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1 Department of Kinesiology, The Pennsylvannia State University, University Park, PA, USA; The Gerontology Center, The Pennsylvannia State University, University Park, PA, USA
2 School of Kinesiology, The University of Illinois at Chicago, Chicago, IL, USA
3 Department of Kinesiology, The Pennsylvannia State University, University Park, PA, USA
* To whom correspondence should be addressed. E-mail: jjs388{at}psu.edu.
The current study examined the influence of aging on the oscillatory activity of a population of motor units during rhythmical force production. Previously, it has been shown that aging humans have greater low frequency and less high frequency electromyographic (EMG) activity during constant and slow ramp force contractions. We hypothesized that more rapid force contractions would reverse the established finding of reduced high- and greater low-frequency EMG activity to greater high- and reduced low-frequency EMG activity in older adults. Intramuscular EMG activity and effector force were recorded while 45 human subjects (20 - 31 and 60 - 88 years of age) rhythmically produced force at four distinct frequencies (1, 2, 3, and 4 Hz) and two force levels (5 and 25% MVC). Spectral and coherence analyses were performed on the force output and EMG activity. In the 3 and 4 Hz targets the older adults had greater 35-50 Hz and reduced 0-5 Hz EMG activity compared to the young adults. There was greater EMG-force coherence in the 0-5 Hz bandwidth for the young subjects. No systematic age difference in the phase relationship between the EMG and force signals were found. Higher frequency force contractions reversed the previously established aging differences in the relative contribution of low- and high-frequency EMG activity. Thus, the frequency properties of the task goals channel the relative contribution of low and high EMG activity. Furthermore, it is proposed that aging humans lose the adaptive capability to coordinate the excitatory and inhibitory activity of multiple neural oscillators.
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