Effects of aging on force variability, single motor unit discharge patterns, and the structure of 10, 20, and 40 Hz EMG activity.

The purpose of this investigation was to examine the discharge properties of single motor units and the structure of the rectified 10, 20, and 40 Hz electromyographic (EMG) activity to determine a physiological correlate for the greater force variability with aging. Young (n=10; mean: 22+/-1 years), old (n=10; mean: 67+/-2 years), and older-old (n=10; mean: 82+/-5 years) adult humans produced isometric second finger abduction force in both constant and sine-wave tasks at 5, 10, 20, and 40% of their maximal voluntary contraction. Force and fine-wire intramuscular electromyography were recorded from the first dorsal interosseous muscle. The amount and time-dependent structure of the discharge rate variability of single motor units and Fourier analysis of the rectified intramuscular EMG was performed. Force output variability increased across the young, old, and older-old groups. The amount and time-dependent structure of the discharge rate variability of single motor units did not differ between the young and aging groups. There was a progressive decrease in the relative power of approximately 40 Hz EMG activity from the young>old>older-old subjects across the 5, 10, 20, and 40% maximum voluntary contraction (MVC) force levels. There was also a progressive increase in the relative power of the approximately 10 Hz EMG activity from young<old<older-old subjects at each target force level. The findings showed that a shift in the relative contribution of approximately 40 Hz to approximately 10 Hz neural activity is related to the reduced capacity of older adults to maintain optimal force control.