INTRODUCTION

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NORMAL AGING RESULTS IN MANY physiological changes in the neuromuscular system, including the loss of motoneurons and the subsequent reorganization of motor unit territories (7, 11, 27). One consequence of these adaptations is a decline in the ability to perform simple motor tasks, such as exerting a constant force during a submaximal isometric contraction (18, 23). For example, the magnitude of the force fluctuations during isometric contractions by a hand muscle is greater for old adults compared with young adults (18). Similarly, when using a hand muscle to lift light loads while performing a position-tracking task with the index finger, the old adults exhibit greater fluctuations in displacement compared with young adults (25). On the basis of such findings, it appears that old adults are less steady than young adults when exerting low forces and lifting light loads.

There are at least two features of the neural drive to muscle that could account for differences in the magnitude of the fluctuations in force or acceleration during the performance of a task: the characteristics of the most recently recruited motor units (1, 8, 13) and the relative activation of the agonist and antagonist muscles (21, 30, 32, 33). Because motor units appear to be recruited in ascending order of size and initial discharge rates are low, fluctuations in the net force are influenced by the discharge behavior of the most recently recruited motor units. This effect of the unfused tetani on the force fluctuations is greatest at low forces, when the relative contribution of a motor unit to the net force is greatest, and is greater in muscles with a narrow recruitment range (17), such as occurs with the decline in motor unit number with age (7, 11). Furthermore, adaptations that exaggerate size differences among motor units, such as short-term synchronization (10) and the reorganization that occurs with aging (7, 11, 27), magnify the force fluctuations by increasing the relative contribution of individual motor units to the net force. Alternatively, fluctuations in force and acceleration could be produced by alternating activation of the agonist and antagonist muscles (32, 33).

On the basis of these reports on coactivation during slow finger movements (32, 33) and heightened coactivation in old adults (30), we hypothesized that the decline in steadiness with advancing age is due to enhanced coactivation of the antagonist muscle. The purpose of the study was to determine the association between steadiness and activation of the agonist and antagonist muscles during submaximal isometric and anisometric contractions. Subjects exerted constant forces by performing isometric contractions and lifted constant loads with anisometric contractions, and the steadiness of each performance was compared with the electromyogram (EMG) of the agonist and antagonist muscles. Some of these results have been presented previously (6, 14).

	METHODS

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Twenty-nine healthy subjects (14 men, 15 women) participated in the study. The subjects were assigned to one of two experimental groups: an old group [n = 15; 74 ± 1.5 (SE) yr, range 66-82 yr] and a young group (n = 14; 23 ± 0.9 yr, range 19-30 yr). There were seven women in the old group and nine women in the young group. All subjects were right-hand dominant and had no known neuromuscular disorders. The Institutional Review Board at the University of Colorado approved the experimental procedures, and each subject provided written consent before participating in the study.

Mechanical Recording

For the isometric contractions, the left hand was positioned so that the proximal interphalangeal joint of the index finger was aligned with a force transducer (model 13, Sensotec) that monitored the abduction force exerted by the index finger. A low-sensitivity transducer (0.053 V/N, range 0-220 N) was used to measure forces >35% of the maximal voluntary contraction (MVC) force, whereas a more sensitive transducer (0.54 V/N, range 0-22 N) was used to monitor forces <35% of MVC force. For the anisometric contractions, the index finger was free to move in the abduction-adduction plane. An electrogoniometer (Biometrics K100, Penny & Giles) was placed on the dorsal surface of the index finger and wrist to measure the angular displacement about the first metacarpophalangeal joint. Additionally, an accelerometer (model 7265A-HS, Endevco; mass = 6 g; linear range of acceleration response: ±20 g; frequency response = 0-500 Hz) was attached to the proximal interphalangeal joint of the index finger to record acceleration in the horizontal plane, which corresponded to an abduction-adduction movement.

Electrical Recording

A bipolar intramuscular electrode was used to record the EMG of the antagonist muscle, second palmar interosseus. The electrode was inserted ~2 cm proximal to the metacarpophalangeal joint of the index finger. The electrode consisted of two Formvar-insulated, stainless-steel wires (50- and 100-µm diameter) that were threaded through a 25-gauge disposable needle. The needle was inserted into the muscle and then subsequently removed, leaving the wires in the muscle for the duration of the experimental session. Insertion of the intramuscular electrode into second palmar interosseus was verified by comparing the EMG during separate contractions of the second palmar and second dorsal interosseus muscles. Only those recordings showing no EMG activity during a maximal contraction of the second dorsal interosseus muscle were included in the analysis. Insertion of the electrode into second palmar interosseus was successful in 10 young and 9 old adults. A reference electrode (4-mm diameter; silver-silver chloride) was placed on the styloid process of the ulna. The surface EMG signals were amplified (×1,000-10,000) and band-pass filtered (20-800 Hz). The intramuscular EMG signals were amplified (×1,000-10,000) and band-pass filtered (0.1-5 kHz) before being recorded on a digital tape recorder.

