1NeuroMuscular Research Center and
2Department of Biomedical Engineering,
Erim, Zeynep,
M. Faisal Beg,
David T. Burke, and
Carlo J. de Luca.
Effects of Aging on Motor-Unit Control Properties.
J. Neurophysiol. 82: 2081-2091, 1999.
It was
hypothesized that the age-related alterations in the morphological
properties of a motor unit would be accompanied by modifications in the
control aspects of the motor unit, as either an adaptive or
compensatory mechanism to preserve smooth force production. In
specific, the objective of the study was to investigate the age-related
alterations in the concurrent firing behavior of multiple motor units
in the first dorsal interosseous (FDI) muscle in isometric contractions
at 20 and 50% of the subject's voluntary contraction level. Analysis
of the data collected from 10 young (24-37 yr of age) and 10 elderly
(65-88 yr of age) subjects led to three novel observations regarding
the firing behavior of aged motor units. 1) Among
elderly subjects, there is a decrease in the common fluctuations that
are observed among the firing rates of motor units in the young.
2) The relationship observed between the firing rate and
recruitment threshold of young subjects is disturbed in the elderly.
Although in young subjects, at any point in a given submaximal
contraction, earlier recruited motor units have higher firing rates
than later-recruited units; in aged subjects this dependency of firing
rate on recruitment rank is compromised. 3) The
progressive decrease observed in the firing rates of concurrently
active motor units in constant-force contractions in the young is not
seen in the aged. In addition to these original findings, this study
provided support for earlier reports of 1) decreased
average firing rates probably reflecting the slowing of the muscle,
2) a shift in recruitment thresholds toward lower force
levels in line with the shift toward type I fibers, and 3) multiphasic action potential shapes indicative of the
reinnervation process that takes place during aging. Taken as a whole,
these findings indicate significant age-related modifications in the control properties of human motor units.
It is well-known that the physiological
characteristics of human nerves and muscle fibers change with age. The
vast bulk of research in this area has been on identifying the changes
in the morphological (Kanda and Hashizume 1989 The Precision Decomposition technique (De Luca 1993 Experimental design
SUBJECTS.
Data were collected from a total of 20 subjects. The group of 10 subjects 20-37 yr of age (30.2 ± 5.66 yr, mean ± SD) were classified as "young," and the group of 10 subjects 65-88 yr of age (76.9 ± 6.56 yr) were classified as "elderly." The
elderly subjects were screened by a practicing physiatrist for
neuromuscular disorders that may interfere with the study. Local and
institutional review board approval was obtained, and all the subjects
gave informed consent. Force and myoelectric data were collected from the FDI muscle of the dominant hand.
ISOMETRIC FORCE RECORDINGS.
The hand of the subject was immobilized by placing it in a special mold
so that the FDI was constrained to contract isometrically. The force of
isometric abduction and flexion were measured by placing a
high-stiffness strain-gauge force transducer against the proximal
interphalangeal joint of the index finger. The force signals were
amplified and filtered (DC-300 Hz) before acquisition and storage on a
PC as well as being recorded on instrumentation tape for backup.
PROTOCOL.
The MVC was acquired by instructing the subject to maximally abduct the
index finger and measuring the force produced. This was repeated three
times, and the maximum force measured was taken as the abduction MVC.
Subsequently, the same procedure was repeated for the flexion
direction, and the flexion MVC was determined. Flexion force was
recorded (as a percentage of the flexion MVC) to ensure that the
subject was performing the prescribed task of contracting only in the
abduction direction.
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
),
physiological (Roos et al. 1997
), and histochemical
(Ansved and Larsson 1995
) properties of the motor unit
(MU) with age. In light of the significant changes in the properties of
the elements of a motor unit, it can be hypothesized that the control
aspects of the motor unit would be modified as an adaptive or
compensatory mechanism to preserve force production. Indeed, the firing
rates of aged motor units were found to be lowered in several studies
(Howard et al. 1988
; Nelson et al. 1984
;
Newton et al. 1988
; Soderberg et al.
