CATTERFELD, G.E.; DUTTO, D.J.; WICKLER, S.J.; HOYT, D.F.; California State Polytechnic University, Pomona: Deviant Forces in Trotting Horses

Previous kinematic studies in horses, which normally trotted from 1.8 to ca. 5.0 m/s, suggested there might be deviations from the trends of forces and impulses seen at higher trotting speeds when speed decreased below 2.5 m/s. Six horses were filmed at 250 Hz as they trotted across a force plate at speeds of 1.5-3.0 m/s. Stride period (Tstr) was obtained from the video and time of contact (Tc) from the force plate. These data were combined with previous higher speed data (2.2- ca. 5.0 m/s) to examine trends. The peak vertical ground reaction force (Fz) of the hind limb increased 40% as speed increased from 1.5 m/s to ca. 2.7 m/s and then was fairly constant. The peak Fz of the forelimb increased as a power function of speed. Vertical impulse decreased as a second order polynomial of speed in both limbs. From average Fz, force distribution was 57% fore and 43% hind. Tc decreased as a power function of speed for both limbs, and Tc of the forelimb always exceeded Tc of the hind by 13%. Tstr decreased linearly as speed increased. Duty factor (DF) decreased as a power function of speed, with forelimb DF exceeding the hind by ca. 14%. DF was above 0.5 at speeds below ca. 2.2 m/s in the forelimb and ca. 1.6 m/s in the hind limb. DF seemed to predict Fz for the hind, but not the forelimb, at speeds of 1.5-2.7 m/s. The greatest deviation in kinetics of low speed trotting was seen in hind limb peak Fz at speeds below ca. 2.7 m/s. Since this is well within the normal trotting range, it suggests that leg compression must change if leg stiffness remains constant, as assumed by the spring mass model. (Supported by NIH S06-GM53933 to DFH & SJW).