Both heads of the gastrocnemius muscle contribute to ankle plantar flexion. This study utilized integrated electromyography to investigate whether the percent electrical activity contributed by each head remained constant or changed during isometric contractions at five different resistance levels. Fifty healthy volunteers ranging in age from 19 to 34 years, with no history of musculoskeletal or neuromuscular disorders involving the right lower extremity, were studied. All tasks were performed in the prone position, knee in extension, with the leg and foot in neutral with respect to rotation. Motor points of the medial head and lateral head were identified and surface electrodes were placed just distal to them. The subjects maintained 20° of plantar flexion under five conditions: a maximal isometric plantar flexion contraction (one trial only), and with a 5-, 10-, 15- and 20-lb cuff weight attached to the right foot (three trials each). EMG recordings, 8 s in length, were taken during the isometric contractions. Integrated EMGs were averaged for each cuff weight and the resulting values used in the analysis. A repeated measures ANOVA was performed and a significance level of p≤0.05 was used to determine statistical significance. As weight increased, the absolute value of the integrated EMG recorded over both muscles increased, but the percent contributed by each head remained essentially equal (50%) within the four submaximal loads tested. However, for the maximal isometric contraction, the medial head contributed a significantly higher percentage of the total integrated EMG (58%). Therefore, in the open-chain activity described, the two heads of the gastrocnemius demonstrate similar neural drive at submaximal levels of contraction, but this changes as maximum isometric levels are reached.
Keywords: Gastrocnemius; Integrated EMG; Isometric; Plantar flexion.