Experimental Procedures

Isometric MVC force . With the index finger abducted 5°, subjects performed an isometric contraction with the first dorsal interosseus muscle that consisted of a gradual increase (~3 s) in abduction force from zero to maximum. The force was held at maximum for 2-3 s. Subjects were instructed to follow a verbal count for the timing of the contraction and performed two to three trials until the maximum force exerted for at least two trials varied by <5%. The maximum value was used in the analysis. Subjects were given 60 s between trials. The MVC force exerted by the index finger in the adduction direction was performed in the same manner with the second palmar interosseus muscle.

1-RM load. The passive range of motion for the index finger about the metacarpophalangeal joint in the abduction-adduction plane was determined (15-25°), and the displacement signal from the goniometer was displayed on the monitor. Cursors were positioned on the monitor to delineate the range of motion. A string attached around the subject's index finger and threaded over a pulley was connected to an inertial load that opposed abduction movements. Subjects were instructed to lift and lower the load slowly through the range of motion. The load was increased after each repetition until the subject could no longer complete the task. The maximal load that could be lifted over the complete range of motion was identified as the 1-RM load. Subjects were given 60 s between each attempt.

Constant-force task. Five target forces (2.5, 5, 20, 50, and 75% of MVC force) were determined for the isometric contractions. The target force and the force exerted by the index finger were displayed on the monitor. In a randomized order, subjects performed an isometric contraction and exerted an abduction force with the index finger to match the target force and were instructed to maintain the force as steady as possible for ~15 s. Once the exerted force reached the target, subjects were asked not to breathe for the duration of the contraction. This was done to minimize contamination of the force record by the respiratory rhythm. Subjects performed one trial at each force level, and all trials were performed with the index finger abducted by 5°.

Constant-load task . Five loads (2.5, 5, 20, 50, and 75% of 1-RM load) were raised and lowered with anisometric contractions. The angular position of the index finger was displayed on the monitor along with a triangular displacement template. Each subject was instructed to match the template by moving the index finger through the range of motion from the initial position (0° abducted). The duration of the movement was ~12 s, that is, 6 s to lift the constant load and 6 s to lower it. Each subject performed three consecutive trials with a load, and the order of the five loads was randomized.

Data Analysis

For the MVC task, the dependent variables for each muscle were the peak force and the average amplitude of the full-wave rectified EMG (AEMG) for a 0.5-s window centered at the peak force. For the constant-force task, the dependent variables were the SD of force within a 10-s window, the coefficient of variation for force fluctuations (SD/mean × 100), and the AEMG for both the first dorsal interosseus and the second palmar interosseus muscles within a 2-s window when the abduction force was relatively steady.

For the constant-load task, the dependent variables were the SD of acceleration and the AEMG for first dorsal interosseus and second palmar interosseus during the anisometric contractions. The SD of acceleration was measured for the middle 4 s of the lifting and lowering phases of the task. The SD was normalized to the load that was lifted (m · s²/kg). The AEMG was quantified as both a time course and as an average value for each phase of the trial. The AEMG time course was expressed as a set of ~150 average amplitudes, one for each contiguous 80-ms interval during the task. The average level of EMG during each phase was determined from the middle 0.5 s. The AEMG data were normalized to the value obtained for each muscle during the MVC task. Each subject performed three trials with each load, which were averaged for statistical comparisons.

The acceleration records from each experimental session were examined with spectral analysis to identify the location of peaks in the power density spectrum. This was done with fast Fourier transformation and a customized commercial software package (CED Spike2). For this analysis, the acceleration was sampled at 1,000 Hz. The fast Fourier transformation block size was 1,024 points, corresponding to an approximate bin width of 0.98 Hz. Separate spectral analyses were performed on the middle 4 s of the shortening and lengthening contractions in each trial. The power spectrum for each phase was derived from the average of seven contiguous data blocks that overlapped by one-half block size.

