CATTERFELD, GE; HOYT, DF; DUTTO, DJ; WICKLER, SJ; COGGER, EA; California State Polytechnic University, Pomona: Trotting at low vs. high speeds: different gait dynamics of fore- and hind limbs

Statistical analysis of our previously reported ground reaction force (GRF) and stride parameter data from horses trotting from ca. 1.5-5.0 m/s identified some deviations in the trends of these variables with speed. Most variables were continuous over the entire speed range (i.e. best fit with linear, power, or second-order polynomial relationships). However in the hind limb, peak vertical GRF, time of contact, and duty factor (DF) were discontinuous with speed ? they were better described by two-line segmented relationships. These variables had inflection points at an average speed of 2.53 m/s, suggesting a shift in the mechanics of the hind limb at low speeds. This shift was correlated with the fact that, at trotting speeds below 2.31 m/s, the DF of the forelimb was greater than 0.5 (no aerial phase), unlike the hind limb in which DF was less than 0.5 at all trotting speeds. The DF in the hind limb would have been even smaller at low speeds were it not for a prolonged GRF that appears to serve to dissipate some of the elastic energy stored during the first part of contact at speeds below ca. 2.5 m/s. Additionally, the hind limb applies a small braking GRF near the end of contact at speeds below ca. 2.0 m/s. These strategies, which are presumably necessary to trot at low speeds, may account for the increased cost of transport at speeds below the preferred speed of the horse (3.3 m/s). As costly as these strategies are, trotting is still the preferred gait at speeds above 1.8 m/s, where, it seems likely, the cost of transport in walking would be even higher. Supported by NIH #S06 GM53933 to DFH & SJW and an Agricultural Research Initiative grant to SJW.