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Postexercise potentiation of the H-reflex in humans. Med. Sci. Sports Exerc., Vol. 30, No. 6, pp. 933-941, 1998. Post-muscle activation effects on segmental reflexes reveal divergent results dependent upon the manner in which the muscle is activated. Electrically activating triceps surae invokes a potentiation of the Achilles' tendon reflex and the soleus (S) H-reflex termed posttetanic potentiation. In contrast, brief volitional activation produces a subsequent potentiation of tendon reflexes, whereas H-reflexes become depressed.The present investigation explored the effect of an intense bout of volitional resistance exercise on the S and lateral gastrocnemius (LG) H-reflexes to determine if a potentiation of the H-reflex could be induced with physiological stimuli.LG and S H-reflexes were obtained from 10 college age men and women before and after a vigorous bout (eight sets of 10 repetitions) of concentric-eccentric triceps surae exercise.Every subject displayed an initial depression of the LG (P < 0.01) and S H-reflex (P < 0.05) immediately postexercise, consistent with postactivation depression. As a group, there was a significant (P > 0.01) potentiation of the LG H/M ratio following the depression. Five of 10 subjects demonstrated this potentiation, which often lasted 10 min postexercise. The other five subjects displayed a longer and more profound early depression followed by a return to control levels.The data suggest that at least two overlapping processes are occurring, a brief depression followed by or superimposed over a longer lasting potentiation. Possible neural mechanisms and implications to strength training are discussed.