Statistical Analysis

	RESULTS

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The results from this study extended our previous work (18, 23, 25) by examining a greater range of forces, evaluating anisometric contractions, and assessing the role of coactivation as a mechanism that could contribute to the decline in performance with age. We found that the old adults were less steady than the young adults when performing submaximal isometric and anisometric contractions with the first dorsal interosseous muscle and that the old but not the young subjects were less steady when performing lengthening contractions compared with shortening contractions. Although the old adults tended to coactivate the antagonist muscle more often during these tasks, there were no associations between the difference in steadiness and the amount or the pattern of coactivation.

Isometric Steadiness

Representative records from a young and an old subject performing the constant-force task, which involved an isometric contraction, are shown in Fig. 1, A and C, respectively. The fluctuations in force were quantified as the SD and as the coefficient of variation. There was no effect of gender within the young or old groups on any of the measures of steadiness (coefficient of variation: gender main effect, P = 0.62; age × gender × force interaction, P = 0.76) or muscle activity (first dorsal interosseus: gender main effect, P = 0.98; age × gender × force interaction, P = 0.89; second palmar interosseus: gender main effect, P = 0.74; age × gender × force interaction, P = 0.54) during the isometric contractions. Therefore, for subsequent analyses, the data were collapsed across gender for both the young and old subjects.

View larger version (32K): [in this window] [in a new window]   Fig. 1.   Representative performances by a young (A and B) and an old subject (C and D) during isometric constant-force (A and C) and anisometric constant-load (B and D) tasks. A and C: isometric contractions at a target force of 75% maximal voluntary contraction (MVC). Top trace, full-wave rectified and filtered electromyogram (EMG) for second palmar interosseus (SPI); middle trace, EMG for first dorsal interosseus (FDI); bottom trace, abduction force exerted by the index finger. B and D: anisometric contractions with a load of 75% of 1 repetition maximum (1-RM). Top 2 traces, EMG for SPI and FPI muscles, respectively; bottom 2 traces, acceleration of the index finger in abduction-adduction plane and index finger displacement, respectively.

The SD of the force fluctuations increased as a function of target force for both the old and young subjects (Table 2; young: SD = 0.024 × force + 0.01, r² = 0.86; old: SD = 0.032 × force + 0.073, r² = 0.47). The SDs at the 2.5, 5, and 50% target forces were greater for the old subjects compared with the young subjects. Post hoc analyses indicated that the SDs at the 2.5, 5, and 20% target forces were significantly lower than at the 50 and 75% target forces for both groups. Furthermore, the SD at the 50% force was lower than that at the 75% force for the young subjects. Similarly, the coefficients of variation of the force during the 2.5, 5, 50, and 75% contractions were greater for the old subjects compared with the young subjects. Post hoc tests indicated that the coefficients of variation for the old subjects at the 2.5 and 5% forces were greater than at the 20, 50, and 75% forces. For the young subjects, the coefficient of variation at the 2.5% force was greater than at the 20, 50, and 75% forces.

The AEMG for first dorsal interosseus increased linearly as a function of target force for both groups (young: AEMG = 0.93 × force + 2.49, r² = 0.72; old: AEMG = 0.80 × force + 6.69, r² = 0.68). The relative amplitude of the AEMG for first dorsal interosseus was similar for the two groups of subjects at each target force, except at the two lowest forces (Table 2). Post hoc comparisons indicated that the AEMG for first dorsal interosseus was greater at the 50 and 75% target forces than at the 2.5, 5, and 20% forces for the old subjects. For the young subjects, the AEMG was significantly different between all target forces, except for the comparisons between 2.5 and 5% and between 5 and 20%.

The AEMG for second palmar interosseus also increased as a function of target force for both groups of subjects, although at a much lower rate and with more variability compared with the AEMG for first dorsal interosseus (young: AEMG = 0.18 × force + 3.55, r² = 0.36; old: AEMG = 0.25 × force + 7.50, r² = 0.25). There were no significant differences between the old and young subjects in the AEMG for second palmar interosseus across all target forces (Table 2). However, there were differences within the groups as a function of target force. Post hoc analysis indicated that the AEMGs for second palmar interosseus at the 2.5, 5, 20, and 50% forces for both the old and the young subjects were significantly less than that for the 75% force.

