Changes in spinal reflexes preceding a voluntary movement in young and old adults

Age-related differences in spinal excitability during response preparation were assessed by eliciting either a 50% H-reflex or an Achilles tendon reflex preceding the onset of a right plantar flexion contraction in 20 young adults (23.1 +/- 1.64 yrs) and 20 old adults (68.5 +/- 5.53 yrs).

On each simple reaction time trial, the test reflex was elicited at a specific test interval during either the foreperiod or the response period. The foreperiod test intervals were 500, 600, 700, 800, 900, and 1000 msec after the presentation of the warning stimulus. The response period test intervals were 50, 100, 150, 200, 250, and 300 msec after the presentation of the response stimulus. Control reflexes were randomly elicited between the simple reaction time trials.

Changes in reflex excitability were not observed during the foreperiod in either age group. During the response period, the percentage of H-reflex facilitation as compared to control H-reflexes was similar for the young (68%) and the old (61%) adults, but the magnitude of Achilles tendon reflex facilitation with respect to control reflex responses was greater in the young adults (74%) than in the old adults (38%). The time course of H- and tendon reflex facilitation was delayed in the old group during the response period.

The results indicate that processes underlying the preparation and generation of a motor response are similar in young and old adults. However, these processes occur at a slower rate in old adults.