Kinematics of the distal hindlimb during stance phase in the fast trotting standardbred.

Fast trotting Standardbred horses were filmed along a straight on an oval dirt track. Five consecutive stance phases were analysed to describe the planar kinematics of the distal hindlimb. The rapid changes in the geometry of the distal hindlimb that occur during the early stance phase were studied. The hoof segment was initially braked vertically and moved in the direction of the horse. The hoof moved forward on the track surface for more than 20% of the stance time (ST). Two specific deviations in the otherwise smooth course of the fetlock joint angle appeared at 16 and 29% of ST. Tarsal angular joint displacement was, on the other hand, more smooth throughout the stance phase. Segment angular velocity was greatest in the proximal pastern segment, while the metatarsus was almost totally braked in its forward rotation during the early stance. Tibial angular velocity was more smooth and greater than that of the metatarsus. Initial vertical braking of the hoof was related to the rapid rotation of the proximal pastern segment, while the metatarsal and proximal pastern segment angular velocities decreased as the hoof was braked horizontally. Also coincident with horizontal braking of the hoof was an increase in the angular velocity of the tibial segment. It was concluded that the horizontal as well as the vertical braking of the hoof affect the disto-proximal braking of the segments of the distal hindlimb during the early stance phase. The early stance phase changes in the distal hindlimb suggest rapid changes in the internal forces of the limb and should be of importance to the orthopaedic health of Standardbred trotters. These rapid changes at the fetlock joint and hock joint during the early stance may be important in lameness as excessive rapid and repetitive loading and movement are thought to induce joint damage (Radin et al. 1991).