Interindividual variability in performance of fast movements is commonly explained by a difference in maximal muscle-shortening velocity due to differences in the proportion of fast-twitch fibers. To provide a better understanding of the capacity to generate fast motion, this study aimed to 1) measure for the first time in vivo the maximal fascicle-shortening velocity of human muscle; 2) evaluate the relationship between angular velocity and fascicle-shortening velocity from low to maximal angular velocities; and 3) investigate the influence of musculo-articular features (moment arm, tendinous tissues stiffness, and muscle architecture) on maximal angular velocity. Ultrafast ultrasound images of the gastrocnemius medialis were obtained from 31 participants during maximal isokinetic and light-loaded plantar flexions. A strong linear relationship between fascicle-shortening velocity and angular velocity was reported for all subjects (mean R 2 ϭ 0.97). The maximal shortening velocity (V Fmax) obtained during the no-load condition (NLc) ranged between 18.8 and 43.3 cm/s. V Fmax values were very close to those of the maximal shortening velocity (V max), which was extrapolated from the F-V curve (the Hill model). Angular velocity reached during the NLc was significantly correlated with this V Fmax (r ϭ 0.57; P Ͻ 0.001). This finding was in agreement with assumptions about the role of muscle fiber type, whereas interindividual comparisons clearly support the fact that other parameters may also contribute to performance during fast movements. Nevertheless, none of the biomechanical features considered in the present study were found to be directly related to the highest angular velocity, highlighting the complexity of the upstream mechanics that lead to maximalvelocity muscle contraction. maximal unloaded velocity; muscle-tendon unit; muscle mechanics; muscle architecture; stiffness; ultrasound THE CAPACITY OF HUMAN SKELETAL muscle to achieve maximal power production is functionally very important during ballistic movements such as sprinting or jumping. The maximal angular velocity an individual can produce represents one of the key determinants of this ability and hence of human performance in various explosive tasks such as a sprint while running or cycling (31, 57). In the literature, this ability to reach extreme angular velocities in unloaded conditions is related mainly to a higher proportion of fast-twitch fibers (5, 11, 56) or a longer fascicle length, or both, which implies a higher number of sarcomeres in series (9, 53). These considerations suggest that subjects who are able to produce high velocity at the joint level should also be able to develop high muscle fascicle-shortening velocity. Consequently, the muscleshortening velocity should be a key determinant of the ability to perform very-high-velocity movements.To our knowledge, no previous study has measured actual maximal fascicle-shortening velocity in vivo in humans. Although measurement of fascicle-shortening velocity is classically performed using ul...