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1 Faculty of Kinesiology, University of Western Ontario, London, Ontario, Canada N6A 3K7; and 2 Neuromuscular Unit, Department of Neurology, New England Medical Center and Tufts University, Boston, Massachusetts 02111
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Doherty, Timothy J., and William F. Brown. Age-related
changes in the twitch contractile properties of human thenar motor
units. J. Appl. Physiol. 82(1):
93-101, 1997.
The purpose of this study was to examine the
effects of aging on the contractile and electrophysiological properties
of human thenar motor units (MUs). Percutaneous electrical stimulation
of single motor axons within the median nerve was used to isolate and
examine the twitch tensions, contractile speeds, and surface-detected
MU action potential (S-MUAP) sizes of 48 thenar MUs in 17 younger
subjects (25-53 yr) and 44 thenar MUs in 9 older subjects
(64-77 yr). A wide range of twitch tensions, contractile speeds,
and S-MUAP sizes was observed in both age groups. However, older
subjects had significantly larger MU twitch tensions and slower MU
twitch contraction and half-relaxation times. These changes were
accompanied by increased S-MUAP sizes. These findings suggest that the
human thenar MU pool undergoes significant age-related increase in MU
size and slowing of contractile speed. Such adaptation may help to
overcome previously reported age-related losses of thenar MUs.
aging; electromyography; skeletal muscle; motor unit force
INTRODUCTION AGING IN HUMANS is associated with decline in strength
and atrophy of skeletal muscles (16, 18, 21, 26). These
changes may be attributed, in part at least, to losses of motor units (MUs). For example, by using one electrophysiological technique, we
recently showed that the numbers of MUs comprising the thenar muscles
of healthy subjects between 60 and 80 yr of age were reduced, on
average, by about one-half compared with subjects between 20 and 40 yr
of age (13). Such an order of magnitude of age-related loss of MUs was
similar to previous reports for the thenar muscles (4, 38) and other
distal (9, 38) and proximal muscle groups (7, 11, 16).
Other studies have shown significant, although generally less-marked,
age-related reductions in the force output of muscles (9, 16, 24, 48).
Losses of close to one-half of the normal complement of MUs in skeletal
muscles in the elderly may be partially offset by adaptations such as
collateral sprouting from surviving motor axons to reinnervate muscle
fibers dennervated by the loss of their parent motoneuron (3, 5, 31,
32). The presence of such a positive age-related adaptation is
supported by the finding of increases in MU action potential size,
whether detected with concentric needle electrode, macroneedle
electrode, or surface electrodes (7, 11, 13, 22, 31, 41).
Few studies (9, 20, 36) have examined age-related changes in the
contractile properties of human MUs, probably because of the formidable
technical problems these studies present in vivo. Such studies,
however, are important because assessments of the overall force output
of muscles or muscle groups may mask differences in the effects of
aging on different physiological types and properties of the MUs. The
purposes of this study were, therefore, to
1) examine the contractile
properties of single thenar MUs as collected with threshold
percutaneous stimulation of single motor nerve fibers,
2) look for age-related differences in these properties, and 3) examine
whether there were any significant differences in the impact of aging
on MUs of differing physiological type.
