single motor unit and EMG recordings from two subjects during such ramp contractions in
the knee extended (long) and knee flexed (short) positions are presented in Fig. 2.
Motor Unit Recordings
Using the concentric recording electrode, 229 single motor units were recorded from MG. A
little more than half of these recordings were achieved with the MG at the long length (121
out of 229, 53%) while the remaining units (108 out of 229, 47%) were recorded with the
MG at the short length. For a specified level of plantar flexor torque (20 Nm), the level of
SOL EMG rms observed at the short length was significantly greater than that recorded at the
long length (P < 0.01). Moreover, with GAS in the shortened position, motor unit
recruitment occurred at significantly higher levels of SOL EMG rms activity and torque
output (P < 0.01). This behaviour was consistent across all subjects and the difference in
plantar flexor torque (torqueShort – torqueLong) that corresponded to the onset of MG single
motor unit activity in the long and short lengths ranged between 17 to 38 Nm. This was
similar to the difference in SOL EMG rms (rmsShort – rmsLong), which ranged between 0.09
mV to 0.30 mV. The means ± SD of the SOL EMG rms level for a torque output of 20 Nm
in the knee extended and flexed positions corresponded to 0.10 ± 0.05 mV and 0.19 ± 0.09
mV (increase of ~ 47%). The mean torque and SOL EMG rms corresponding to the onset of
MG single motor unit activity are illustrated in Table 1 and Fig. 3, respectively.
The mean amplitude for the MG units recruited at the short length (0.47 ± 0.43 mV) was
significantly greater than the amplitude of those units recruited at the long length (0.30 ± 0.24
5
mV) (P < 0.02). Additionally, at the onset of motor unit activity the initial firing frequency
for the first ten action potentials recorded at the short length (5.8 ± 2.2 Hz) was significantly
less than the discharge rate for the same number of units recorded at the long length (7.4 ±
2.6 Hz) (P < 0.01).
Fig 2. Concentric needle recordings of single motor unit activity from MG in the extended
and flexed knee position for subjects 6 (I) and 7 (II). The vertical lines superimposed on the
raw data indicate the onset of plantar flexor force production (A) and the onset of motor unit
activity (B). Dotted lines superimposed on the voluntary torque trace illustrate the 2 Nm · s-1
ramps. Action potentials below (n = 10) were superimposed to demonstrate the continuous
sampling of the same unit.
6
Plantar Flexor Torque versus EMG Thresholds
The levels of both torque and SOL EMG rms corresponding to the onset of MG motor unit
activity are plotted in Fig. 4. In the long length, MG motor units were generally recruited at
low levels of plantar flexor torque (< 5Nm) and SOL EMG rms levels generally less than 0.1
mV. However, there were 9 out of 121 units (7.4% and for the most part from subject 7)
recorded in this position that were associated with an elevated level of SOL activation and
higher torque. These units were recruited at torque outputs that ranged between 10 to 35 Nm
(mean 27 ± 10 Nm) and SOL EMG rms levels of 0.12 to 0.38 mV. On the other hand, there
was a greater distribution in the recruitment levels of MG motor units in the flexed knee
position, especially pertaining to the SOL EMG rms thresholds. Only a few units (3 out of
108, 2.8% and mostly from subject 8) were recorded with thresholds lower than 10 Nm
(ranging between 2 to 8 Nm). At the short length, there were at least 9 trials in which a
continuous torque ramp of up to 50% MVC failed to recruit any motor unit activity from the
MG muscle.
Fig 3. The means ± SD (n = 9) for the torque and SOL EMG rms that is required to recruit a
motor unit in both the long (µ) and short (_) positions. See Table 1 for the subject means.
The recruitment thresholds reached a level of statistical significance (∗) between knee
positions (P<0.05).
Plantar Flexor Torque
To estimate relative torque production, data from a previous experiment on a similar
population was used (Cresswell et al. 1995). In this prior study, maximum voluntary
contractions (MVC) were performed in both flexed and extended knee positions. The
maximal voluntary torques were 135 ± 24 Nm and 104 ± 24 Nm at the long and short lengths,
respectively. As such, in the present study a mean torque recruitment of 3 Nm ± 8 in the
extended knee position would be equivalent to 2 ± 6% of the maximal voluntary effort at that
same position (range between ~ 0 to 26%). Similarly for the flexed knee, a torque output of
32 ± 10 Nm equated to approximately 24 ± 9 and 31 ± 10% of the MVC at the long and short
lengths, respectively (ranging between 3 to 46% for the short length). It is important to note,
however, that these values are based on the calculated MVC for the appropriate position.
7
Fig 4. Scatter plot of the torque threshold versus the SOL EMG rms threshold for the total
number of motor unit recordings in this study (n = 229). Thresholds were plotted for motor
units recorded in the long (Ø) and short (O).
