Dynamically similar locomotion in horses

It is possible for animals of very different sizes to use the same patterns of locomotion, i.e. to move in a 'dynamically similar fashion'. This will only occur, however, if relevant biomechanical parameters scale with size in such a way that they compensate for the effects of size differences. Here we apply this principle to understanding the effects of size on locomotion within a species: the domestic horse. We predict that, without any factor to compensate for size differences, detectable deviations from dynamically similar locomotion would occur over the size range present in adult horses. We measured relative stride length (RSL) and duty factor (DF) in 21 trotting horses (body mass: 86-714 kg), and interpolated the data to predict RSL and DF at equivalent speeds (Froude numbers: 0.5, 0.75, 1.0). RSL and DF at equal Froude number were not significantly related to body mass. This is consistent with the hypothesis that horses trot in a dynamically similar fashion at equal Froude number. We show that the nonlinear stress-strain relationship of tendon can contribute to reducing deviations from dynamic similarity, 'buffering' the effects of variation in body mass, but conclude that this effect is unlikely to explain fully our results. This suggests that a 'compensatory distortion' may occur in horses, counteracting the effects of size differences. The approach used here is also applicable to understanding the consequences of size changes within an individual during growth.