METHODS
Force-collection system. Twitch forces exerted by thenar MUs vary widely with respect to their primary vector (43). We, therefore, employed a two-axis force-collection system to measure the force generated by individual thenar MUs in the abduction as well as flexion plane. This enabled the resultant prime vector of the MU to be derived and the associated twitch force to be determined. This was essential because, except for those instances where the presumed location of the MU could be estimated from the electrode detection site where the S-MUAP was maximal, or those few MUs that generated large enough twitches to dimple the skin overlying the MU, it was impossible to predict with accuracy the prime vector for many individual MUs. Earlier attempts to determine the prime force vector for single MUs by searching with a single-axis system were found to be too tedious and time consuming. Two force transducers (model FT10, Grass Instruments, Quincy MA; stiffness 0.2 mm/N) were mounted at right angles to each other on adjustable micrometer stages, which allowed each to be precisely positioned independently of the other. Bridge excitation and signal amplification were provided by two separate strain-gauge signal conditioners (Durham Instruments, Pickering, ON; DC 6 kHz, 3 dB down), which had been previously calibrated. The force transducers contacted the comfortably extended thumb at the level of the interphalangeal joint via slightly curved aluminum cups ~3 by 10 mm in size, and 0.5 N of passive force was applied in the direction of thumb abduction and flexion (Fig. 1). Isolation of single MU responses. During the initial search for suitable single thenar motor axons, the median nerve was stimulated at frequencies of
1 Hz by using a
hand-held bipolar stimulating electrode. The electrode incorporated two
saline-soaked felt pads ~5 mm in diameter and 30 mm apart (model
13L36, Dantec) and was used in conjunction with a constant-voltage
stimulator (model S48m, Grass Instruments) and stimulus isolation unit
(model SIU5).
The search for single MUs was carried out by looking for sites at which
a single thenar motor axon could be stimulated along the course of the
median nerve between the thenar motor point and the distal forearm.
More proximal sites along the course of the median nerve were often
unsuitable for finding single thenar motor axons because it was too
difficult to find sites in this region where a single thenar motor axon
could be found with a stimulus threshold sufficiently below that of any
median motor axons supplying forearm muscles. The latter was necessary
for the purposes of these studies to avoid distorting the thenar MU twitches by contraction of the forearm muscles.
Criteria for accepting an electrophysiological and force response
as generated by a single thenar MU in response to stimulation of the
median nerve.
We have previously described in detail the electrophysiological
criteria required to ensure that single MU responses are obtained by
threshold percutaneous stimulation of the median nerve (13, 14). These
electrophysiological criteria and the additional criteria required to
ensure that the twitch force represented a single thenar MU were as
follows.
1) The presence of
"all-or-nothing" MUAPs as detected by surface electrodes (and in
some instances an intramuscular needle electrode) in response to small
increments and decrements in the stimulus intensity delivered to the
nerve at the selected site along its course (see Fig. 1, Ref.
14).
2) Successive "all" responses,
or S-MUAPs, were identical in shape, size, and latency in response to
successive stimuli exceeding threshold, and the S-MUAPs exhibited no
fractionation or alternation such as might indicate the presence of two
or more thenar motor axons with overlapping thresholds (see Fig.
2; Refs. 13, 14).
3) The all-or-nothing responses of the S-MUAP were matched by all-or-nothing twitch responses in the corresponding MU in response to the same graded changes in the stimulus intensity. This particular criterion, however, could not be reliably depended on as a mandatory selection criterion for all MUs because the small size of some twitches precluded reliable visual identification of their twitch without averaging 5-20 successive responses. Many MUs, however, did generate large enough forces for the twitch response to be sufficiently defined without signal averaging. In these latter instances, the all-or-nothing twitch response of the MU corresponded with the equivalent all-or-nothing S-MUAP (Fig. 2). 4) The excitation threshold of the chosen motor axon was sufficiently separate from that of neighboring thenar motor axons to allow stimulation frequencies as high as 30 Hz or more without exceeding the thresholds of other thenar motor axons. In practice, this meant that the stimulus threshold of the next higher threshold thenar motor axon(s), at a given stimulus site, often exceeded that of the lowest threshold thenar motor axon by
2-10
V for stimulus durations of 0.05-0.1 ms. This degree of separation
in excitation thresholds was more than sufficient to confidently stimulate the desired thenar motor axon without fear of stimulating other thenar motor axons (Fig. 2; Ref. 14).
5) Stimulation of other than the
test motor axon was signalled by an increase or change in the size or
shape of the S-MUAP. Instances where the thresholds of two or more
thenar motor axons overlapped were readily detected by making repeated
small increments and reductions in the stimulus intensity or in some
cases by delivering a sufficiently long train of constant-intensity
stimuli to reveal fractionation of the composite S-MUAP or alternation
(6). Both of the latter phenomena could be taken as reliable evidence
that the threshold of the selected MU, at that stimulus site, was
insufficiently separate from the thresholds of other thenar motor axons
to allow reliable study of the contractile properties of the targeted
MU.