Anisometric Steadiness

Representative records from a young and an old subject performing the constant-load task with anisometric contractions are shown in Fig. 1, B and D, respectively. The steadiness with which subjects could lift and lower submaximal loads was quantified as the SD of acceleration relative to the load that was lifted. As with the coefficient of variation for the constant-force task, the normalized SD of acceleration declined as a function of load for both groups of subjects (Table 3). For both groups of subjects, the normalized acceleration fluctuations during the shortening and lengthening contractions were greater with the 2.5 and 5% loads compared with the respective higher loads. The old subjects had significantly greater normalized acceleration fluctuations than the young subjects for both the shortening (lifting the load) and lengthening (lowering the load) contractions with the three lightest loads (2.5, 5, and 20%) and during the lengthening contractions with the 50% load. Furthermore, the old subjects were less steady when performing the lengthening contractions compared with the shortening contractions with every load except 75%. Conversely, the young subjects were less steady with the shortening contractions compared with the lengthening contractions with both the 50 and 75% loads.

To assess the EMG activities of the first dorsal interosseous and second palmar interosseous muscles during the anisometric contractions, we used two approaches. First, the AEMGs (normalized to the AEMG obtained during the MVC task) were determined for 150 consecutive epochs and during the middle 0.5 s of the shortening and lengthening contractions. Second, we examined the experimental records on a magnified time scale (1-s duration) to determine whether the pattern of activation was characterized by alternating bursts of the first dorsal and second palmar interosseous muscles.

Average EMG. The ensemble AEMG activity for the agonist muscle (first dorsal interosseus) during the constant-load task involved a progressive increase in the amplitude during the shortening contraction and a progressive decrease during the lengthening contraction (Fig. 2). The AEMG for first dorsal interosseous at the transition from the shortening to the lengthening contraction increased as a function of load. For the antagonist muscle (second palmar interosseus), the AEMG was relatively constant during the lifting of the load but increased gradually when the load was lowered by a lengthening contraction of first dorsal interosseus. These average patterns were consistent in the young subjects across all loads. In the group of old subjects, however, the amplitudes of the activation patterns of the first dorsal interosseous muscle were more variable, as shown by the amplitude of the SE bars in Fig. 2. For example, the normalized AEMG for the first dorsal interosseous in the old subjects lifting the 2.5% load reached ~50% at the transition from the shortening to the lengthening contraction. This was due, primarily, to two old subjects whose AEMGs peaked at 78 and 149%, respectively, whereas the range of peaks was 19-55% for the other old subjects when lifting the 2.5% load. For these two subjects, the peak AEMGs increased as a function of load to 83 and 170% with the 75% load. These differences were not associated with differences in the displacement of the index finger, average velocity of the movement, or the steadiness of the contractions. Moreover, the AEMG patterns for the second palmar interosseous muscle in those two subjects were not different from the patterns exhibited by the other subjects.

View larger version (25K): [in this window] [in a new window]   Fig. 2.   Average amplitude of the full-wave rectified EMGs (AEMGs) over consecutive 80-ms intervals for FDI and SPI muscles of young (left) and old (right) subjects during constant-load task with 2.5% load. Duration is expressed as a percentage of the time for the movement (~12 s). Load was lifted with a shortening contraction by FDI for the first 50% of the task and lowered with a lengthening contraction by FDI for the second 50% of the task. Values are group means ± SE.

View larger version (20K): [in this window] [in a new window]   Fig. 3.   AEMG for agonist (A; FDI) and antagonist (B; SPI) muscles of young and old subjects during the middle 0.5 s for anisometric contractions as a function of load. Loads were 2.5, 5, 20, 50, and 75% of 1-RM load. Values are group means ± SE.

View larger version (19K): [in this window] [in a new window]   Fig. 4.   Associations between the normalized SD of acceleration and the AEMG of SPI. A: shortening contractions with 2.5% load. B: lengthening contractions with 2.5% load. C: shortening contractions with 75% load. D: lengthening contractions with 75% load. Each data point represents the average of 3 trials for a subject. AEMG corresponds to the average value over the middle 0.5 s of each contraction.

Patterns of EMG activity. Although the time course for the AEMG of second palmar interosseus was reasonably consistent during the constant-load task (Fig. 2), the subjects exhibited three distinct patterns (Fig. 5). Each subject consistently used a selected pattern across all loads. The first strategy involved minimal activation of second palmar interosseus during both the shortening and lengthening phases of the contractions (Fig. 5A). The young subjects used this strategy most often (8 of 10 subjects), compared with only 1 old subject. The second pattern, which was used by four old subjects and one young subject, involved a progressive increase in AEMG for second palmar interosseus muscle throughout the contraction (Fig. 5B). The third pattern, which was used by four old subjects and one young subject, comprised a marked and sustained increase in the amplitude of second palmar interosseus at the start of the lengthening phase (Fig. 5C).

