Comparison of the morphology of the limbs of juvenile and adult horses (Equus caballus) and their implications on the locomotor biomechanics

Abstract

We analyzed the morphology and the walk-trot and trot-gallop transition velocities of nine juvenile horses and compared them with their mothers. We also compared the relative stride length and the duty factor of the juveniles with respect to adults at three equivalent trotting speeds (Froude numbers 0.5, 0.75, and 1.0), to determine dynamic similarity. Juveniles had a negative allometry in their leg bones, mainly because of little size changes of the distal portions. The negative allometry of extremities allows juveniles to increase stride length without increasing step frequency, which can be biomechanically advantageous. The Froude number during the walk-trot velocity transition of juveniles was similar to that of adult horses, but walk-trot transition velocity in juveniles was greater than expected for their mass. However, during the change trot-gallop, the trot-gallop velocity transition was conserved, but the Froude number was lower. Thus, juvenile horses did not move in a manner that was dynamically similar to the adult horses. At low speed (walk-trot), the gait approaches the behavior predicted by the inverted pendulum model, but at high speed (trot-gallop) dominates the inertial forces. The trot-gallop gait change would be conducted at speeds that would minimize energy costs of transport owing to collisions and changes in the trajectory of the center of mass.