One of the most obvious locomotory behaviors is gait transition (changing from walk to trot/run and changing from trot to gallop). There have been numerous attempts to explain gait transitions. These include considerations of muscle function (Taylor, 1978(Taylor, , 1985 and bone strain (Biewener and Taylor, 1986;Rubin and Lanyon, 1982), theoretical explanations based on mathematical models (Alexander, 1989;Alexander and Jayes, 1983), psychological factors (Diedrich and Warren, 1995) and engineering models (Schoner et al., 1990;Vilensky et al., 1991).The walk-trot and trot-gallop gait transitions were originally explained on the basis of metabolic economy (Hoyt and Taylor, 1981). In ponies (Equus caballus), metabolism increased curvilinearly for walking and trotting, and the gait transitions occurred at the speeds where the metabolism curves intersected. This is referred to as the 'energetically optimal transition speed' (EOTS; Hreljac, 1993) because, when the animals extended their gaits beyond the normal transition speeds, the metabolic rate was higher in the extended gait than in the normal gait. Hoyt and Taylor concluded that ponies changed gaits to minimize energetic costs. However, one limitation of this study was that gait transition speeds were not rigorously determined.Subsequently, this explanation was challenged by the 'force trigger' hypothesis. Farley and Taylor (1991) showed that the transition from trotting to galloping in ponies is correlated with musculoskeletal forces by demonstrating that the transition occurs at a slower speed when a pony carries a load. Measurements of oxygen consumption (again observed to be a curvilinear function of speed) indicated that the ponies were making the transition to a gallop at speeds where it is energetically more expensive to gallop than to trot -at speeds slower than the EOTS. In some studies, the walk-run transition in humans occurs at the EOTS (Mercier et al., 1994;Diedrich and Warren, 1995) and in others it does not (Hreljac, 1993; Minetti et al., 1994a,b). Hreljac (1993) ruled out muscle stress as the trigger for the walk-run transition in humans and suggested that the trigger is kinematic (Hreljac, 1995).In a study of horses and preferred speed , the energetics of trotting were measured on the level and up a 10% incline. In the preliminary portion of this study, we determined the speeds at which the horses would trot. We noted that, when trotting up an incline, the horses made the transition to a gallop at a slower speed than they would when on the level. Because forces are not expected to be higher when Two studies have focused on potential triggers for the trot-gallop transition in the horse. One study concluded that the transition was triggered by metabolic economy. The second study found that it was not metabolic factors but, rather, peak musculoskeletal forces that determine gait transition speeds. In theory, peak musculoskeletal forces should be the same when trotting up an incline as when trotting at the same speed on the level. Assuming this is ...
