Fig.8. Average biceps shortening velocity (A) and vastus lengthening velocity (B) as a function of locomotor speed. Biceps shortening velocities are calculated by dividing the total distance shortened by the biceps by the duration of shortening. Only shortening velocities during walking and trotting are shown because, during these gaits, shortening velocity remained relatively constant over most of the shortening period (unlike during galloping, for which shortening velocities varied substantially). Negative values are used to indicate shortening velocities. Shortening velocities increase with speed and are highest during fast trotting. Average vastus lengthening velocities are calculated by dividing the total distance lengthened during the first half of the stance phase (the ‘yield’ phase) by the duration of this lengthening phase. Positive values are used to indicate lengthening velocities. Lengthening velocities increase with speed and are, therefore, greatest during fast galloping. Different symbols represent the average values from locomotor sequences at various speeds and gaits. Different symbol types represent different individuals. L, resting fascicle length.