1991
). Other studies reported decreased firing rates
specifically at high level contractions (Kamen et al.
1995
). Another study on the first dorsal interosseous (FDI)
reported an altered mode of recruitment/derecruitment in the elderly
(Kamen and De Luca 1989
). In contrast, Galganski
et al. (1993)
reported no age-related change in the firing
behavior of FDI motor units during a threshold task. Apart from these
few studies, there is little published data on the control aspect of
the motor unit, especially at higher force levels.
;
LeFever et al. 1982
) allows accurate identification of
the firing times of motor units enabling investigation of the firing
behavior of several concurrently active motor units as a function of
time. The objective of this study was to investigate the age-related alterations in the firing behavior of concurrently active human motor
units in the FDI muscle at force levels that were significantly different from threshold [20 and 50% maximal voluntary contraction (MVC)], using the accuracy offered by the Precision Decomposition technique. At these force levels, especially at 50%MVC, it is possible
to observe a motor unit sample that is representative of the whole population.
METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
MYOELECTRIC SIGNAL RECORDINGS AND ANALYSIS. The specially designed 25-gauge quadrifilar needle electrode used to acquire myoelectric data carries in its bore four pickup wires (cross-sectional diameter 50 µm), which are exposed from a side port on the cannula and separated by 200 µm. The three differential pairs of intramuscular signals (band-pass filtered 1-10 kHz) derived from these four pickup surfaces were recorded in addition to the electrical activity detected by the cannula of the needle with respect to the reference (band-pass filtered 10 Hz to 1 kHz). The surface EMG signal was recorded using a bipolar surface electrode (band-pass filtered 10 Hz to 1 kHz). A moistened velcro strap placed around the forearm near the elbow and an electrode with a gel interface placed on the middle finger served as reference electrodes.
The intramuscular EMG signals were resolved into the individual motor-unit firing trains using the Precision Decomposition technique (De Luca 1993Data analysis
DETERMINATION OF CONSTANT FORCE INTERVAL. To ensure a common basis for comparison of motor-unit firings, a 5-s-long stable force interval was determined for each trial in which the force at any instant in the interval was within ±10% of the mean force in the interval, and the coefficient of variation (CV) of force was <0.4 over this period. Average firing rates and cross-correlation functions between mean firing rates were calculated over the first such stable force interval.
MOTOR-UNIT PARAMETERS. The recruitment threshold of a motor unit was calculated by averaging the force over 15 samples (corresponding to a window of 7.5 ms) beginning at the first discharge time of the motor unit. The continuous mean firing rate of a motor unit was obtained by passing a Hanning window of duration ranging from 0.4 to 2 s over an impulse train consisting of the discharge times of the motor unit. The average firing rate was defined as the average of the continuous mean firing rate signal in the stable force interval. The firing rate slope was calculated as the slope of the regression line fit to the mean firing rate signal in the region starting from where the force and mean firing rates stabilized at the target level to the end of the force plateau. The mean and standard deviation of the interpulse interval were calculated in the same stable force region, and the coefficient of variation of the interpulse interval was defined as the ratio of the standard deviation of the interpulse interval to the mean interpulse interval. The initial firing rate was calculated by inverting the average of the first three interpulse intervals corresponding to the first four firing instances of the motor unit. The choice of four spikes represents a compromise between reducing the variance of the estimate and capturing transient changes in the firing rates, given the high instability of motor-unit firings at recruitment. The coefficient of variation of force in a given interval was defined as the ratio of the standard deviation of the force samples to the mean of the force signal in the interval.
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RESULTS |
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Data from a total of 231 motor units were acquired and analyzed during isometric contractions at 20% (69 motor units from the young, 68 from the elderly) and 50%-MVC (43 motor units from the young, 51 from the elderly) force levels.
Interaction among firing rates of concurrently active motor units
It has been proposed that motor units of a given motoneuron pool
respond to a "common drive" (De Luca et al. 1982b;
De Luca and Erim 1994
; Erim 1992
) and
that the response of an individual motor unit is prescribed by its
inherent drive/firing rate characteristics (Erim et al.
