Influence of fascicle length on twitch potentiation of the medial gastrocnemius across three ankle angles

Samantha L. Kuzyk, Rowan R. Smart, Carey L. Simpson, Andrey Fedorov, Jennifer M. Jakobi

Length dependence of post-activation potentiation (PAP) is a well-established phenomenon in animal models but less certain in intact whole human muscles. Recent advances in B-mode ultrasonography provide real-time imaging and evaluation of human muscle fascicles in vivo, thus removing the assumption that joint positioning alters fascicle length and influences the extent of PAP. The purpose of this study was to determine whether a conditioning maximal voluntary contraction (MVC) would influence the return of medial gastrocnemius (MG) fascicles to baseline length and alter the extent of twitch potentiation between three ankle positions. Ultrasonography was used to measure MG fascicle length for baseline and potentiated twitches at angles of 10° dorsiflexion (DF), 0° neutral (NEU-tibia perpendicular to the sole of the foot), and 20° plantar flexion (PF). A MVC was used as a conditioning contraction and PAP determined for each ankle angle. PAP of the plantar flexors was greater in PF (28.8 ± 2.6%) compared to NEU (19.8 ± 1.8%; p < 0.05) and DF (9.3 ± 2.8%; p < 0.0001). In PF, fascicle lengths (4.64 ± 0.17 cm) were shorter than both NEU (5.78 ± 0.15 cm; p < 0.0001) and DF (6.09 ± 0.15 cm; p < 0.0001). Fascicle lengths for the baseline twitches were longer (5.92 ± 0.11 cm) than the potentiated twitches (5.83 ± 0.10 cm; p < 0.01) at all joint angles. Although PAP is greatest in PF compared to NEU and DF, the higher PAP in the PF joint angle cannot be attributed to fascicles remaining shortened following the MVC because across all joint positions, fascicles are similarly shortened following the MVC.