CHANGES IN SPINAL REFLEXES PRECEDING A VOLUNTARY MOVEMENT IN YOUNG AND OLD ADULTS

Background. Age-related differences in spinal excitability during resp onse preparation were assessed by eliciting either a 50% H-reflex or a n Achilles tendon reflex preceding the onset of a right plantar flexio n contraction in 20 young adults (23.1 +/- 1.64 yrs) and 20 old adults (68.5 +/- 5.53 yrs). Methods. On each simple reaction time trial, the test reflex was elicited at a specific test interval during either th e foreperiod or the response period. The foreperiod test intervals wer e 500, 600, 700, 800, 900, and 1000 msec after the presentation of the warning stimulus. The response period test intervals were 50, 100, 15 0, 200, 250, and 300 msec after the presentation of the response stimu lus. Control reflexes were randomly elicited between the simple reacti on time trials. Results. Changes in reflex excitability were not obser ved during the foreperiod in either age group. During the response per iod, 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 id the young adults (74%) than in the old adults (38%). The time course of H- and tendon reflex facilit ation was delayed in the old group during the response period. Conclus ions. The results indicate that processes underlying the preparation a nd generation of a motor response are similar in young and old adults. However, these processes occur at a slower rate in old adults.