Effect of trotting speed, load and incline on hindlimb stance-phase kinematics.

The objective was to understand how the stance-phase kinematics of the hindlimb of trotting horses change with speed under 3 conditions (level, loaded and incline), to compare our results with the predictions of the spring-mass model and to help focus our future studies of muscle function. Video recordings were made of 5 Arabian horses trotting on a treadmill. Five consecutive strides were digitised and averaged for each trial. The angle-time diagrams were qualitatively similar to those reported previously. As speed increases, the range of motion of the hindlimb increases, as predicted by the spring-mass model. This is the result of increased range of motion in the coxofemoral and tarsal joints. The hindlimb does not 'land short-take off long'. When trotting up an incline, the hindlimb undergoes a greater range of motion because of increased retraction resulting from increased extension of the coxofemoral joint. At hoof contact on an incline, the 3 proximal joints are more flexed than on the level. Carrying a load had no effect on kinematics. These results suggest that there may be larger changes in strain with speed in muscles acting at the coxofemoral and tarsal joints than at the femorotibial joint, and that locomotion up an incline will change muscle strain more than carrying a load.