DUTTO, D.J.*; HOYT, D.F.; WICKLER, S.J.; California State Polytechnic Univ., Pomona CA: Ground Reaction Forces During Level, Incline, and Decline Trotting

Little is known of the relative contributions of quadruped forelimbs and hind limbs to locomotion up and down hills. Two runways, one level and one with a 5.7 deg slope were used to record force during locomotion on sloped (incline or decline) and level surfaces. Horizontal and vertical forces were measured for six horses during trotting over a range of speeds (2.2 to 5.0 m/s) for each condition. To facilitate comparison, ground reaction forces from sloped trials were corrected to true vertical and horizontal. The following variables were determined for each trial: peak vertical (Fz), braking (Fb), and propulsive (Fp) forces; vertical (Iz), braking (Ib) and propulsive (Ip) impulses; and duty factor (DF). For all three conditions, forelimb Fz increased linearly with speed, with the greatest forces on the level and decline. Hind limb Fz remained nearly constant across speeds for the level and decline, but increased with speed on the incline. Iz tended to decrease with increased speed and be greater for the forelimb. Fb tended to be greater for forelimb, particularly on the declined slope. Ib was 5 times greater on the decline. Fp was higher for the hind limb, except for the decline where fore- and hind limb forces were similar. On the level, Ip was similar between the fore- and hind limbs, but on the incline the hind limb generated greater Ip. Over a half stride on the level, the force necessary to resist gravity is distributed as 57%/43% (forelimb and hind limb), on the incline as 53%/47% and on the decline as 61%/39%. DF appears to predict Fz for all three conditions for the fore- but not the hind limb. In general, the relative contribution of the “downhill” limb is increased relative to the level. (Supported by NIH grant #2S06 GM53933 to DFH and SJW and an Agricultural Research Initiative)