6) The direct response of the S-MUAP
was identical in shape and size to the F response, when evoked, for
that particular MU (Fig. 1; Ref. 14).
When a site was found meeting all of the above criteria for a single
thenar motor axon and MU, the stimulating electrode was usually taped
in place for the remainder of the studies on that MU. In some cases,
however, it was actually preferable for the operator to hold and make
minor adjustments in the pressure and position of the stimulating
electrode as required to maintain isolation of a particular motor axon.
Force and EMG data collection.
At the gains required to detect single MU twitch forces, baseline
fluctuations related to respiration, pulse pressure waves, and other
movements often exceeded the size of the MU twitches. These
fluctuations were minimized in two ways, based on methods described by
Westling et al. (49). The pressure waves associated with the cardiac
pulse were minimized by detection of the latter by using an infrared
finger pulse monitor from which a trigger pulse was delivered to the
stimulator after a delay of 50-150 ms. In this way, the
contractile response of the MU was collected in the cardiac pulse-free
period between successive heart beats. Movement related to respiration
was dealt with simply by asking the subjects to hold their breath for
5-15 s during data collection. Finally, a computer algorithm was
used to sample the DC offsets in the force signals and reset the
baseline to zero just before the delivery of each stimulus.
After isolation of a stable single motor axon as outlined in
Criteria for accepting an electrophysiological and
force response as generated by a single thenar MU in response to
stimulation of the median nerve, typically 5-20
responses were ensemble averaged to obtain the MU twitch
force. Initially, it was hoped that single raw force
responses would be acceptable, but despite the above measures the
background noise level relative to the small size of many of the
twitches mandated the use of averaging for most MUs.
Averaging was limited to the number of traces required to produce a
stable twitch response.
Force and EMG signals were collected on-line (12-bit analog-to-digital
converter) and viewed on a four-channel split-screen display. The force
signal was sampled at 500 Hz, whereas the surface EMG was sampled at 5 kHz with a band width of 2 Hz to 2 kHz. All data were stored on a hard
disk for future analysis. Channel 1 sampled only the stimulus artifact, which served as a triggering source
to generate time-locked delayed sweeps of channels
2-4. Channel 2 displayed the surface-detected EMG, and channels
3 and 4 displayed the
abduction and flexion components of the twitch, respectively (Fig. 2).
Additionally, the unaveraged surface EMG signal was monitored on a
storage oscilloscope to ensure that the same single MU as indicated by
its unique S-MUAP was evoked with each stimulus. If an F response
occurred, the trial was discarded and restarted to avoid distortion of
the twitch by the additional force contribution of the recurrent
discharge of the MU.
Force data analysis.
The force signals were analyzed on-line by a separate computer
algorithm. The algorithm derived the resultant composite twitch force
from the abduction and flexion force components. The peak twitch force
(mN), contraction time (CT; time in ms from onset of force response to
peak twitch tension), and half-relaxation time
(RT1/2; recovery time in ms from
peak force to a value one-half of the peak) were determined by a
computer algorithm from the resultant composite force record.
EMG data analysis.
The negative peak areas (µV · ms) of the associated
S-MUAPs were determined by a computer algorithm. As well, S-MUAP
negative peak areas were normalized to the size of the maximum
M-potential negative peak area as detected with the same electrode
configuration. Finally, after collection of the force response,
repeated stimuli (often >100) were delivered to the motor axon in an
attempt to evoke an F response. When an F response occurred, the motor
axon conduction velocity (CV) was calculated as described previously (15).
Values are means ± SD throughout the text. Comparisons between mean
values were made with two-tailed
t-tests, and relationships between
associated variables were analyzed with linear regression.
