Effects of fatiguing isometric exercise upon achilles tendon reflex and plantar flexion reaction time components in man.

Effects of three different fatiguing local muscular exercises upon plantar flexion reaction time and achilles tendon reflex time have been studied in 24 normal males. The Exercise Conditions, each involving a series of 30 maximal voluntary isometric contractions (MVC) of the plantar flexors, differed by allowing either 5, 10 or 20 sec rest interval between each MVC. Decrements in strength ranged from 15% to 34% MVC. Trend analysis of the fatigue patterns revealed that a cubic orthogonal polynomial equation was sufficient to describe the profile of MVC decrement for all conditions. Following the fatiguing exercise, simple visual reaction time (plantar flexion), and its two components, premotor and motor time, failed to demonstrate any change from Pre Exercise Conditions. Achilles Tendon Reflex Times, however, demonstrated a marked augmentation, as manifest in reduced total reflex times, contraction times and half relaxation times. These results may suggest the differential fatigue of motor units employed in the three motor tasks, viz. MVC, voluntary reaction and achilles tendon reflex. A plausible explanation for the augmentation of the reflex contraction resides in the known potentiating effect of elevated intramuscular temperature. Alternatively, one might postulate a neurally mediated increase in gain of the stretch servomechanism. The possibility of both mechanisms being operative is not excluded.