HOYT, DF; WICKLER, SJ; DUTTO, DJ; California State Polytechnic University, Pomona; California State Polytechnic University, Pomona; California State Polytechnic University, Pomona: The trot-gallop transition: insights from energetics, muscle function and biomechanics

Recently, it was shown that the trot gallop transition occurs at a lower speed when going up an incline than on the level, but under both conditions, it occurs at the energetically optimal transition speed. This correlation with energetics does not provide a completely satisfactory explanation because at the highest speed where the animal switches from a gallop to a trot, the energy cost of trotting is higher. We hypothesized that differences in muscle function might contribute to the decision to change gait. The trot-gallop transition speed was determined in 4 horses before training them to extend both gaits. Muscle strain of the vastus lateralis (by sonomicrometry) and muscle activity (EMG) were measured in horses trotting and galloping on the level and up a 10% incline. Stance time was signaled by an accelerometer attached to the hoof. Synchronized ground reaction force (GRF-1000 Hz) and digital video (250 Hz) were recorded while horses trotted at constant speeds on a level runway and up a 10% incline. Inverse dynamic analysis was used to determine the joint moment, power, and work during the stance phase of trotting for the joints of both limbs. Net strain of the vastus did not change significantly with speed or gait under either condition and strain rate was the same in both gaits. Integrated EMG (IEMG) was higher on an incline as expected. IEMG under both conditions was lower in galloping, indicating a smaller volume of active muscle, consistent with the decrease in peak ground reaction force previously reported. Compared with the level, peak GRF on an incline is lower in the forelimb and higher in the hind limb. It still is not completely clear what causes a horse to change from galloping to trotting. Supported by NIH # S06 GM53933 to DFH & SJW.