A method to determine mechanical energy conservation and efficiency in equine gait: a preliminary study

Metabolic and mechanical energy costs of locomotion can be combined to calculate locomotor efficiency, which is the quotient of the mechanical energy and metabolic costs. The purpose of this pilot study was to evaluate the mechanical and metabolic energy costs of locomotion at a range of 7 trotting speeds (2.5 to 6.2 m/s) on a level treadmill. A single, sound Thoroughbred horse was modelled as a system of 15 linked segments incorporating all 4 limbs, head, neck and trunk. The horse performed a continuous incremental exercise test at increasing trotting speeds while VO2 was recorded using a breath-by-breath gas analysis system. Positional data were recorded concurrently at 100 Hz using a 2-camera infrared kinematic system. Mechanical energy cost was calculated for 3-6 strides per speed increment, and metabolic data were obtained during the last 15 s of each speed step. Mechanical energy cost increased linearly from 3.3 J/kg/m at 2.5 m/s to a value of 5.31 J/kg/m at 6.2 m/s, and the within-subject variability was low at each of the speed steps. This analysis accounted for the important energy-conserving mechanisms of energy exchange within and between segments of the link segment model. Within-segment energy conservation remained approximately constant as speed increased, whereas between-segment conservation increased from 1040 to 4502 J/stride. The combination of both metabolic and mechanical costs of locomotion yielded an inverted bell-shaped curve of 'apparent' efficiency across the speed increments, with the maximum value occurring when metabolic cost was lowest at 3.8 m/s.