We studied nine adult horses spanning an eightfold range in body mass (M(b)) (90-720 kg) and a twofold range in leg length (L) (0.7-1.4 m). We measured the horses' walk-trot transition speeds using step-wise speed increments as they locomoted on a motorized treadmill. We then measured their rates of oxygen consumption over a wide range of walking and trotting speeds. We interpreted the transition speed results using a simple inverted-pendulum model of walking in which gravity provides the centripetal force necessary to keep the leg in contact with the ground. By studying a large size range of horses, we were naturally able to vary the absolute walking speed that would produce the same ratio of centripetal to gravitational forces. This ratio, (M(b)v2/L)/(M(b)g), reduces to the dimensionless Froude number (v2/gL), where v is forward speed, L is leg length and g is gravitational acceleration. We found that the absolute walk-trot transition speed increased with size from 1.6 to 2.3 m s(-1), but it occurred at nearly the same Froude number (0.35). In addition, horses spontaneously switched between gaits in a narrow range of speeds that corresponded to the metabolically optimal transition speed. These results support the hypotheses that the walk-trot transition is triggered by inverted-pendulum dynamics and occurs at the speed that maximizes metabolic economy.
Disuse (inactivity, bed rest, and spaceflight) may lead to a loss of muscle mass and a decrease in oxidative capacity in skeletal muscle. If such changes were to occur in hibernating animals, both locomotor and thermogenic function would be compromised. Muscle masses and oxidative capacities (as assessed by citrate synthase activity) were measured in the gastrocnemius and semitendinosus muscles, cardiac muscle (ventricle), and brown fat (axillary pad) in a group (n = 7) of prehibernating ground squirrels (Spermophilus lateralis) and after 6 mo of hibernation (n = 8). Hibernation produced significant atrophy in the gastrocnemius (14%) and semitendinosus (42%) muscles. Cardiac tissue increased (21%) in mass, as did brown adipose tissue (150%). That such changes were not due simply to fluid shifts was evidenced by similar protein concentrations between groups. In contrast to many other disuse studies, oxidative capacity was increased significantly in the gastrocnemius (65%) and semitendinosus (37%). Citrate synthase was also higher in cardiac tissue of hibernators (20%) but was not significantly different in brown fat.
Although the forces required to support the body mass are not elevated when moving up an incline, kinematic studies, in vivo tendon and bone studies and kinetic studies suggest there is a shift in forces from the fore-to the hindlimbs in quadrupeds. However, there are no wholeanimal kinetic measurements of incline locomotion. Based on previous related research, we hypothesized that there would be a shift in forces to the hindlimb. The present study measured the force produced by the fore-and hindlimbs of horses while trotting over a range of speeds (2.5 to 5·m·s -1 ) on both level and up an inclined (10%) surface.On the level, forelimb peak forces increased with trotting speed, but hindlimb peak force remained constant. On the incline, both fore-and hindlimb peak forces increased with speed, but the sum of the peak forces was lower than on the level. On the level, over the range of speeds tested, total force was consistently distributed between the limbs as 57% forelimb and 43% hindlimb, similar to the weight distribution of the horses during static weight tests. On the incline, the force distribution during locomotion shifted to 52% forelimb and 48% hindlimb.Time of contact and duty factor decreased with speed for both limbs. Time of contact was longer for the forelimb than the hindlimb, a finding not previously reported for quadrupeds. Time of contact of both limbs tended to be longer when traveling up the incline than on the level, but duty factor for both limbs was similar under both conditions. Duty factor decreased slightly with increased speed for the hindlimb on the level, and the corresponding small, predicted increase in peak vertical force could not be detected statistically.
SUMMARY The net work of the limbs during constant speed over level ground should be zero. However, the partitioning of negative and positive work between the fore- and hindlimbs of a quadruped is not likely to be equal because the forelimb produces a net braking force while the hindlimb produces a net propulsive force. It was hypothesized that the forelimb would do net negative work while the hindlimb did net positive work during trotting in the horse. Because vertical and horizontal impulses remain unchanged across speeds it was hypothesized that net work of both limbs would be independent of speed. Additionally because the major mass of limb musculature is located proximally,it was hypothesized that proximal joints would do more work than distal joints. Kinetic and kinematic analysis were combined using inverse dynamics to calculate work and power for each joint of horses trotting at between 2.5 and 5.0 m s–1. Work done by the hindlimb was indeed positive (consistently 0.34 J kg–1 across all speeds), but, contrary to our hypothesis, net work by the forelimb was essentially zero (but also independent of trotting speed). The zero net work of the forelimb may be the consequence of our not being able to account, experimentally, for the negative work done by the extrinsic muscles connecting the scapula and the thorax. The distal three joints of both limbs behaved elastically with a period of energy absorption followed by energy return. Proximal forelimb joints (elbow and shoulder) did no net work, because there was very little movement of the elbow and shoulder during the portion of stance when an extensor moment was greatest. Of the two proximal hindlimb joints, the hip did positive work during the stride,generating energy almost throughout stance. The knee did some work, but like the forelimb proximal joints, had little movement during the middle of stance when the flexion moment was the greatest, probably serving to allow the efficient transmission of energy from the hip musculature to the ground.
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 ...
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