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Department of Integrative Physiology, University of Colorado, Boulder, Colorado
Submitted 12 October 2004 ; accepted in final form 28 January 2005
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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contraction type; first dorsal interosseus; manual dexterity; steadiness; strength training
One possible consequence of changes in motor neuron properties and the synaptic inputs they receive is an increase in the variability of motor unit discharge rate, which appears to be attributable to the amount of synaptic noise that is imposed on the membrane potential of the motor neuron during the afterhyperpolarization trajectory (4, 34). Although increased discharge rate variability of motor units does not appear to influence maximal strength, it does contribute significantly to the fluctuations in muscle force during submaximal contractions (29, 35). The functional significance of the increased fluctuations in muscle force is that it impairs the ability of an individual to exert a constant trajectory (6) or to move the limb accurately to a desired target (19). Experimental evidence from the first dorsal interosseus muscle during the performance of isometric, shortening, and lengthening contractions demonstrated that older adults have larger fluctuations in motor output compared with young adults (17, 25) and that these fluctuations were associated with greater variability in motor unit discharge rate (29). Computer simulations further supported these experimental findings (35, 45). For example, when the coefficient of variation for motor unit discharge increased from 10 to 40%, which corresponds to the range of values often observed experimentally (15, 35), there was a parallel increase in the coefficient of variation in the simulated force.
Auspiciously, the amplitude of the force fluctuations can be reduced through practice and strength training in healthy subjects (22, 25, 26, 30) and patient populations (2). For example, Keen and colleagues (25) demonstrated that strength training of the first dorsal interosseus muscle is an effective intervention to reduce the fluctuations in the abduction force exerted by the index finger in healthy old adults. Subsequently, it was shown that the training reduces the fluctuations during anisometric (shortening and lengthening) contractions of the same muscle and that practice is as effective as strength training (30). Reductions in force fluctuations through training have also been related to improvements in hand function of older individuals (41).
The above observations suggest a hypothesis that variability of motor unit discharge is a significant contributor to the impaired ability of older adults to perform steady muscle contractions (29, 45) and the capacity of both strength training and practice of a task to lower the enhanced fluctuations exhibited by older adults (25, 30). The present study used an intervention that required the older adults to practice the task for 2 wk followed by strength training for 4 wk to identify the relative contributions of practice and strength training to improvements in the fluctuations of motor output. The purpose of the study was to examine the association between discharge rate variability and the fluctuations in motor output by reducing the fluctuations and examining the effects on the discharge rate variability of motor units. The results indicated that reductions in motor unit discharge rate variability, primarily associated with the light-load training, were accompanied by lower fluctuations in motor output and improved manual dexterity of old adults. Some of these data have been presented in abstract form (27).
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METHODS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Experimental Arrangement
The experiments were performed on the left hand (nondominant). Each subject was comfortably seated in an upright position facing a 17-in. computer monitor, which was positioned 1.5 m in front of the subject at eye level. The monitor was used to display the position of the index finger for the subject. Both arms were slightly abducted and flexed to 
90° at the elbow. The forearms and hands were supported in the prone position by platforms with the right hand resting comfortably. The left hand was placed in a manipulandum with the third to fifth digits flexed and secured around a handle. The left index finger was kept extended with a splint secured to the lateral aspect of the finger, and thumb extension was maintained by a support (Fig. 1). This arrangement allowed abduction of the index finger about the metacarpophalangeal joint in the horizontal plane, a movement produced almost exclusively by contraction of the first dorsal interosseus muscle (32).