RESULTS
From examination of the median innervated thenar muscles of 17 younger subjects, a total of 48 MUs were collected from which satisfactory contractile and electrophysiological data could be derived. The yield per subject ranged from 1 to 5 MUs, with a mean yield of 2.8 ± 1.2. From 9 older subjects, 44 MUs were studied with the yield per subject varying from 1 to 7 MUs. The mean yield in the latter subjects was 4.8 ± 1.8. The results for younger and older subjects are summarized in Table 1.
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S-MUAP negative peak areas varied widely in size from 30 to 1,208 µV · ms (Fig. 7). The distribution of S-MUAP sizes was similar to the distribution of MU twitch forces, there being relatively greater numbers of responses generating smaller S-MUAPs and forces. MU twitch force was significantly correlated with S-MUAP negative peak area (r = 0.582; P < 0.001).
In 15 of the 48 MUs, an F response was evoked and the CV of the MU was calculated. CVs ranged from 49 to 67 m/s, with a mean value of 59 ± 4 m/s. CVs were not significantly correlated with either the twitch force, CT, or RT1/2. Furthermore, twitch force was not significantly correlated with CT or RT1/2. However, the CT and RT1/2 were significantly correlated (r = 0.566; P < 0.001). Comparison between MU contractile and electrophysiological properties in younger and older subjects. As was the case in younger subjects, MUs in older subjects exhibited a wide range in their twitch tensions and a relatively greater preponderance of MUs generating smaller twitch tensions (Fig. 4, Table 1). In older subjects, however, the distribution was shifted to the right and the mean MU twitch force was significantly greater than that for the younger subjects (34%; P < 0.05). The CT and RT1/2 of MUs in older subjects exhibited widely ranging values with somewhat greater variability in twitch duration compared with the younger subjects (Figs. 5 and 6). On average, the time course of the MU twitch was prolonged, as indicated by the modest yet significantly increased mean CT (P < 0.01) and RT1/2 (P < 0.01) (Table 1). S-MUAPs from older subjects, as for the younger subjects, exhibited a wide range of negative peak areas with a positively skewed distribution (Fig. 7). The mean S-MUAP size did not significantly differ, possibly because of the small sample size and wide range of values in both age groups. However, when the S-MUAP sizes were normalized to their respective maximum M-potential negative peak area, the mean value for older subjects was significantly larger compared with the younger subjects (43%; P < 0.05). Only 6 of 42 MUs in older subjects were found to have an F response, despite the delivery in each case of >100 stimuli. This subset of data was considered too small to analyze statistically with regard to age-related changes in MU axonal CVs and contractile properties (but see Ref. 15). As for the younger subjects, there were no significant correlations between MU force and either CT or RT1/2 in older subjects. CT and RT1/2, however, were significantly related (r = 0.566; P < 0.001) as were MU force and S-MUAP size (r = 0.404; P = 0.007).
DISCUSSION
In this study, the electrical and contractile properties of single thenar MUs in younger and older subjects were studied. It proved possible, by using carefully graded percutaneous stimulation, to excite between one and five single median motor axons in most subjects and measure the size of the associated MU twitch responses and S-MUAPs. However, before our findings in younger and older subjects are discussed, the underlying assumptions and potential biases in our method of collecting single MUs need to be addressed.