Discussion
The main findings of this study were the significantly increased levels of SOL EMG rms and
plantar flexor torque corresponding to the onset of firing of MG motor units during ramped
voluntary isometric plantar flexor efforts with the GAS muscle shortened. The observed
changes are credited to increased inhibition of the MG motoneurone pool due to the
diminished force producing capabilities of the MG motor units as a result of their reduced
length by means of knee flexion.
Methodological Considerations
Concentric needle electrodes were used rather than fine-wire electrodes, as used by Miles et
al. (1986), Vander Linden et al. (1991) and Christova et al. (1998), for the recording of single
motor unit activity in the MG muscle. The use of indwelling fine-wire has an advantage in
that it may result in the same motor unit being recorded even once the muscle has undergone
a length change. However, a limitation is the difficulty to relocate the wires between trials,
especially deeper, in order to obtain recordings from different regions within the same
muscle. In the present study, the concentric needle was randomly positioned within the
muscle between all trials to obtain recordings from a larger muscle volume. Using this
technique it was not possible to monitor the same motor unit at different muscle lengths,
however, we were able to record from at least 20 different sites in all subjects and thus were
able to obtain a larger random sample of motor units. Due to the consistency of MG onset
thresholds at both the long and short lengths, we are confident that the differences in onset
thresholds are a direct result of changes in muscle length and are not complicated by the
positioning of the needle within the muscle.
Torque Production at shortened GAS lengths
The control of torque production by the triceps surae muscle group is complex, mainly by the
fact that the GAS muscle is bi-articular, crossing both the knee joint and ankle while the
soleus muscle crosses the ankle only. As such, any change in knee angle will independently
8
alter the length of GAS in relation to SOL. Furthermore, it is believed that the GAS muscle,
which is like the majority of muscles that are involved in stretch–shortening actions, operates
primarily on the ascending limb of the force-length relationship (Rassier et al. 1999) and that
when it is maximally shortened via knee flexion, and independent of ankle angle, is incapable
of producing force (cf Herzog 2000).
We, and other authors, have previously demonstrated that with a progressive shortening in
the GAS muscle via knee flexion, there is a reduction in both voluntary and involuntary
maximal plantar flexor torque output (Fugl-Meyer et al. 1979; Sale et al. 1982; Herzog et al.
1991; Cresswell et al. 1995). Recently, Kawakami et al. (1998) reported that as the knee is
moved to a more flexed position the medial GAS fascicles decrease their length up to 24 mm
and thereby increase their pennation angle by approximately 22ϒ. Such a change may limit
the amount of slack that can be taken up in the muscle, thereby resulting in the overall force
transmitted to the tendon being reduced. To accommodate for the increased slack and
maintain torque output in a shortening muscle the CNS may increase the firing frequency of
motor neurons (Vander Linden 1991). However, once the muscle fibre reaches a critical
shortened length, the overall contribution of the muscle to torque output will be minimal,
even if fully activated. In this condition, the muscle is called ‘actively insufficient’ and it
would therefore seem appropriate to reduce the drive to, or neural outflow from, spinal
motoneurones whose muscle fibre reside at compromised shortened lengths (cf. Suter and
Herzog 1997; Herzog 2000).
In earlier studies (Cresswell et al. 1995; Pinniger et al. 2000) we observed a significant
reduction in surface EMG from both heads of the GAS muscle during voluntary plantar
flexor efforts with the knee flexed to shorten the GAS. From these studies, it was uncertain if
the EMG reduction was due to electrode-muscle configuration changes, thereby recording
from different volumes and possibly different numbers of fibres within the muscle, or due to
inhibition of GAS motor units induced by the reduction in their fibre lengths. To further
investigate the latter of these theories we have endeavoured to more specifically determine
the effect of changing GAS muscle length on the recruitment and firing behaviour of its
motor units.
Muscle activations at shortened GAS lengths
Similar to previous findings, a greater level of voluntary drive, as evidenced by a significant
increase in SOL EMG rms, was required by the plantar flexors to achieve the same level of
torque in the flexed knee position. This finding was expected, as the SOL muscle must now
be activated to a greater extent to compensate for the limited force contribution of the
shortened GAS.
It was clearly evident when trying to record single motor units from the shortened MG that
the onset of their activity was significantly delayed when performing the voluntary torque
ramps. This meant that the MG units at short lengths were recruited at significantly greater
levels of plantar flexor drive and torque production. This finding corroborates earlier results
of decreased surface EMG amplitude from the GAS and triceps surae at short as compared to
long lengths when performing both submaximal and maximal voluntary isometric plantar
flexions (Fugl-Meyer et al. 1979; Sale et al. 1982; Cresswell et al. 1995; Pinniger et al. 2000).