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For the single motor unit recordings, electrodes were custom fabricated using three Formvar-insulated, stainless steel wires (50-µm diameter; California Fine Wire, Grover Beach, CA), fixed together with medical-grade cyanoacrylate glue. A custom coiling apparatus (29) was used to coil the recording end of the fine wires around a mandrel (diameter of 0.13 mm) for 3 mm. The wires were then threaded through a single-use, 27-gauge hypodermic needle, and a barb of 2 mm in length was created at the tip of the recording end. The wires were cut perpendicularly with surgical grade scissors to expose the recording surfaces. The electrode was inserted into the first dorsal interosseus muscle with the hypodermic needle; up to two electrodes were inserted in each experiment. Recordings were obtained from two wires within each electrode; the third wire was used as an alternative bipolar configuration to optimize the recording quality. The possibility of recording from the same motor unit with the two electrodes in one experiment was minimized by positioning the electrodes in different parts of the muscle, making large (5 mm) adjustments in the location of the electrode in the search for single motor units and by comparing the behavior of the two motor units. The signals were amplified (1,000–2000x) and band-pass filtered (0.2–8 kHz), displayed on an oscilloscope, and stored on tape (Sony PC 116 DAT recorder, Sony Magnescale, Montvale, NJ). Motor unit potentials were detected online using an amplitude window discriminator (DIS 1, BAK Electronics, Rockville, MD).
Experimental Procedures
Each subject participated in a familiarization session and three experimental sessions. In the familiarization session, subjects received written and oral descriptions of the project, watched a visual demonstration of the protocol, and were given practice trials of the experimental task. The three experimental sessions included an initial session before the onset of training, a second session after 2 wk of light-load training, and a third session after an additional 4 wk of heavy-load training. Therefore, each subject acted as his or her own control. Four experimental measurements were made in these sessions: 1) manual dexterity, a test of the ability to work quickly and precisely with the hands and fingers; 2) recruitment threshold, determination of the minimal inertial load that had to be supported by an isometric contraction of the first dorsal interosseus for the isolated motor unit to discharge action potentials repetitively; 3) anisometric task, shortening and lengthening contractions of the first dorsal interosseus muscle to lift and lower a light inertial load; and 4) one-repetition maximum (1-RM) load, identification of the maximal load that could be lifted by a shortening contraction of the first dorsal interosseus muscle. The discharge of motor units was recorded during the anisometric task. Each experimental session lasted 1.5 h.
Manual dexterity. The manual dexterity of the subjects was assessed at the beginning of every experimental session, before the insertion of the fine-wire electrode, with the Purdue pegboard test (Lafayette Instrument, Lafayette, IN). This standardized, reliable (9) means of evaluating hand and finger function involved retrieving small metal pegs from a cup and placing them in a line of holes. The number of pegs placed in the holes within 30 s was recorded. The test was repeated twice with each hand, and an average value for each hand was determined.
Recruitment threshold.
This measurement was denoted as the minimal load at which a motor unit discharged action potentials repetitively. To determine this load, the subject attempted to maintain a constant finger position (5° of abduction) with the help of a target line on the feedback monitor, while small increments of mass (
5 g) were added to load the index finger in the direction of adduction. Mass was added until an electrode detected the discharge of action potentials from one motor unit. The load was adjusted further to ensure repetitive discharge of the unit without saturating the signal with activity from neighboring motor units.
Anisometric task. After a rest period, subjects lifted and lowered the same load identified by the recruitment threshold by using abduction-adduction movements of the index finger that were produced with shortening and lengthening contractions of the first dorsal interosseus. Subjects were encouraged to match index finger displacement to a triangular template shown on the monitor; they were required to produce slow, constant-velocity (1.7°/s) abduction-adduction movements over a 10° range of motion (Fig. 2). Each subject raised the load during 6 s of abduction (shortening contraction) and lowered the load during 6 s of adduction (lengthening contraction). Subjects repeated this movement five times. The experimenters carefully monitored the hand to ensure the movement was limited to abduction of the index finger about the metacarpophalangeal joint. Only those trials where motor units discharged repetitively throughout both the shortening and lengthening phases of the anisometric task were accepted for analysis. As well, only those trials in which the discharge of a single motor unit could be measured were evaluated for steadiness. When a motor unit could not be reliably followed through both phases of movement, the recruitment threshold and anisometric tasks were repeated or attempts were made to follow the discharge of another motor unit. To record from a different motor unit, the pairs of recording wires were either switched or pulled to a more superficial location, and the recruitment threshold task was repeated.
