Age-related impaired reflex sensitivity in a human hand muscle.

1. The rectified and averaged electromyogram (EMG) was recorded from the first dorsal interosseous muscle (FDI) in normal male and female human subjects, ranging in age from 18 to 67 yr. It was elicited by a brief stretch, given to the outstretched forefinger. 2. The responses to stretch consisted of components W30, the monosynaptic stretch reflex and W60, which is likely to arise in the skin and nonmuscular structures. The figures 30 and 60 refer to the mean latencies, in milliseconds, of the respective waveforms (W). 3. For a group of subjects > 30 yr of age, W30 was significantly smaller than in a group under this age. The size of W60 was not related to age and the W30/W60 ratio was < 0.45 in the older subjects. In the younger group, the ratio was always above 0.5. 4. The fact that the age-related reflex impairment affects only W30 and not W60, indicates that central processing in the motor neuron pool is unlikely to be the mechanism involved in the impairment. 5. Control experiments show that changes in frictional resistance in muscles, joints, and tendons with age, are not large enough to account for these results. 6. Neuromuscular block did not occur in these experiments and could not be implicated in the impaired reflex sensitivity.