View larger version (25K): [in this window] [in a new window]   Fig. 5.   Three patterns of muscle activation during shortening and lengthening contractions. AEMG for agonist (FDI) and antagonist (SPI) muscles of 1 young subject (A) and 2 old subjects (B, C) performing constant-load task with a 50% 1-RM load. Each data point represents the AEMG of the muscle for an ~80-ms interval. Duration of shortening (0-50% of time) and lengthening (50-100% of time) contractions are represented as a percentage of the total movement time. Both young and old subjects exhibited these patterns.

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View larger version (20K): [in this window] [in a new window]   Fig. 6.   Example of alternating activity of FDI and SPI muscles in an old subject performing a lengthening contraction with the 2.5% load. Traces depict, from top to bottom, EMGs of FDI and SPI muscles, acceleration of the index finger, and index finger displacement. This pattern of activation was found in the experimental records of 5 of 10 young subjects and 7 of 9 old subjects but was not related to the steadiness of shortening and lengthening contractions, as indicated by normalized SD of acceleration.

Spectral Analysis of Acceleration

View larger version (14K): [in this window] [in a new window]   Fig. 7.   Power density spectra for acceleration during anisometric shortening contractions. A: power density spectrum for a young subject performing a shortening contraction with 2.5% load. B: power density spectrum for a young subject performing a lengthening contraction with 20% load. C: power density spectrum for a young subject performing a lengthening contraction with 75% load. Records represent the average of 3 trials for each load.

In general, the shape of the power spectrum for acceleration comprised either a single low-frequency peak (5-12 Hz) or a low-frequency peak in combination with elevated power at higher frequencies. The spectra with a single low-frequency peak (Fig. 7A) were associated with the lightest loads (2.5 and 5%) for the shortening and lengthening contractions of both the young and old subjects. There was no difference due to age in the frequency at which the peak occurred. As the load increased to moderate levels (20 and 50%), an additional peak emerged at intermediate frequencies (Fig. 7B), again for both contractions and age groups. For five of the old subjects, however, this additional peak first appeared when lifting the 5% load. At the highest load (75%), the low-frequency peak was usually diminished and accompanied by a broad elevation of power at higher frequencies (Fig. 7C).

	DISCUSSION

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The main findings were that the old adults were less steady than the young adults when exerting low forces and lifting light loads with a hand muscle. The old adults were most unsteady when lowering light loads with lengthening contractions. There was no association, however, between steadiness and coactivation of the antagonist muscle (second palmar interosseus), neither in its average amplitude nor in alternating activation of the agonist and antagonist muscles.

Although we typically find a difference in strength for first dorsal interosseus between young and old adults (18, 23, 25), there was no difference in MVC force or 1-RM load between the two groups in the present study. Whereas most studies report that the muscles of older adults are weaker (20), there is substantial variability in motor performance such that some old adults are comparable to young adults (7, 24, 26). The finding of no difference in MVC force or 1-RM load, therefore, is probably a consequence of this variability. Nonetheless, the absence of a difference in strength underscores the interpretation that differences in steadiness and strength are not related.

Isometric Steadiness

Anisometric Steadiness

The greatest differences in steadiness between the young and old subjects during the anisometric contractions were for the lightest loads (Table 2). When these light-load tasks were performed, the old subjects used greater activation of both the agonist and antagonist muscles (Fig. 3). Furthermore, coactivation of second palmar interosseus during both the shortening and lengthening contractions was greater at all loads for the old subjects compared with the young subjects, although not statistically significant during the shortening contractions at 2.5, 5, and 20% of 1-RM load. Because the antagonist muscle exerts a force in the opposite direction to the agonist muscle, the quantity of motor unit activity in the agonist muscle must have been greater in the presence of coactivation. Accordingly, larger motor units would likely have been recruited in the presence of coactivation. Nonetheless, the distributions for SD and AEMG for second palmar interosseus (Fig. 5) indicated that there was no association between steadiness and the average level of coactivation of the antagonist muscle during the anisometric contractions.