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Steadiness Training
Subjects performed steadiness training by lifting and lowering loads under identical conditions as in the experimental session. The LVDT was calibrated over a 10° range of motion before all training sessions, and subjects attempted to match the experimental template by displacing the index finger. There were 5 s of rest after each trial. All training was done in the laboratory under supervision, where emphasis was placed on performing steady contractions. Training was executed three times every week and consisted of 6 sets of 10 repetitions for a total of 60 trials per training session. Verbal encouragement was provided during the training sessions. The training involved a light load (10% 1-RM) for the first 2 wk and then a heavy load (70% 1-RM) for the final 4 wk. 1-RM load was tested every week, and training loads were adjusted accordingly. On average, each training session lasted 30 min.
Data Analysis
The data collected during the experiments were stored on tape and later downloaded to a computer and analyzed offline. The sampling rate was 200 samples/s for the position and acceleration signals and 20,000 samples/s for the single motor unit recordings.
Single motor unit discharges were identified on the basis of action potential shape and amplitude. A computerized, spike-sorting algorithm (Spike2, Cambridge Electronic Design, Cambridge, UK) assisted in this endeavor. The interspike intervals were examined for every trial, and those trials that contained abnormally short or long interspike intervals due to discrimination error were reanalyzed on a spike-by-spike basis. Motor units exhibited a tendency for a systematic increase or decrease in the discharge rate during shortening and lengthening contractions. Therefore, the slope of the linear regression line for the change in the interspike interval over time was subtracted from the data to remove the trend. Subsequently, the SD and coefficient of variation for the interspike intervals were determined.
For the trials in which single motor unit discharges were discriminated, the fluctuations in motor output were measured with an accelerometer attached to the lateral surface of the proximal interphangeal joint of the index finger. Each anisometric trial was analyzed by marking the beginning, middle, and end of the movement to indicate the lifting and lowering phases of the task. The SD of the acceleration for both the whole phase (6 s) and middle 4 s of each phase was determined and used to quantify the fluctuations in motor output.
Statistical Analysis
The dependent variables were the maximal load lifted (1-RM load); the Purdue pegboard test score; the SD of acceleration; the mean, SD, and coefficient of variation for the interspike intervals of motor unit discharge; and the slope of the mean interspike intervals during each contraction.
The effect of training on strength (1-RM load) and manual dexterity (Purdue pegboard test score) were assessed with one-way, repeated-measures ANOVA (3 training sessions). The effect of training and contraction type on steadiness (SD of acceleration) and motor unit activity (mean, SD, coefficient of variation, and slope of discharge trend) was assessed with a repeated-measures two-way ANOVA (2 contraction types x 3 time points).
Bivariate linear regressions were performed to examine the association between the coefficient of variation for motor unit discharge and the SD of acceleration, the association between the change in the coefficient of variation for motor unit discharge and the change in Purdue pegboard test scores, the association between the change in the SD of acceleration and the change in Purdue pegboard test scores, and the association between the change in 1-RM strength and the change in Purdue Pegboard scores. The 
level for all statistical tests (except post hoc analysis) was set at 0.05, and all significant interactions were examined with appropriate post hoc analyses; these included dependent t-tests with Bonferroni corrections to locate differences between the shortening and lengthening contractions and differences between sessions. Unless otherwise stated, means ± SD are reported in the text, whereas the figures depict means ± SE.
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RESULTS |
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All subjects achieved a higher score on the Purdue pegboard test (P = 0.002) for the right hand (13 ± 3 pegs) compared with the left hand (11 ± 3 pegs) at the beginning of the study. Furthermore, the score for the women with the left hand (12 ± 1 pegs) was greater than that for the men (9 ± 3 pegs) at the beginning of the study (P = 0.041). The score for the right hand did not change (P = 0.116) over the 6 wk of training. However, the score for the left hand improved significantly, with the greatest changes occurring during the final 4 wk of training (P = 0.004). The left and right hands achieved similar scores on the Purdue pegboard test at week 6. There was a weak, yet significant relation between improvements in 1-RM strength and improvements in Purdue pegboard test scores (r2 = 0.17, P = 0.03).
