The effect of exercise, thyroid status and insulin-induced hypoglycaemia on the Achilles tendon reflex time in man.

The effect of moderate exercise on the Achilles tendon reflex half-relaxation time (ART) was observed in 28 hypothyroid, 35 euthyroid, and 20 hyperthyroid subjects of comparable age and sex. Both the resting ART and the decrease after exercise (delta ART) were longer in hypothyroid patients and shorter in hyperthyroid patients as compared to the euthyroid group. delta ART was strongly related to the resting ART rather than to the free thyroxine index. In four hypothyroid subjects, the half-time of recovery of the ART after exercise was similar, approx. 24 min. Insulin-hypoglycaemia in eight resting endocrine patients also caused a fall in ART similar to the post-exercise delta ART in the same patients. A possible catecholamine effect is indicated by the smaller post-exercise delta ART in nine hypertensive patients receiving chronic propranolol treatment, 320 mg daily, than that of a control group matched for age, sex, and resting ART. These studies suggest that submaximal exercise largely removes the differential effect of thyroid status on skeletal muscle contractility as measured indirectly by the ART. The effects of exercise and induced insulin-hypoglycaemia on delta ART may be mediated in part by endogenous catecholamines.