1996
). The common fluctuations observed in the mean firing
rates of concurrently active motor units in healthy, young subjects
were attributed to the fluctuations in this common drive to the
motoneuron pool. In data collected from young subjects in this study,
common fluctuations were consistently observed in mean firing rates at
both the 20% and the 50%-MVC levels. Figure 1, A and B,
displays the firing behavior of young motor units at two contraction
levels. In addition to the common firing rate fluctuations, the
top panel of the figure exemplifies other collective firing
properties of motor units, which will be discussed. In most aged
subjects and in most of the time during the contractions, the common
fluctuations in the mean firing rates of motor units were found to be
decreased as seen in Fig. 1, C and D. Note that decreased commonality in MU firing rates is also seen in Figs. 3 and 4,
where in addition to the out-of-phase behavior of fluctuations, motor
units even exhibited different trends (some increasing at the same time
as others decreasing) in their firing rates.
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To study the commonality of firing rate fluctuations in the elderly, the pair-wise cross-correlation of the dc-removed mean firing rates of concurrently active motor units was calculated over the 5-s constant force interval. The cross-correlation values obtained from a young and elderly subject are shown in Fig. 2. The cross-correlation values in the elderly are reduced as is evident in Fig. 2B, and random phase values between the firing rates are observed. In performing statistical comparison of the common drive behavior in the two age groups, the cross-correlation function was calculated between the mean firing rates of all the motor units observed in a given contraction. This resulted in 146 motor unit pairs in the young and 182 pairs in the elderly group. Because there was no effect (P > 0.05) of force level on cross-correlation peak values, the peak cross-correlation values obtained at 20 and 50%MVC levels were grouped together within each age group. The group means ± SD were 0.49 ± 0.16 for the young, and 0.43 ± 0.14 for the aged group. Because the distribution of the peak values did not conform to a normal distribution, the nonparametric analogue of the two-sample t-test, the Mann-Whitney U test, was used to compare the two age groups. The hypothesis that the two groups had the same distribution was rejected at P < 0.005 level of significance. Although the group means appear to be close, the distribution of the cross-correlation peak values were different. For instance, in the young 48% of the pairs had peak values >0.5, whereas in the aged only 30% of the pairs had peak values above 0.5. In fact, this difference in distributions is what is captured by the Mann-Whitney test, with the high number of samples available for both groups yielding a high power for the comparison. The location of the peak values, or the delay between the mean firing rates of motor units (0.16 ± 0.28 s for the young, and 0.23 ± 0.31 s for the aged group) were also found to be different at P < 0.005 using the Mann-Whitney test. Increased delays and increased variability in delay are to be expected from the reinnervation process, which alters the distance the action potentials travel along the neurons that have picked up orphan fibers.
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Firing rate-recruitment threshold dependence
Another significant property displayed by concurrently active
motor units in healthy, young subjects is the inverse relationship between their firing rates and recruitment thresholds (De Luca et al. 1982a; De Luca and Erim 1994
; Erim
et al. 1996
; Person and Kudina 1972
). This
relationship manifests itself as a nestling of the mean firing rate
curves within one another, with the earlier recruited motor units
achieving a higher firing rate than the ones recruited subsequently,
resulting in the term "onion skin phenomenon" (De Luca and
Erim 1994
). In the elderly, the "onion skin" behavior was
disrupted in almost all the trials with the mean firing rate curves of
the later recruited motor units "crossing over" the mean firing
rate curves of the earlier recruited motor units. Figure
3 shows this altered firing behavior in
the elderly subjects at the two force levels studied. "Cross-over"
is also evident in the bottom panel of Fig. 1 and in Fig.
4.
