Department of Kinesiology, McMaster University, Hamilton, Ontario, Canada L8S 4K1.

Twitch contractions of the ankle dorsiflexors were evoked before and after applied 7-s tetanic stimulation at 100 Hz in 20 young adults. Torque decreased 15% during the tetanus. At 5 s after tetanus, twitch peak torque had potentiated 45%. Potentiation declined to 28% after 1 min, rose slightly to 33% at 2 min, and declined slowly with potentiation still 25% after 5 min. There was large intersubject variation in the amount of potentiation (5-140%) and its persistence (5 to >/=20 min). The muscle compound action potential (M wave) did not change significantly (from pretetanic value) at 5 s after tetanus but increased sharply (26%) at 2 min and then subsided. Twitch half relaxation time (23%) decreased significantly more than twitch rise time (13%) 5 s after tetanus and recovered more slowly. Twitch rates of torque development (75%) and relaxation (71%) increased similarly 5 s after tetanus and were still elevated (approximately 25%) at 5 min. The extent of twitch torque potentiation was significantly inversely correlated with pretetanic twitch rise time (r = -0.69), half relaxation time (r = -0.61), and twitch-to-tetanus ratio (r = -0.66). The data indicate that posttetanic potentiation has a greater effect on twitch half relaxation time than on time to peak torque and is more prominent in muscles with a short twitch time course and small twitch-to-tetanus ratio.