Validation of vertical ground reaction forces on individual limbs calculated from kinematics of horse locomotion

The purpose of this study was to determine whether individual limb forces could be calculated accurately from kinematics of trotting and walking horses. We collected kinematic data and measured vertical ground reaction forces on the individual limbs of seven Warmblood dressage horses, trotting at 3.4 m s(-1) and walking at 1.6 m s(-1) on a treadmill. First, using a segmental model, we calculated from kinematics the total ground reaction force vector and its moment arm relative to each of the hoofs. Second, for phases in which the body was supported by only two limbs, we calculated the individual reaction forces on these limbs. Third, we assumed that the distal limbs operated as linear springs, and determined their force-length relationships using calculated individual limb forces at trot. Finally, we calculated individual limb force-time histories from distal limb lengths. A good correspondence was obtained between calculated and measured individual limb forces. At trot, the average peak vertical reaction force on the forelimb was calculated to be 11.5 +/- 0.9 N kg(-1) and measured to be 11.7 +/- 0.9 N kg(-1), and for the hindlimb these values were 9.8 +/- 0.7 N kg(-1) and 10.0 +/- 0.6 N kg(-1), respectively. At walk, the average peak vertical reaction force on the forelimb was calculated to be 6.9 +/- 0.5 N kg(-1) and measured to be 7.1 +/- 0.3 N kg(-1), and for the hindlimb these values were 4.8 +/- 0.5 N kg(-1) and 4.7 +/- 0.3 N kg(-1), respectively. It was concluded that the proposed method of calculating individual limb reaction forces is sufficiently accurate to detect changes in loading reported in the literature for mild to moderate lameness at trot.