The aim of the present study was to investigate the behaviour of human muscle fascicles during dynamic contractions. Eight subjects performed maximal isometric dorsiflexion contractions at 6 ankle joint angles and maximal isokinetic concentric and eccentric contractions at 5 angular velocities. Tibialis anterior muscle architecture was measured in vivo using B-mode ultrasonography. During maximal isometric contraction fascicle length was shorter and pennation angle larger compared to that at rest (P < 0.01). During isokinetic concentric contractions from 0 to 4.36 rad s^-1, fascicle length measured at a constant ankle joint angle increased curvilinearly from 49.5 to 69.7 mm (41%; P < 0.01), whilst pennation angle decreased curvilinearly from 14.8 to 9.8 deg (34%; P < 0.01). During eccentric muscle actions, fascicles contracted quasi-isometrically, independent of angular velocity. The behaviour of muscle fascicles during shortening contractions was believed to reflect the degree of stretch applied to the series elastic component, which decreases with increasing contraction velocity. The quasi-isometric behaviour of fascicles during eccentric muscle actions, suggests that the series elastic component acts as a mechanical buffer during active lengthening.