Regulation of motor output between young and elderly subjects

Objectives: Considerable information exists concerning the differences in motoneuron pool (MP) excitability between young and elderly subjects. A recent study demonstrated decreased heteronymous Ia facilitation with aging, suggesting increased presynaptic inhibition (PI) with increasing age as a mechanism for this change (Morita et al., Exp Brain Res 104 (1995) 167). It has been suggested that during voluntary movement, supraspinal, and possibly, segmental mechanisms (Hultborn et al., J Physiol 389 (1987) 757) modulate this inhibition. It is theorized that PI can modulate the recruitment gain of the MP during movement without altering the excitability of the motoneurons. Therefore, the purpose of this study was to examine the roles of PI and volitional volleys in modulating MP output in young and elderly subjects. Methods: Twenty apparently healthy females participated in this study, 10 college aged (mean age, 22.4±2.8 years) and 10 independent, community dwelling elderly (mean age, 77.6±5.4 years). All subjects were tested in a semi-recumbent position. H-reflexes were elicited at rest, and at 10 and 20% of maximal voluntary contraction. To assess MP output, background electromyography (EMG) was monitored prior to stimulation. The stimulus intensity was adjusted during volitional contractions to ensure similar control reflexes (25% of the maximal motor response (M-max)) at each level of contraction. Results: Control reflexes at each level of volitional contraction (rest, 10 and 20%) were similar for both groups. To assess PI and to estimate the extent to which a change in the H-reflex amplitude reflects a change in MP gating, the common peroneal nerve was stimulated at 1.5 times the motor threshold 100 ms prior to stimulation of the tibial nerve. Significantly greater PI was observed for the young subjects at rest (5 vs. 13% M-max). At both 10 and 20% levels of voluntary contraction, the conditioned reflex was significantly different from rest for the young subjects. The elderly subjects, in contrast, failed to modulate the conditioned reflex until the 20% of maximal voluntary contraction (MVC) condition. When examining the recruitment gain in the MP during the PI condition (H-reflex amplitude as a function of EMG levels), a significant group effect was observed, with the young subjects demonstrating significantly higher PI gain. Conclusions: These results indicate differential control of MP output (e.g. PI vs. volitional volleys) in young and elderly subjects.