Steadiness Improvements
The intervention decreased the fluctuations in acceleration during shortening and lengthening contractions (Fig. 4). The data for the full 6 s and the middle 4 s of each contraction were similar, and, therefore, only the 6-s data are reported. The SD of acceleration was greater for the lengthening contractions (0.286 ± 0.102 m/s2; P < 0.001) compared with the shortening contractions (0.245 ± 0.124 m/s2) and remained so throughout training. The SD of acceleration decreased by similar amounts at week 2 for both the shortening (0.125 ± 0.077 m/s2) and lengthening (0.140 ± 0.056 m/s2) contractions (P < 0.001). There was no change in the SD of the acceleration from week 2 to week 6 (shortening: 0.116 ± 0.084 m/s2, lengthening: 0.136 ± 0.045 m/s2).
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Discharge rates were determined for 111 motor units in the three experimental sessions (week 0: 38 units; week 2: 39 units; week 6: 34 units). The absolute load at which motor units were recorded was similar for the three experimental sessions; the mean load was 97.2 ± 34.3, 72.3 ± 22.4, and 109.2 ± 62.6 g for week 0, week 2, and week 6, respectively. Typically, motor units displayed a systematic decrease in mean interspike interval during the shortening contractions and an increase during the lengthening contractions. Therefore, the slope of a regression line was subtracted from the data to remove the trend before the variability of motor unit discharge was calculated. Although the trend differed with contraction type (shortening: 6.5 ± 6.4 ms/s, lengthening: 10.7 ± 2.9 ms/s; P < 0.001), it was similar for all three sessions for each contraction type.
The mean, SD, and coefficient of variation for interspike interval were calculated for each trial, and these values were averaged over the trials in which the motor unit was recorded (mean 4 ± 1 trials). Mean interspike interval was longer for the lengthening contractions (115 ± 18 ms) compared with the shortening contractions (93 ± 18 ms; P < 0.001), which corresponded to 9.1 ± 2.1 pulses/s (pps) and 11.2 ± 2.2 pps, respectively. Mean discharge rate did not significantly change across the three experimental sessions (Fig. 5A).
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Association Between Discharge Rate Variability and Steadiness
There was a weak yet significant positive correlation between the SD of acceleration and the coefficient of variation of interspike intervals for both the shortening (r2 = 0.24; P = 0.003) and lengthening (r2 = 0.14; P = 0.002) contractions (Fig. 6). Approximately 20% of the variability in the SD of acceleration was explained from the variability of single motor unit discharge. Furthermore, data obtained from 9 of the 10 subjects revealed a moderate significant negative correlation (r2 = –0.26; P = 0.005) between the change in coefficient of variation for interspike interval (combined for both the shortening and lengthening contractions) and the changes in manual dexterity scores (Fig. 7B). Thus 26% of the improvement in manual dexterity was explained by the reduction in the variability of single motor unit discharge. There was a stronger significant negative correlation (r2 = –0.56; P < 0.001) between the change in SD of index finger acceleration with training and the change in manual dexterity scores (Fig. 7A). Approximately 56% of the improvement in manual dexterity could be explained by the reduction in the SD of index finger acceleration.
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Practice vs. Strength Training
Six weeks of training resulted in significant improvements in strength, as indicated by an increase in 1-RM load. The initial 2 wk of light-load training was equally effective in increasing the strength of the first dorsal interosseus muscle as the following 4 wk of heavy-load training. The time course of these improvements in strength was similar to that reported in earlier training studies involving abduction of the index finger (2, 25, 30). For example, Laidlaw and colleagues (30) observed increases of almost 23% in 1-RM strength of the first dorsal interosseus muscle after 4 wk of training, which compares with a 44% increase in 1-RM load after the first 2 wk of practice and a further increase of 27% after 4 wk of strength training in the present study. Because improvements in strength occurred so rapidly and with light loads, it is likely that the gains in strength were due to an improved ability of the nervous system to activate the involved muscles (1, 11).
