In the present study we investigated in vivo length changes in the fascicles and tendon of the human gastrocnemius medialis (GM) muscle during walking. The experimental protocol involved real–time ultrasound scanning of the GM muscle, recording of the electrical activity of the muscle, measurement of knee– and ankle–joint rotations, and measurement of ground reaction forces in six men during walking at 3 km h–1 on a treadmill. Fascicular lengths were measured from the sonographs recorded. Musculotendon complex length changes were estimated from anatomical and joint kinematic data. Tendon length changes were obtained combining the musculotendon complex and fascicular length–change data. The fascicles followed a different length–change pattern from those of the musculotendon complex and tendon throughout the step cycle. Two important features emerged: (i) the muscle contracted near–isometrically in the stance phase, with the fascicles operating at ca. 50 mm; and (ii) the tendon stretched by ca. 7 mm during single support, and recoiled in push–off. The behaviour of the muscle in our experiment indicates consumption of minimal metabolic energy for eliciting the contractile forces required to support and displace the body. On the other hand, the spring–like behaviour of the tendon indicates storage and release of elastic–strain energy. Either of the two mechanisms would favour locomotor economy.