Long-latency enhancement of quadriceps excitability from stimulation of skin afferents in young and old adults.

This study examined the role of cutaneous input on spinal excitability in young and old adults. Patellar tendon reflexes were elicited in 15 old adults (M = 75 yrs) and 25 young adults (M = 26 yrs) at intervals ranging from 15 to 175 ms following cutaneous stimulation to one of four skin sites: (a) ipsilateral calf, (b) contralateral calf (old adults only), (c) ipsilateral anterior thigh, and (d) contralateral anterior thigh. The younger adults had a more vigorous reflex response than the older adults, as indicated by peak force. Force latencies were also faster for the younger adults than the older adults. However, both age groups showed a long-latency facilitation of quadriceps excitability regardless of cutaneous stimulation site. Thus, it seems that low threshold cutaneous afferents contribute an excitatory input to the extensor motoneuron pool that is maintained with age.