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To quantify the extent of the violation of the onion skin rule in the elderly, motor units observed from a given subject were pooled for each contraction level. Regression lines were fit to sample points representing the recruitment threshold and average firing rate of each motor unit identified for the subject at a given force level. Although due to the small sample sizes statistical significance for the regression fit was reached only a quarter of the time, for all of the young subjects (100% of the subjects at both force levels), the slope of the regression line was negative, indicating the onion skin phenomenon. For the elderly subjects, at 20% of MVC 56%, and at 50% of MVC 50% of the subjects had negative slopes, whereas the rest of the subjects displayed positive slopes indicating violation of the onion skin phenomenon.
It is well-known that in the young, the order of de-recruitment of motor units is the reverse of the order of recruitment (see Fig. 1). In the elderly, the same order of recruitment and de-recruitment was noted to be preserved in almost all the cases, i.e., the motor units that were recruited later were de-recruited earlier.
Firing rate trends during sustained contractions
During 8-15 s constant-force contractions in young subjects, the
firing rates of motor units gradually decrease with time and the rate
of decrease is positively correlated to the recruitment threshold of
the motor unit (De Luca et al. 1996). This behavior was
observed in the data collected from the young subjects in this study
(see Fig. 1, top panel). In the data collected from the
elderly, few motor units decreased their firing rates with time during
the constant force plateau, whereas other concurrently active motor
units increased or maintained their mean firing rates as seen in Fig.
4. Note the increasing trend after the first 20 s following the
decreasing trend in the mean firing rates in Fig. 4A. Firing
rate trends in different directions among concurrently active motor
units are also seen in the bottom panel of Fig. 1 and in
Fig. 3. In comparing firing rate trends, we used the slopes of the
regression lines fit to mean firing rate signals. In agreement with our
previous findings, firing rate drops were significantly higher at the
higher force level for both age groups (P < 0.005). As
the distribution of firing rate slopes did not conform to a normal
distribution (P > 0.05), the nonparametric
Mann-Whitney U test was used to compare the age groups. For
the 20% MVC contractions, there was no statistically significant
difference (P > 0.05) between the two age groups
(
0.050 ± 0.049 pps2 for the young;
0.040 ± 0.049 pps2 for the aged).
However, at 50% MVC, there was a significant difference at
P < 0.005 (
0.28 ± 0.27 pps2 for the young;
0.16 ± 0.24 pps2 for the aged).
Average firing rates of motor units
The results from all the subjects in each group were pooled
together for each force level. The average firing rate of each identified motor unit is plotted against its recruitment threshold in
Fig. 5 for 20% (A) and for
50%-MVC (B) levels, with squares representing data from
young subjects and triangles corresponding to aged subjects. Table
1 summarizes the parameters of the linear regression analyses at the two force levels. The firing rates of the
motor units in the elderly were depressed in comparison to those
observed in the young at both force levels as displayed by the
regression lines fit to each group. The coefficients for the regression
equations were compared at a significance level of 0.05 at each
contraction level using the statistical tests described by Zar
(1984). For the 20%-MVC case, the null hypothesis of
equal regression line slopes for the young and the elderly was
rejected, resulting in the rejection of the null hypothesis that the
data from the young and elderly were generated by the same regression
model. For the 50%-MVC case, the null hypothesis of equal slopes could
not be rejected. The intercepts of these lines were then compared, and
the null hypothesis of equal intercepts was rejected, thus resulting
again in the rejection of the null hypothesis that the data from the
young and elderly were generated by the same regression model.
Averaging of the firing rates across motor units of all thresholds
yielded 18.33 ± 2.76 pps for the young, 15.17 ± 3.31 pps
for the aged at 20% MVC; and 26.08 ± 4.52 pps for the young,
22.11 ± 5.57 pps for the aged at 50% MVC. The difference between
the age groups were significant (P < 0.001) for both
force levels.
