Force shoes have been used in horses with some success( Björk, 1958 Frederick, Jr and Henderson,1970 Hugelshofer,1982 Kai et al.,2000 Ratzlaff et al.,1985 Ratzlaff et al.,1990 Roepstorff and Drevemo,1993) but their mass and size may influence locomotion. Force measuring treadmills have been used in horses ( Weishaupt et al.,2002) but treadmill gait is not completely normal and it would be difficult to study fit top class racehorses at their maximum attainable speed in that environment. Overlap time (when more than one leg is on the ground) dropped with speed and approached zero at maximum speed.ĭirect measurement of ground reaction force during highspeed locomotion in large animals is extremely difficult. The duration of the aerial phase of the stride (when no limbs are in contact with the ground) was independent of speed. 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. Equivalent values for protraction duration were 364 and 342 (fore) and 355 and 326 ms (hind). 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. 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.
Measurements were made on nine elite Thoroughbred racehorses during gallop locomotion over a range of speeds from 9 to 17 m s -1. 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. Limited measurements of these variables and predictions of peak limb force have been undertaken for large mammals performing high-speed over-ground exercise. When an animal changes speed, these variables change at different proportions. 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. Stride duration, stance duration and protraction duration are key variables when describing the gaits of terrestrial animals.