Regarding the strain and elongation distribution along the tendon and aponeurosis the literature is reporting different findings. Therefore, the purpose of this study was to examine in vivo the elongation and the strain of the human gastrocnemius medialis tendon and aponeurosis simultaneously at the same trial during maximal voluntary plantarflexion efforts. Twelve subjects participated in the study. The subjects performed isometric maximal voluntary contractions of their left leg on a Biodex-dynamometer. The kinematics of the leg were recorded using the Vicon 624 system with 8 cameras operating at 120 Hz. Two ultrasound probes were used to visualise the tendon (myotendinous junction region) and the distal aponeurosis of the gastrocnemius medialis respectively. The main findings were: (a) the absolute elongation of the gastrocnemius medialis tendon was different to that of the aponeurosis, (b) the strain of the gastrocnemius medialis tendon did not differ from the strain of the aponeurosis, (c) during the "isometric" plantarflexion the ankle angle exhibited significant changes, and (d) the non-rigidity of the dynamometer arm-foot system and the coactivity of the tibialis anterior both have a significant influence on the moment exerted at the ankle joint. Thus the strain of the human gastrocnemius medialis tendon and aponeurosis estimated in vivo using two-dimensional ultrasonography is uniform. To calculate the elongation of the whole tendon it is necessary to multiply the strain calculated for the examined part of the tendon by the total length of the tendon.