Post Activation Potentiation Is Greater In Human Triceps Brachii Versus Triceps Surae Muscles

Post activation potentiation (PAP) is an intriguing characteristic of muscle physiology in which an increase in electrically-evoked twitch force is observed after voluntary activation. This phenomenon is believed to be largely due to phosphorylation of the myosin regulatory light chain by skeletal muscle light chain kinase. In animal models, PAP has been recognized as an inherent property of fast twitch muscle fibers. However, it has been suggested that in humans PAP occurs more or less independently of muscle fiber type. PURPOSE: To compare the magnitude of PAP in slow and fast human muscles. METHODS: Eleven men and women (18-44 y) underwent an electrical muscle stimulation protocol in which two sets of a series of four twitches were elicited both before and after a 6 s maximal voluntary contraction (MVC) of both the triceps surae (~25% fast twitch) and the triceps brachii (~75% fast twitch) muscles. Peak twitch torque (PTT), relative rate of torque development (RTD), and PAP were determined. RESULTS: Both MVC (89.8 ± 38 vs. 44.9 ± 15.1 Nm; p < 0.0001) and unpotentiated PTT (13.7 ± 5.8 vs. 3.6 ± 2.4 Nm; p < 0.0001) were greater in the larger triceps surae vs. the smaller triceps brachii. On the other hand, unpotentiated RTD was higher in the triceps brachii vs. the triceps surae (2622 ± 1151 vs. 1416 ± 93 %/s; p < 0.0001). Post activation potentiation averaged 164 ± 132% in the triceps brachii versus 20 ± 17% in the triceps surae (p < 0.0001). CONCLUSION: The triceps surae, a slow muscle, exhibits greater absolute voluntary and twitch torques whereas the triceps brachii, a fast muscle, exhibits faster RTD and greater PAP.