The neural adaptations that contribute to rapid increases in strength for the first dorsal interosseus muscle likely involve the discharge characteristics of motor units within a muscle and the coordination of muscles involved in establishing the posture of the body during the task. Although we did not measure motor unit activity during the 1-RM task because of technical limitations, it has been found that the maximal discharge rate of motor units, which is reduced with aging (23), can be increased with strength training (39). Even in young adults, peak discharge rates of motor units do not appear to occur on the plateau of the force-frequency relation (16, 33) and adjustments in discharge rate can have a profound effect on strength activities (46). The strength gains can also be influenced by adjustments in the activation of support musculature (31, 42). Although we purposely chose abduction of the index finger due to the relative simplicity of the action, a muscle can only exert an effect on its surroundings when the body can provide the relevant reaction forces (24, 36). For example, the inability to match the maximal voluntary contraction (MVC) force by electrical stimulation of a muscle in vivo (8, 12) is likely due, at least in part, to the difference in postural support between the two conditions. Therefore, changes in the maximal force exerted in the abduction direction by the index finger depends not only on the contraction of the first dorsal interosseus muscle but also on the postural contractions of other support musculature.
Consistent with findings in other studies involving older subjects and light loads (3, 18, 29), the lengthening contractions initially exhibited greater fluctuations in acceleration. After 2 wk of practice, however, the fluctuations decreased for both contraction types while still maintaining differences in the SD of acceleration between the shortening and lengthening contractions. Because of the rapid adaptation (2 wk), it is likely that these improvements were also mediated by changes in the activation signal generated by the nervous system (6).
Motor Unit Discharge Characteristics
In an earlier study that used a similar experimental arrangement and task (29), old adults exhibited greater fluctuations in position and greater variability in motor unit discharge rate than young adults during anisometric contractions. It also has been demonstrated, in young women, during force matching to a sinusoidal target that the accuracy of force production with the first dorsal interosseus muscle is highly dependent on discharge rate variability (26). The findings of the present study demonstrated a weak association between the fluctuations in acceleration and variability of motor unit discharge rate. The relation was characterized by the parallel declines in discharge rate variability and the SD of acceleration fluctuations in response to training in individual subjects, as well as by the correlations between the two variables when all the experimental sessions were combined.
Although the observed associations between the fluctuations in acceleration and discharge rate variability were statistically weak, experimental limitations likely masked the magnitude of the effect. Studies that combine experimental measurements and computer modeling underscore the functional significance of discharge rate variability. When experimentally measured values for discharge rate variability were included in the model, the variation in force fluctuations from 2 to 95% MVC force was statistically similar to that measured experimentally (35). Importantly, this result was achieved by manipulating discharge rate variability of the entire motor unit population. In contrast, the present study compared the discharge rate variability of a single motor unit with the fluctuations in motor output due to the activity of many motor units. Furthermore, the discharge was recorded with the motor unit operating close to its recruitment threshold when the coefficient of variation for discharge rate was the greatest and likely unrepresentative of the average discharge rate variability for all the active motor units. Taken together, these studies provide compelling evidence that the variability in motor unit discharge has a strong influence on the fluctuations in motor output during steady contractions.
There were no changes in the mean discharge rate for each contraction type across the three sessions. The load supported during the anisometric contractions was set to ensure a minimal repetitive discharge rate for the isolated motor unit during both the shortening and lengthening contractions. Discharge rate, however, changed during each contraction and differed between the two contraction types. The systematic change in discharge rate during each contraction was likely due to the length-tension properties of the muscle rater than changes in the length of the moment arm for first dorsal interosseus relative to metacarpophalangeal joint of the index finger throughout the range in motion. Because the moment arm is maximal when the finger is fully abducted (5), the moment arm increased during the shortening contraction as muscle length decreased. The converse occurred during the lengthening contractions. Consequently, the change in discharge rate during each contraction type complemented the change in muscle length rather than the change in moment arm.
