Postactivation potentiation, fiber type, and twitch contraction time in human knee extensor muscles

In small mammals, muscles with shorter twitch contraction times and a predo minance of fast-twitch, type II fibers exhibit greater posttetanic twitch f orce potentiation than muscles with longer twitch contraction times and a p redominance of slow-twitch, type I fibers. In humans, the correlation betwe en potentiation and fiber-type distribution has not been found consistently . In the present study, postactivation potentiation (PAP) was induced in th e knee extensors of 20 young men by a 10-s maximum voluntary isometric cont raction (MVC). Maximal twitch contractions of the knee extensors were evoke d before and after the MVC. A negative correlation (r = -0.73, P < 0.001) w as found between PAP and pre-MVC twitch time to peak torque (TPT). The four men with the highest (HPAP, 104 +/- 11%) and lowest (LPAP, 43 +/- 7%) PAP values (P < 0.0001) underwent needle biopsies of vastus lateralis. HPAP had a greater percentage of type II fibers (72 +/- 9 vs. 39 +/- 7%, P < 0.001) and shorter pre-MVC twitch TPT (61 +/- 12 vs. 86 +/- 7 ms, P < 0.05) than LPAP. These data indicate that, similar to the muscles of small mammals, hu man muscles with shorter twitch contraction times and a higher percentage o f type II fibers exhibit greater PAP.