SUMMARY Stride duration, stance duration and protraction duration are key variables when describing the gaits of terrestrial animals. Together, they determine the duty factor (the fraction of the stride for which the limb maintains contact with the ground surface), from which the peak vertical force can be estimated. When an animal changes speed, these variables change at different proportions. Limited measurements of these variables and predictions of peak limb force have been undertaken for large mammals performing high-speed over-ground exercise. This study set out to make such measurements, employing a previously validated system consisting of limb-mounted accelerometers and a Global Positioning System data logger. Measurements were made on nine elite Thoroughbred racehorses during gallop locomotion over a range of speeds from 9 to 17 m s -1 . No statistically significant differences were seen in any variables between the lead and non-lead limbs for either the fore or hind pairs of limbs. Mean stance durations of 131 and 77 ms in the forelimbs and 143 and 94 ms in the hindlimbs were recorded at speeds of 9 and 17 ms -1 , respectively. Equivalent values for protraction duration were 364 and 342 (fore) and 355 and 326 ms (hind). Peak limb forces (from duty factor) at 17 ms -1 were 24.7 N kg -1 body weight (range 22.6 to 26.0 N kg -1 body weight) for the forelimbs and 15.3 N kg -1 (range 13.7-16.2 N kg -1 body weight) for the hindlimbs. The duration of the aerial phase of the stride (when no limbs are in contact with the ground) was independent of speed. Overlap time (when more than one leg is on the ground) dropped with speed and approached zero at maximum speed.
SUMMARY Measurement of peak vertical ground reaction force (GRFz) from multiple limbs simultaneously during high-speed, over-ground locomotion would enhance our understanding of the locomotor mechanics of cursorial animals. Here, we evaluate the accuracy of predicting peak GRFz from duty factor (the proportion of the stride for which the limb is in contact with the ground). Foot-mounted uniaxial accelerometers, combined with UHF FM telemetry, are shown to be practical and accurate for the field measurement of stride timing variables,including duty factor. Direct comparison with the force plate produces a mean error of 2.3 ms and 3.5 ms for the timing of foot on and foot off,respectively, across all gaits. Predictions of peak GRFz from duty factor show mean errors (with positive values indicating an overestimate) of 0.8±0.04 N kg –1 (13%; N =42; mean ± s . e . m .) at walk, –0.3±0.06 N kg –1 (3%; N =75) at trot, –2.3±0.27 N kg –1 (16%; N =18) for the non-lead limb at canter and+2.1±0.7 N kg –1 (19%; N =9) for the lead limb at canter. The substantial over- and underestimate seen at canter, in the lead and non-lead limbs, respectively, is attributed to the different functions performed by the two limbs in the asymmetrical gaits. The difference in load experienced by the lead and non-lead limbs decreased with increasing speed.