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

INTRODUCTION/AIMS: Voluntary muscle contractions result in a temporary increase in twitch force, a phenomenon termed post activation potentiation (PAP). In rodents and other species, PAP is observed predominantly or exclusively in fast twitch muscles. However, it has been suggested that in humans, PAP occurs more or less independently of muscle fiber type. METHODS: Eighteen healthy men and women (27 ± 8 years) underwent an electrical stimulation protocol during which two sets of four twitches were elicited both pre and post 6 s maximal voluntary contractions of the triceps surae (60%-70% slow twitch) and triceps brachii (60%-70% fast twitch) muscles. RESULTS: Unpotentiated peak twitch torque (PTT) was higher in the larger triceps surae versus the smaller triceps brachii (i.e., 13.4 ± 5.3 vs. 3.4 ± 2.1 nm; p < 0.001), but time to peak torque was shorter (i.e., 84 ± 7 vs. 132 ± 14 ms; p < 0.001) and relative rate of torque development (RTD) was greater in the triceps brachii (2294 ± 257 vs. 1425% ± 102%/s; p < 0.001). PAP increased PTT by 172% ± 124% in the triceps brachii versus 20% ± 20% in the triceps surae (p < 0.001). Absolute RTD also increased more in the triceps brachii (i.e., 240% ± 170% vs. 31% ± 24%; p < 0.001). However, PAP-induced changes in half-relaxation time and relative rate of relaxation did not differ between muscle groups. DISCUSSION: We conclude that PAP influences contraction but not relaxation of human muscle in a fiber type dependent manner. This should be kept in mind when interpreting individual differences in the results of neuromuscular testing, response to varying warm-up protocols, etc.