Meeting Abstract
Locomotion energy costs can be affected by the substrate underfoot and the mechanics of movement. For example, in some cases substrate properties can exacerbate energy expenditure while in other instances those properties, if skilfully exploited, such as by arboreal primates, can attenuate transport costs. We present the first data on metabolic costs of horizontal jumping in humans, and interpret the differing costs of jumping between two distinct substrates in terms of kinematic alterations. The substrates were either ‘firm’ or ‘compliant’ and jumps over two distances were measured: 1.2 and 1.8 m. Participants jumped at 0.2 Hz, back and forth between surfaces, wearing a portable respiratory gas analyser, and metabolism remained aerobic. The cost for a human to jump horizontally is, per unit distance, around 18-fold the cost to walk and around 12-fold the cost to run. The main findings concerning the effects of substrate properties on jumping energy costs were: (1) for long jumps, jumping from a compliant surface is energetically less costly than jumping from a firm surface; (2) the difference in energy costs associated with a compliant versus firm take-off surface is not present at shorter jumping distances. Kinematic analysis indicates possible explanations for these findings. Firstly, the calf muscle is likely used more, and the thigh muscles less, to take-off from a firm springboard during 1.8 m jumps, which may result in the power required to take-off being produced less efficiently. Secondly, the angle of take-off from the compliant surface during 1.8 m jumps is closer to the optimal for energetic efficiency (45°); possible due to the impulse provided by the surface as it returns stored energy during the final stages of the take-off. The theoretical effect on energy costs due to a different take-off angle for jumps of only 1.2 m is close to negligible.
