Contralateral influences on patellar tendon reflexes in young and old adults.

In an effort to more fully investigate age-related changes in spinal reflex parameters, we measured force-time characteristics of the patellar tendon reflex in aged subjects and contrasted these with data obtained from college-age individuals. We also conditioned the tendon jerk with a tap to the contralateral tendon. The results showed a marked tendon reflex enhancement in the old group, consisting of greater overall reflex force produced by the quadriceps. In both groups, the contralateral conditioning stimulus produced a short-latency inhibition (at 25 msec) followed by a longer-latency facilitation (beginning at 75 msec). Both the early inhibition and the later reflex enhancement were greater in the aged subjects. We suggest that some age-related change may occur at the spinal level to compensate for decrements in more complex motor functioning.