Locomotion in the Horse: Kinematics and External and Internal Forces in the Normal Equine Digit in the Walk and Trot

SUMMARY Kinematics and the external and internal forces acting on the digit during the support phase of the walk and trot were studied in 11 normal horses. Kinematics were obtained from high speed movie films, external forces by means of a force platform, and internal forces were computed using an analytical model of the equine digit. The general motion of the digit is much the same for the walk and the trot except that the angular velocities increase as the speed of the horse increases and the support time decreases. The minimum value of the pastern angle is less for the trot than for the walk (the fetlock joint sinks more for the trot than for the walk). The maximum vertical force of the hoof on the ground is about 0.6 × body weight for the walk and about 0.9 × body weight for the trot. The peak values of the horizontal component of force are about equal (opposed to, and in the direction of motion) and the location of the resultant force with respect to the hoof remains constant during the middle portion of the support phase. The peak axial compressive load in the metacarpal bone is about 2.4 × body weight for the walk and about 4.9 × body weight for the trot. These values are consistent with existing experimental measurements. The compressive force in P2 is 1.6 × body weight for the walk and 2.0 × body weight for the trot. The force of the deep flexor tendon on the navicular is 0.67 × body weight for the walk and 0.77 × body weight for the trot. The peak load on the deep flexor tendon is 0.83 × body weight for the walk and 1.4 × body weight for the trot. Fetlock joint extension is resisted mostly by the deep flexor for the lightly loaded digit during the walk. As the speed of the animal increases, the contributions of the suspensory ligament and the superficial flexor tendon increase.