Age-related changes in monosynaptic reflex excitability.

The excitability of the H-reflex was tested in a younger group (19 to 31 years) and an older group (60 to 72 years) of subjects. The threshold for the H-reflex was higher and the reflex latency was 7 msec longer in the older group of subjects. Also, the H-reflex amplitude was significantly smaller, longer in duration and polyphasic in shape whereas the time delay between the M-response and the H-reflex (central latency) was 5 msec longer in the older group. The H-reflex recovery curves of the older subjects demonstrated a longer primary inhibition period (94 msec in older subjects, 44 msec in younger subjects) and was slower, more inhibited, and took a longer time for maximal recovery. In accordance with previous findings, these results are attributed to possible central as well as peripheral and muscle fiber defects in old subjects. Such changes must be taken into account when using H-reflex tests in the clinic.