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Initial firing rates
The initial firing rates of motor units in the elderly subjects
were lower as compared with the young at both 20% and 50%-MVC force
levels. Figure 6 shows the initial firing
rate plotted as a function of the recruitment threshold for 20%-MVC
(A) and 50%-MVC (B) contractions. As seen from
the figure, the slight positive correlation between the initial firing
rates and recruitment threshold (Clamann 1970; De
Luca and Erim 1994
) is maintained in the elderly. Table
2 presents the parameters of the
regression lines calculated for the two age groups at the two force
levels. The regression equations were compared at a significance level
of 0.05 at each contraction level using the same statistical tests
employed for the comparison of average firing rate versus recruitment
threshold comparisons. For both force levels, the null hypothesis of
equal slopes for the two equations could not be rejected. The
intercepts of these lines were then compared, and the null hypothesis
of equal intercepts was rejected.
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Coefficient of variation of interpulse intervals
In analyzing the variability among interpulse intervals, units
recruited within 5%MVC of the target level were not considered. Such
units, operating too close to their recruitment threshold, would be
effectively turning on and off with slight fluctuations in drive, and
hence were considered unstable. At 50% of MVC, the CV of interpulse
intervals of a given motor unit was independent of the recruitment
threshold of the unit in both age groups (P > 0.10),
in agreement with our previous findings (Erim et al. 1995). Student's t-test revealed no statistically
significant difference (P > 0.05) between coefficients
of variation of the two groups at this force level (young: 0.283 ± 0.071; elderly: 0.301 ± 0.083). In contrast, at 20% of MVC,
the CV of interpulse intervals appeared to be linearly related to
recruitment threshold (P < 0.005) for the aged group,
whereas it was independent of recruitment threshold for the young group
(P > 0.10). A t-test between the two groups
pointed to a statistically significant (P < 0.005)
difference between the groups (young: 0.188 ± 0.078; elderly:
0.234 ± 0.074).
Distribution of recruitment thresholds
Recruitment was observed throughout the range up to 50% of MVC in both the young and the elderly. However, the recruitment threshold distribution in the elderly reflected a shift toward lower values. Figure 7 shows the percentage count of recruitment thresholds of motor units observed throughout the range in 20% (A) and 50%-MVC (B) trials. The recruitment thresholds were observed to be almost the same for motor units sampled from a single trial in many instances in the elderly. On the other hand, the recruitment thresholds of motor units sampled in a given trial were found to vary in a wider range in the young.
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Maximal voluntary contraction force
The maximal voluntary contraction force measured at the beginning of each experiment was significantly different in the young and the aged subjects. The maximal force of abduction is plotted as a function of age in Fig. 8. A t-test between the groups indicated that the mean abduction forces in the two populations (37.5 ± 8.253 N in young and 27.4 ± 10.55 N in the elderly) were significantly different (P < 0.05). There was a high variation in the maximum strength among both the young and the elderly groups.
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Fluctuations in force
The elderly subjects were able to track the presented trapezoidal
trajectories with fair amount of accuracy in force. However, in many
cases, the elderly subjects' force contained high-frequency fluctuations. The power spectral density estimate on the force plateau
(60 s duration at 20% and 20 s duration at 50% of MVC) was
calculated (Welch 1967) using 10-s duration windows with
50% overlap to examine whether the spectral content differed in some fashion from the young. Even though there was variability in each group, power was in general distributed over a greater bandwidth in the
elderly as exemplified in Fig. 9.
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The CV of force (defined as the ratio of the standard deviation to the mean of the force signal) was calculated over the entire force plateau at both 20%-MVC level (young: 0.031 ± 0.012; elderly: 0.030 ± 0.010) and 50%-MVC level (young: 0.028 ± 0.005; elderly: 0.030 ± 0.013). A t-test on the CV of force between the young and the elderly groups indicated that it was not significantly different (using significance level P < 0.05) at either force level.
Satellite potentials and polyphasic potentials
Satellite and polyphasic motor-unit action potentials (MUAPs) were
observed in the signals recorded from the elderly muscle. Figure
10 shows MUAPs recorded via the
intramuscular needle electrode from two elderly subjects. In Fig.
