Effects of aging on force variability, single motor unit discharge patterns, and the structure of 10, 20, and 40 Hz EMG activity
Introduction
Research on the aging neuromuscular system has focused on the histochemical and physiological changes associated with the mean level of force produced (for review, see [21], [33]), and less attention has been given to the neuromuscular changes associated with the dynamic, time-dependent properties of force control. A general finding is that there is an increase in the force output variability of older compared to young adults [13], [20]. This aging-related change in force variability has been postulated to arise from the discharge properties of single motor units [13], [17], [20], [34], however, there is little neurophysiological evidence to support this hypothesis [33].
Studies have found different results for the relation between discharge rate variability of single motor units and force output variability. For instance, Laidlaw et al. [20] reported greater variability in the discharge pattern of single motor units and greater force variability in older adults compared with young controls. However, although Erim et al. [11] found that single motor unit discharge variability at 20% maximum voluntary contraction (MVC) was greater in older compared to young adults, the authors did not find a difference in force output variability with aging. The present study directly tests the hypothesis that greater force variability in older adults is related to changes in the discharge rate variability of single motor units.
The discharge pattern of a pool of motor units grouped together could also influence force output variability [2]. For instance, the oscillatory activity in the 10, 20, and 40 Hz bands of the rectified electromyogram (EMG) of muscle represents the collective behavior of many motor units that create an oscillation at multiple frequency rhythms [2], [4], [10], [14], [15], [19], [25], [32], [36], [37]. During wrist extension, it has been shown that the 40 Hz EMG band is reduced in patients with Parkinson’s disease compared with control subjects [4], and that there is greater 10 Hz EMG power during wrist extension of patients with Parkinson’s disease. Additionally, the 40 Hz EMG band was correlated with activity from the hand region of the primary motor cortex [5] and levodopa therapy given to persons with Parkinson’s disease increases 40 Hz EMG activity [24]. These findings support the postulation that multiple oscillatory activity plays a critical role in the physiology of motor control. The second goal of this study is to examine the hypothesis that greater force variability is related to a loss of 40 Hz EMG activity and an increase in 10 Hz EMG activity in older adults.
These two hypotheses about the relation between force variability, motor unit discharge behavior, and the frequency structure of EMG activity were examined in young (20–24 years), old (64–69 years), and older-old (75–90 years) adult humans in vivo during isometric second finger abduction force of the first dorsal interosseous muscle. Although previous studies have shown greater force variability in older adults compared with young adults during constant force production, the generality of this finding across different tasks, that might yield different motor unit discharge behavior, has yet to be determined. Therefore, we had subjects produce force across varying force levels (5, 10, 20, and 40% MVC) at both constant and sine-wave targets. Detailed time and frequency analyses of the dynamics of force output, single motor unit discharge behavior, and EMG activity are presented to determine which hypothesis provides a physiological correlate for the greater force output variability in older humans.
Section snippets
Subjects
A total of 30 subjects were assigned to three different age groups: young group (n=10; range: 20–24 years; mean: 22±1 years; five females and five males), old group (n=10; range: 64–69 years; mean: 67±2 years; five females and five males), and older-old group (n=10; range: 75–90 years; mean: 82±5 years; five females and five males). Twenty-nine out of the 30 subjects were right hand dominant. The subjects were naive to the purpose of the experiment and none had a history of a neurological
Results
MVC was compared between age groups in a one-way between subjects ANOVA. The MVC was greater in the young (mean=20.10 N), compared with the old (mean=18.33 N) and older-old (mean=12.99 N) groups, F(2,27)=4.50; P<0.05. Tukey’s HSD post-hoc test revealed that the aging effect was due to the older-old group having a lower MVC compared with the young and old groups. The force target levels used in the experiment were scaled to each individual subject’s MVC for both the constant and sine-wave target.
Discussion
It was observed that aging humans in contrast to young adult control subjects produce more variable force contractions at 5, 10, 20, and 40% of their MVC at both constant and sine-wave targets. These findings are similar to the two-fold increase in force variability with aging described by Galganski et al. [13] and Laidlaw et al. [20] during isometric second finger abduction force of the first dorsal interosseous muscle. However, the findings from this study extend these previous observations
Acknowledgements
This research was supported in part by grants from the National Institute of Aging (T32-AG00048), the Gerontology Center at The Pennsylvania State University (GERO 423-141001), and Grants-in-Aid Research from the National Academy of Sciences, through Sigma Xi. We also thank the nursing care provided by the staff of the Penn State General Clinical Research Center at the Noll Physiological Research Laboratory (NIH Grant M01-RR-10732). The authors thank Dr. Daniel Corcos for his comments regarding
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