Moreover, as the recordings in this study were made intramuscularly from many regions
within the muscle, the increased threshold of MG units is unlikely a result of electrodemuscle configuration changes but due to an increased level of inhibition or disfacilitation of
motor units within the MG motoneurone pool.
9
Interestingly, other studies have demonstrated opposite changes in motor unit behaviour with
changes in muscle length. For the biceps brachii, Christova et al. (1998) reported an increase
of motor unit discharge rate as it underwent shortening via elbow flexion. In that study, it is
unlikely that the biceps brachii was activated on the initial portion of the ascending limb of
the length-tension relationship as the upper-arm was maintained parallel to the body and not
flexed which would have additionally shortened the muscle. Similarly, Vander Linden et al.
(1991) showed increased motor unit discharge rates in the shortened tibialis anterior muscle
and concluded that this was due to the decreased peak tension and shorter one-half relaxation
times observed in shortened muscle. However, it may well be that the tibialis anterior muscle
does not becomes ‘actively insufficient’ at their test position of 20o dorsiflexion.
The underlying control mechanisms behind the increased recruitment thresholds for GAS
motor units at shortened muscle lengths are not clear. It may well be that cortical drive to the
MG motoneurone pool is reduced, regardless of an increased drive to the synergistic SOL.
However, more likely candidates responsible for the reduced excitability of the MG
motoneurone pool, or even inhibition of specific motor units whose fibres are at less than
optimal lengths, are peripheral afferent inputs receptors with inputs at the spinal level.
Muscle spindles, which can accurately detect length change, have the ability to reduce
motoneurone excitability through reduced spindle input, and as such, are potential candidates
for the excitability changes we have seen. However, reduced Ia-afferent input, via increased
presynaptic inhibition of the Ia terminals through the possible heightened activation of group
II, III and IV peripheral inputs, may also result in disfacilitation of the MG motoneurone
pool. Direct and indirect inhibitory effects from cutaneous and joint afferents are also
potential mechanisms, as it has been shown that the former receptor can have strong
influences on motoneurone excitability (Kanda et al. 1977; McNulty et al.1999).
If afferent input is responsible for modulating GAS motoneurone excitability with changes in
length, then the questions still remains as to whether the total motoneurone pool is receiving
equivalent inhibitory input or whether this input is directed toward specific motor units. The
latter may be the case if non-uniformity of muscle fibre length exists and the CNS can
adequately detect motor units or populations of motor units who are no longer capable of
producing force. The result of greater MG motor unit amplitudes at shorter muscle lengths
suggests that higher threshold, or faster, motor units were the first activated and that the
slower units that were recruited first at the longer lengths were now inhibited. The initial
slower firing frequency of these units at short lengths appears to support this notion (Kudina
1999). However, it must also be pointed out that changes in muscle geometry can have an
effect on motor unit amplitude with larger amplitudes being recorded at shorter muscle
lengths (Gerilovsky, 1989; Garland et al. 1994)
Regardless, this study has demonstrated that with a reduction in muscle length, the onset of
motor unit activity in the MG muscle occurs at significantly higher levels of both plantar
flexor torque and SOL EMG rms activity. This alteration in recruitment may reflect a general
increase in the recruitment threshold of MG motoneurones, or specific inhibition of motor
units with muscle lengths that are no longer capable of producing force. At present it is
unclear if motor units of varying types are equally affected or if there is a somewhat selective
inhibition of low threshold motoneurones. Nonetheless, by limiting the activity of a muscle
that is ‘actively insufficient’ the CNS is able to minimise metabolic costs while maximising
the force output within a functional group.
10
Subject
1
2
3
4
5
6
7
8
9
Mean
SD
Onset of MG Single Motor Unit Activity
Knee Extended (Long)
Knee Flexed (Short)
Torque
SOL EMG
Torque
SOL EMG
(Nm)
rms (mV)
(Nm)
rms (mV)
1.70
0.79
0.79
0.72
1.15
0.94
8.73
1.41
0.49
2.97
7.78
0.046
0.028
0.023
0.018
0.023
0.023
0.110
0.018
0.015
∗
0.045
0.075
•
34.81
34.45
38.16
37.98
32.04
31.61
39.07
19.00
32.48
0.344
0.231
0.316
0.332
0.190
0.226
0.313
0.112
0.121
32.14 ∗
10.25
0.231
0.129
•
Table 1. The mean (±SD) plantar flexor torque and SOL EMG rms levels corresponding to
the onset of MG single motor unit activity for both knee positions for each subject. The (∗)
and (•) indicate significant differences between the torque and SOL EMG rms variables,
respectively. A mean of 12 (±1.3) MG units were recorded in each condition for each subject.
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