Mean discharge rate also differed for the two types of contractions, as observed previously (28, 29, 44). Two factors contributed to the lower mean discharge rates during the lengthening contractions. First, muscles are able to exert a greater force during lengthening contractions compared with shortening contractions (13, 48). Second, the net muscle torque must exceed the load torque to lift the load with a shortening contraction, whereas the net muscle torque must be less than the load torque to lower the load with a lengthening contraction. Therefore, the net muscle torque during the lengthening contractions must have been less than that for the shortening contractions, which must have involved lesser motor unit activity.
Training and Changes in Discharge Rate Variability
In addition to the observation of associations between discharge rate variability, fluctuations in index finger acceleration, and manual dexterity, the present study also demonstrated that the training intervention had an influence on the variability in motor unit discharge rate. Reductions in discharge rate variability, both the SD and coefficient of variation, occurred over the first 2 wk of light-load training for both the shortening and lengthening contractions. No further improvements were detected during the subsequent 4 wk of heavy-load training, despite continued increases in muscle strength. Thus the improved capacity of motor units to discharge action potentials regularly was related more to practice of the task, rather than the strength gains achieved by the muscle.
The time between successive action potentials discharged by a motor neuron depends on the recovery of its excitability after the preceding action potential (40). The dominant factor influencing this recovery is the prolonged afterhyperpolarization potential that occurs after each action potential, which is due to the fast rise and slow decline of a calcium-dependent potassium conductance (43, 47). As a result, the afterhyperpolarization comprises a rapid shift of the membrane potential away from threshold and a gradual recovery back toward threshold. Generation of the next action potential occurs when the threshold is reached and depends, therefore, on the rate of change in the membrane potential during the afterhyperpolarization and the magnitude of the instantaneous fluctuations (or synaptic noise) in the potential (4, 34). Consequently, one or more of these mechanisms must mediate the observed reduction in discharge rate variability. Because the effect was observed in a large sample of motor units, the adaptation may have been evoked by a broadly acting mechanism, such as descending monoaminergic input from the brain stem (20) or a reduction of synaptic noise caused by training-related adaptations in afferent input.
Functional Significance
The ability to reach, grasp, manipulate, and transport objects is critical to performing activities of daily living, and, therefore, the Purdue pegboard test is an appropriate functional assessment of hand control (9). Initially, performance on the Purdue pegboard test was better for the right hand than for the left hand and can be attributed to the degree of use and comfort subjects exhibit with their dominant hand (all subjects reported being right-hand dominant). Because training was performed with the left hand, there was no effect on the performance of the Purdue pegboard test with the right hand. However, as training with the left hand progressed, subjects were better able to use the left hand, and, by the conclusion of the 6 wk of training, subjects were equally effective at using the left hand as with the right hand on the Purdue pegboard test. Therefore, the present steadiness-training intervention improved manual function in older subjects, even with a protocol that was nonspecific to the task. Ranganathan et al. (41) previously found that nonspecific skill training of the hands by healthy old adults improved Purdue pegboard test scores but that it did not increase strength. In the present study, the performance on the Purdue pegboard was related to the strength gains, indicating that strength also contributes to manual dexterity.
The reduction in motor unit discharge rate variability decreased the fluctuations in motor output, which lowers trajectory error (6) and contributed to improved functional performance on the Purdue pegboard test. These interactions were reflected in the significant positive relations between the average variability in discharge rate for each motor unit, fluctuations in index finger acceleration, and performance on the Purdue pegboard test. A remarkable feature of these results is that the discharge characteristics of a few motor units in each experiment were related to fluctuations in the force exerted by the muscle that controlled the finger action, and this behavior was further related to performance of a multimuscle dexterity test.
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ACKNOWLEDGMENTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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Present address of K. W. Kornatz: Dept. of Kinesiology, Arizona State Univ., Tempe, AZ 85287.
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FOOTNOTES |
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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.
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P < 0.001 compared with week 2. B: the scores on the Purdue pegboard test improved for the hand used in training (left) but not the other hand (right). **P = 0.002 left hand compared with right hand. 
P = 0.004 compared with week 0 for the left hand only.
