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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.