10A a distinct satellite potential fires after the first
potential, and in B, two satellite potentials fire after the
first potential. Satellite potentials suggest reinnervation of a group
of orphaned fibers by collateral nerve sprouting from a surviving axon
(Dorfman et al. 1988). Motor units possessing these
satellite potentials were detected more often in the elderly (in all, 8 satellite potentials from 4 elderly subjects were observed as compared
with 2 satellite potentials from 1 young subject).
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DISCUSSION |
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The most important contribution of this work has been to offer
insight into the age-related changes in the time course of motor-unit
firing patterns throughout a contraction. In addition to the decrease
in average firing rates, which had been reported before, our results
reveal that there are significant modifications in the instantaneous
behavior of aged motor units, which are averaged out when average
firing rates over the whole contraction are used. For example, when
only average firing rates are considered, it would seem that the main
effect of aging is similar to that of hand dominance: a decrease in the
firing rates (Adam et al. 1998) that could be explained
by the general slowing of the muscle with a shift toward type I fibers.
However, the investigation of the firing rates as a function of time
throughout the contraction along with the interaction between the
firing rates of concurrently active motor units reveal significant
differences between the two paradigms and suggest important age-induced
alterations in the control of motor units.
Time-varying behavior of motor units
DECREASE IN COMMON DRIVE. The most striking finding of this study is the decrease in the commonality of motor-unit firings in the elderly. This diminished commonality, manifest in the decreased correlation among the fluctuations in motor-unit firing rate as well as the different firing rate trends raises the question as to whether the natural aging process results in significant modifications to the arrangement of the motoneuron pool.
The most direct explanation for the observed decrease in the correlation among the firing activities of concurrently active motor units would be a decrease in the ratio of shared versus unshared inputs received by the motor units. This decrease could come about via a decrease in the common inputs, an increase in the unshared inputs, or a combination thereof. The balance of shared versus unshared inputs to motor units can be altered by age-related changes in spindles including increased capsular thickness, a decrease in the mean number of intrafusal fibers per spindle, spherical axonal swellings and degenerative changes in the spindle neuromuscular end plates (Swash and Fox 1972CROSS-OVER: VIOLATION OF THE ONION SKIN PHENOMENON.
The disturbance to the onion skin phenomenon and the observation of
cross-over as well as different mean firing rate trends among the motor
units of the elderly may be a direct result of a decrease in common
drive. If motor units are responding to significantly different drives,
they cannot be expected to keep the orderly relationship among their
firing rates. However, the possibility of a disturbance in the
intrinsic excitability and firing responses of motoneurons cannot be
ruled out. A decreased sensitivity to excitation has been reported for
aged motoneurons (McComas 1977). Apparently at odds with
this report, but nonetheless demonstrating a disturbance in the
excitation/firing rate characteristics, Morales et al. reported that
motoneuron rheobase current was lower in aged compared with young adult
cats (Morales et al. 1987
).
LACK OF DECREASE IN FIRING RATES DURING SUSTAINED CONTRACTIONS.
In the elderly, the systematic decrease in firing rates of motor units
during a sustained contraction was not observed consistently. In many
cases, concurrently active motor units displayed differing trends in
their firing rates, with some motor units increasing their firing rates
while others decreased or maintained their firing rates. The mechanisms
previously proposed to account for the progressive decrease in the
firing rates include a combination of the late adaptation properties of
the motoneuron (Kernell 1965; Kernell and Monster
1982
) and an accompanying decrease in drive to the motoneuron
pool (De Luca et al. 1996
) due to twitch potentiation of
motor units (Macintosh et al. 1994
; Vandervoort
et al. 1983
). Aging has been shown to be associated with
decreased twitch potentiation in the tibialis anterior muscle of humans
(Hicks et al. 1991
) and the medial gastrocnemius of rats
(Kanda and Hashizume 1989
). If the twitch
potentiation capacity is decreased also in the FDI due to the aging
process, it would be in line with the observed absence of firing rate
decline. Another reason for the lack of decrease in firing rates may be
the selective loss of larger motor units, as reflected in the
significant difference observed in firing rate drops at 50% MVC and
lack thereof at 20% MVC. At the low level, both age groups use
low-threshold motor units that show the least potentiation
(Burke 1981
) that leads to minimal firing rate slopes
(De Luca et al. 1996
) in both age groups. However, at
the 50%MVC level where all motor units are expected to be recruited in
the FDI (De Luca et al. 1982a
,b
; Milner-Brown et
al. 1973
), the elderly are still limited to the small,
low-threshold units, whereas the young rely on high-threshold motor
units that display the most potentiation (Burke 1981
)
and hence the greatest firing rate decrease.