Were the criteria for single MUs applied in this study sufficiently rigorous to justify acceptance of their twitches and associated S-MUAPs as generated by single MUs? Previous studies in both our own and other laboratories have shown that it is entirely possible to stimulate a single motor axon without stimulating other motor axons lying within a motor nerve (1, 2, 4, 8, 9, 13, 14, 30, 39, 49). This may be accomplished by means of a stimulating electrode applied percutaneously or through the insertion of a microelectrode directly into the nerve trunk. To successfully stimulate a single motor axon by using a surface electrode requires the following: 1) the ability to carefully grade and control the stimulus intensity applied to the nerve, 2) an often painstaking search for a location along the course of the nerve at which a single motor axon can be excited with a threshold sufficiently below that of other motor axons to allow the chosen motor axon to be confidently stimulated without fear of exciting neighboring motor axons, and 3) holding the electrode steadily enough for the MU to be studied. At such a carefully chosen site, repeated stimuli delivered to the axon at threshold will either fail to evoke any electrical or mechanical activity, the "nothing" response, or an "all" response signalled by the appearance of the S-MUAP and the corresponding twitch generated by the MU. Furthermore, successive stimuli with an intensity that exceeds the threshold for only the chosen motor axon will evoke successive identical S-MUAPs, which are constant in shape and size throughout the range of stimulus intensities employed in the study (Fig. 2; Refs. 13, 14). In the present study, additional evidence supporting that a single motor axon had been stimulated was the finding that the S-MUAPs generated by the direct and F response were identical in shape and size. However, because not all MUs produced an F response, despite in many cases the delivery of hundreds of successive stimuli to the single motor axon, acquiring an F response was not an absolute criterion for the acceptance of an S-MUAP as representing the potential generated by a single MU. Finally, helpful, but not mandatory, to the determination of single MU status was the presence of a clearly defined all-or-nothing twitch response generated by the putative MU in response to single stimuli. The reason for the latter was the fact that the single twitch responses of some thenar MUs were simply too small relative to the noise level of our force-collection system to be clearly defined without resorting to signal averaging. However, we felt it was entirely reasonable to assume that signal-averaged force responses, corresponding to the rigorously defined all-or-nothing S-MUAPs, indeed represented the twitches of single thenar MUs. Was the sample of MUs representative of the true physiological range of MUs in the thenar muscles? Despite the small yield of MUs per subject, provided there was no systematic selection bias toward motor axons of any particular physiological type or CV, the entire sample should have faithfully approximated the range of contractile characteristics, S-MUAP sizes, and CVs of the whole population of MUs. That there was no such systematic bias is suggested by the wide range of twitch tensions, contractile speeds, and CVs of the MUs in this study. In addition, the distributions and sizes of the thenar S-MUAPs and CVs were similar to previous studies of the same muscle (12, 13, 15, 42-44). The studies of Thomas et al. (43) and Westling et al. (49) of single thenar MU contractile properties provides the most directly comparable results with which to compare our findings. Their force-collection system (43, 49) was similar to that employed in the present study, and the MUs were collected by stimulating single motor axons within the median nerve trunk. They reported a range of MU twitch tensions of 2.9-34.0 mN with a mean value of 11.3 ± 8.2 mN, values very similar to our own range of 1.5-33.2 mN and mean of 8.8 ± 7.4 mN for younger subjects. As well, the range of values of CT (35-80 ms) and RT1/2 (25-108 ms) in their study were very similar to our findings for CT (28-72 ms) and RT1/2 (18-120 ms). On the basis of the foregoing and other previously published evidence (1, 13-15, 38) we, therefore, feel that we were able to successfully isolate and examine the physiological properties of single thenar MUs without any apparent bias toward a particular type of MU. It must be admitted, however, that we do not know what factors apparently govern the fortuitously low threshold of some motor axons at certain sites of stimulation (17). Nevertheless, whatever those factors might be, they apparently have little to do with the CVs or contractile properties of their associated MUs. The most important difference between the study of Thomas et al. (43) and the present study was methodological in that they employed intraneural microstimulation to excite single thenar motor axons, whereas we used percutaneous stimulation. The latter, while at first glance appearing less refined than intraneural microstimulation, is nonetheless capable of stimulating single thenar motor axons and carries the added attractive prospect in some cases of being able to find and longitudinally follow the same MU in serial studies over many weeks and months (14). The method of percutaneous stimulation of single motor axons offers investigators the possibility of examining the influence of training and other interventions on specified single MUs. Studies of this nature are currently underway in our laboratory. What were the relationships between the CV and contractile properties of thenar MUs? No significant correlations were found between the CVs of thenar motor axons and either the twitch tensions or contractile speeds of the corresponding MUs. Additionally, there was no correlation between contractile speed and twitch tension of the MUs. Thomas et al. (43), similarly, found no significant correlation between axonal CV and MU twitch or tetanic tension and only weak correlations between CV and contraction rate measures. Given the apparent lack of any strong correlation in these studies among contractile speed, twitch tension, and motor axon CV, previous assumptions concerning these relationships based largely on studies in other species may have to be reexamined for their relevance in humans. Age-related changes in MU contractile and electrophysiological properties. The mean MU twitch tension was significantly increased in the elderly. This was accompanied by a shift in the distribution toward a greater proportion of MUs generating larger twitch tensions (Fig. 4). For example, in younger subjects, close to one-third of the MUs generated twitch tensions of4 mN and >50% of the twitch tensions were <8
mN. On the other hand, in the older subjects, <10% of the MUs
generated twitch tensions of 4 mN and only one-third of the total
generated twitch tensions of <8 mN. There was also a greater
proportion of MUs generating twitch tensions >16 mN in the older
subjects.