Although the differences in steadiness were not related to the AEMG for second palmar interosseus, the old subjects did use more coactivation during the shortening and lengthening contractions (Figs. 3 and 4). For example, 8 of 9 old subjects but only 2 of 10 young subjects coactivated second palmar interosseus to a significant degree during the lengthening contractions (Fig. 5). These patterns were consistent for a subject across the different trials and loads. Because the AEMG may not be sensitive enough to reveal more subtle features of motor unit activity, we inspected the trials at a magnified time scale for evidence of bursting behavior. As has been shown previously (32), the slow finger movements performed in the present study were characterized by alternating bursts of activity of the agonist and antagonist muscles. However, we did not observe any qualitative differences between the young and old subjects that would explain the differences in steadiness.

Spectral Analysis of Acceleration

Similarly, the fluctuations in force and acceleration during the tasks examined in the present study exhibited a peak in the power density spectrum at 8-12 Hz. This raises the question of whether steadiness is simply a more general description of tremor-related activity? The spectral data suggest that steadiness and tremor are not manifestations of the same mechanism. For example, Vallbo and Wessberg (32) found that when subjects performed slow extension and flexion movements with a finger about the metacarpophalangeal joint, the spectra for acceleration were dominated by an 8- to 10-Hz peak. This peak was present in the spectra for all loads that were used, up to ~10% MVC, as we found in our study (Fig. 7A). The frequency at which the peak occurs in physiological tremor, however, decreases as the load on the limb increases (19, 31). In contrast, the fluctuations in acceleration included increased power in the range of 20-35 Hz when our subjects lifted moderate loads (Fig. 7B). Furthermore, the amplitude and symmetry of the oscillations are different for tremor and slow movements (3, 32) and there is often little association between the magnitude of various tremors and the impairment of functional activities (2, 4, 12).

Mechanisms Contributing to Differences in Steadiness

For the isometric contractions, the reduced steadiness in old adults does not appear to be caused by either enhanced levels of coactivation or reciprocal activation of the antagonist muscle. Furthermore, training-related adaptations suggest that the larger force fluctuations exhibited by old adults, at least for low forces, are not attributable to the use of motor units with larger peak-to-peak forces (23). In contrast, our laboratory has found a strong association at low forces between the force fluctuations and the variability in the discharge rate of motor units (25). At a target force of 2.5% MVC, for example, the coefficient of variation for motor unit discharge was 18% for young adults and 34% for old adults. Furthermore, simulated variations in short-term synchronization among motor units indicated a causative association between discharge rate variability and the coefficient of variation of force (34). However, it is unknown whether motor unit discharge is more variable at higher forces, where we found differences in steadiness in the present study.

Similarly for the anisometric contractions, the difference in steadiness between the young and old adults does not appear to be attributable to enhanced levels of coactivation or differences in alternating activity of the agonist and antagonist muscles. Alternatively, the difference in steadiness of anisometric contractions is likely due to the characteristics of the most recently recruited motor units. A comparison of the shortening and lengthening contractions provides some insight into the significant factors. Because the lowering of a constant load, which involves a lengthening contraction, is accomplished with a reduced level of motor unit activity (Figs. 1, 2, and 3A), differences in motor unit size and discharge rate would enhance the fluctuations in acceleration (16).

When subjects lifted light loads, the average discharge rate of motor units in first dorsal interosseus declined from 15.7 Hz for a shortening contraction to 10.5 Hz for the subsequent lengthening contraction for young subjects and from 17.0 to 13.3 Hz for old subjects (25). Although such reductions in discharge rate would cause a significant decline in the fusion of motor unit force, there was no difference in steadiness between the shortening and lengthening contractions performed by the young subjects. In contrast, there was a strong association between the steadiness of anisometric contractions and the coefficient of variation of motor unit discharge (25). At a target force of 2.5% MVC, for example, the coefficient of variation for the old adults was 33% for the shortening contraction and 43% for the lengthening contraction, compared with 17 and 20%, respectively, for the young adults. Alternatively, the increased size or the decreased number of motor units in first dorsal interosseus with advancing age (18, 27) could increase the fluctuations in acceleration due to a greater relative contribution of individual motor units to the net force (17). However, our laboratory has previously found that abolition of the difference in steadiness of isometric contractions between young and old adults after 4 wk of strength training was not associated with changes in the spike-triggered average force of low-threshold motor units (23) or, presumably, with the number of functioning motor units. Taken together, these findings suggest that the discharge variability of motor units may be the most significant factor contributing to differences in steadiness of submaximal contractions.

In summary, old adults were less steady with submaximal isometric and anisometric contractions with a hand muscle. They were least steady when performing lengthening contractions with light loads. These differences in steadiness were not associated with differences in the average level of agonist or antagonist EMG or in alternating activation of the agonist and antagonist muscles.