Time-averaged motor-unit parameters
In addition to the novel findings regarding the time course of
firing rates during constant-force contractions, our study yielded
several findings that had been reported by other groups (for a review
of age-related changes in motor unit function, see Roos et al.
1997).
DECREASED AVERAGE FIRING RATES.
Several studies (Howard et al. 1988; Nelson et
al. 1984
; Newton et al. 1988
; Soderberg
et al. 1991
) have reported a general decrease in firing rates
in older subjects, in agreement with our findings displayed in Fig. 5.
In contrast to these reports, no age-related change was observed in the
firing behavior of motor units in the FDI during a "threshold" task
(where the force level was just sufficient to maintain a constant, low
discharge rate for a single motor unit value) by Galganski and
coworkers (Galganski et al. 1993
). Others reported no
change at 50%MVC (Kamen et al. 1995
) and decreased
firing rates at higher (Roos and Rice 1996
) or maximal
force levels (Kamen et al. 1995
). These different
results may have arisen from the difference in the controlled parameter (firing rate as opposed to force output), or differences in calculating the average firing rate, neglecting the dependency of firing rates on
recruitment threshold and considering an overall average firing rate
for the whole motor-unit population (thereby allowing motor units with
different thresholds to weigh differently in the average).
Force output
As expected from the obvious signs of muscle atrophy and
waste, the mean MVC force for the young and the elderly subject groups were found to be significantly different (P < 0.05)
using the t-test. However, the CV of force throughout the
contraction revealed no statistically significant difference among the
two age groups at either force level. In contrast, Galganski and
coworkers (Galganski et al. 1993) found a higher CV of
force in the elderly. In a similar study, Keen et al. found higher CV
of force at levels up to 20% MVC, but no significant difference at
50%MVC (Keen et al. 1994
). Galganski et al.
(1993)
found no difference between the two age groups in terms
of the CV of interpulse intervals. In the present study, although there
was no significant difference at 50%MVC, the CV of interpulse
intervals was higher in the elderly at 20% MVC. Although these
conflicting results do not draw a clear picture, our finding of
increased CV of interpulse intervals at 20% MVC, considered in light
of the increased CV observed by Keen et al., may indicate a mechanism
for compromised motor control at lower force levels in the elderly.
This study has revealed that significant changes in the control
properties accompany age-related alterations in the morphology of motor
units. Some of these changes, like the reduction in the average firing
rates in response to a general slowing of muscle fibers, can be
speculated to be a direct result of the changes in mechanical
properties. However, it remains to be understood if other age-related
changes, specifically the decrease in the common drive and onion skin
behavior observed in this study, are compensatory/adaptive mechanisms
to mitigate the diminutive effects of aging on the musculature as
suggested by Roos et al. (Roos et al. 1997); or if they
are independent, age-induced insults to the neural control of muscles
that might be stopped or reversed through appropriate training and
neural "relearning."
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ACKNOWLEDGMENTS |
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The authors are grateful to M. Khouri for technical assistance and J. Meyer for data analysis.
This work was supported by a grant from the Department of Veterans Affairs Rehabilitation Research and Development Service.
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FOOTNOTES |
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Address for reprint requests: Z. Erim, NeuroMuscular Research Center, 19 Deerfield St., Boston, MA 02215.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 15 March 1999; accepted in final form 8 June 1999.
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REFERENCES |
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