Overall, the age-related increase in MU twitch tension was on the order
of 34%. This value was similar to the 37% increase in the mean thenar
S-MUAP size determined by multiple-point stimulation (13) and the 39%
increase in the mean size of thenar S-MUAPs drawn from F responses
(15). An earlier study had shown a significant yet less-marked
age-related increase (23%) in the mean size of biceps-brachii and
brachialis S-MUAPs (16). The extent to which these discrepancies
reflect true differences in the effects of aging on proximal and distal
muscle groups as opposed to any methodological differences in the
collection and detection of S-MUAPs is unknown (31).
Earlier studies of the contractile properties of whole muscles (or
muscle groups) in the elderly have, for the most part, shown increases
in the duration of the twitch response (9, 10, 24, 47, 48). The CT and
RT1/2 observed in the elderly could be explained in one of several ways. For example,
there may be a shift in the relative proportion of slow- and
fast-twitch MUs, with a preponderance of the former. Such a shift, in
turn, may reflect greater losses of fast-twitch MUs or the possible conversion of at least some of the fast-twitch MUs to twitch speeds more characteristic of slow-twitch MUs. For example, in the aged rat,
Pettigrew and Gardiner (37) reported reduced tetanic tensions and
significantly prolonged CTs in the plantaris muscle accompanied by
losses of MUs on the order of 40% and an increase in the proportion of
slow-twitch MUs. Similarly, Kanda and Hashizume (23)
reported a significant age-associated reduction in the mean tetanic
tension of fast-fatigable, fast-intermediate, and fast
fatigue-resistant MUs in the face of substantial increases in the mean
tetanic tension of slow-twitch MUs. Taken together, the foregoing
studies in the rat suggest that there is an age-associated
reorganization of the MU pool characterized by selective losses of fast
MUs and increases in the relative numbers of MUs with slow and
transitional properties. However, the balance of evidence from studies
of human vastus lateralis muscle using histochemical techniques
suggests that fiber type proportions are not significantly altered with aging (28).
Whether such changes as occur with aging in the rat explain changes in
the contractile characteristics of human muscles is not clear on the
basis of this present study. Certainly there is abundant physiological
and some anatomic evidence for significant losses of functioning MUs on
the order of one-third to one-half in the later decades of life in
healthy adults (4, 7, 9, 13, 16, 18, 19, 41, 46). As well, reductions
in the maximum CVs have been reported in various motor nerves in humans (9, 45). However, recent evidence suggests that the latter reductions
in maximum motor CVs may not reflect so much a selective loss of the
more rapidly conducting motor axons but a more or less uniform slowing
of the CVs of all motor nerve fibers with aging (15, 33).
There certainly was no indication in the present study for selective
losses of the fast twitch MUs. Older subjects appeared to maintain the
full range of contractile speeds with regard to both CT and
RT1/2 compared with the younger
adults. The difference between MUs in the young and the elderly was
characterized by a shift in the distribution toward a greater
proportion of MUs exhibiting prolonged CT and
RT1/2. However, there is no way,
based on these data alone, of determining whether this shift reflected preferential losses of fast-twitch MUs as shown in the rat or random
losses of MUs and slowing of the contractile speeds of all MUs
regardless of their physiological type.
In addition, the rates of CT and
RT1/2 of the MU are dependent on
the duration of the active state of its constituent fibers. The
duration of the active state in turn is dependent on the concentration of calcium around the contractile filaments (29). Evidence of damage to
the sarcoplasmic reticulum (SR) with advanced age has been provided
both in human vastus lateralis muscles (25) and in rat muscle (27), in
which a type II fiber-specific decrease in SR volume, rate of calcium
uptake, and calcium pump activity has been reported.
Whatever the underlying mechanism for the slower contractile speeds of
MUs in the elderly, both the present study and the few previous studies
of the physiological properties of single MUs in the elderly all point
toward an increase in the average MU twitch tension coupled with
increases in the CT and RT1/2 of MU twitch contractions (9, 20, 36). These findings provide strong
evidence for age-associated remodeling of the motor unit pool after
significant loss of 
-motoneurons.
The reported increases in the contractile speeds of single MUs in the
elderly may be linked to previously reported reductions in MU firing
rates with aging in both proximal and distal muscles (2, 22, 35, 40).
Prolonged CT and RT1/2 may be a
useful compensatory mechanism enabling MUs in elderly subjects to
achieve fused tetanic forces at lower firing frequencies. In this
fashion, less central drive would be required to reach a given
contractile force. Indeed, Narici et al. (34) reported that the
adductor pollicis muscle of older subjects produced greater relative
forces at lower stimulus frequencies compared with younger subjects. However, this increase in force output per MU may be at the expense of
fine motor control (20), and slower MUs might hinder the ability to
produce large forces rapidly, such as needed in avoiding a potential
fall (47, 48).
In summary, we believe based on these and previous studies that
percutaneous electrical stimulation of single median motor axons
provides an alternative to intraneural microstimulation as a means of
examining the contractile characteristics and CVs of single MUs.
Surface stimulation offers the advantage of being less invasive and the
potential of studying the same MU longitudinally. These advantages are
possibly offset by the relatively few numbers of MUs meeting the
inclusion criteria in individual subjects.
Our findings have shown age-related increases in thenar MU twitch
tension and slower MU contractile speeds. Taken together with previous
findings of significant losses of MUs (7, 9, 13, 16, 39), the increases
in force output and slower contractile speeds probably represent
positive adaptations to the reduced MU numbers. The
question of whether these adaptations affect all MUs to the same extent
regardless of their original physiological type was not resolved in
this study.
A continuum of twitch tensions, contractile speeds, S-MUAP sizes, and
axonal CVs was observed in our study with no clear division of MUs into
subgroups based on their physiological characteristics. The
distributions of MU twitch tensions and S-MUAPs were skewed toward a
preponderance of smaller MUs in both younger and older subjects.
Finally, no significant correlations were found between the CVs of
thenar motor axons and the twitch tensions or contractile speeds, a
finding at variance with studies in other species but consistent with
some recent human studies (43, 49).
ACKNOWLEDGEMENTS
We acknowledge the support of the the Centre for Activity and Ageing (affiliated with Faculties of Kinesiology and Medicine at the University of Western Ontario, London, ON). We also acknowledge Dr. D. W. Stashuk (Dept. of Systems Design Engineering, University of Waterloo, Waterloo, ON) for his expertise in the development of some of the computer software and Robert Petrosenko (Dept. of Biomedical Engineering, University Campus, London Health Sciences Centre, London, ON) for development of the force-collection system.
FOOTNOTES
Address for reprint requests: W. F. Brown, Neuromuscular Unit, Dept. of Neurology, New England Medical Center, 750 Washington St., Box 314, Boston, MA 02111.
Received 10 July 1995; accepted in final form 21 August 1996.
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