Flat and bouncy walking.

Abstract

Walking is one of the major items in human energy budgets. Researchers in the 1950s found that it accounted for 20% of a typical clerk's weekly energy consumption, and 27% of a miner's. Fifty years later, many of us walk less, but walking remains a substantial energy cost. It seems reasonable to expect people to adjust their gaits to keep the cost of moving around as low as possible (see for example Alexander, 2002). Measurements of oxygen consumption have shown that we do indeed adjust many features of gait for economy of energy. We break into a run at the speed (about 2 m s−1) at which running becomes more economical than walking. At any particular walking speed, we spontaneously adjust our strides to the length that minimizes energy consumption at that speed. We do not set our left and right feet down along the same line as if on a tightrope, nor well out to either side of the body in the manner of toddlers, drunkards and sailors on a rolling ship; the intermediate strategy that we actually adopt minimizes energy costs (Donelan et al. 2001). In this issue of The Journal of Physiology, Massaad et al. (2007) report a study of unusually flat or bouncy human gaits. We bob up and down in normal walking because we keep each leg almost straight while its foot is on the ground. Accordingly, we rise as the supporting leg becomes vertical and descend again towards the end of the step, so as to move forward along a series of arcs of circles. The force the foot exerts on the ground stays in line with the leg, so we slow down as we rise and speed up as we fall. Kinetic energy is converted to gravitational potential energy and back again in the manner of a pendulum. In principle, no work is required until the other foot lands on the ground and we set out on a new arc. At that stage, the downward motion of the body has to be halted, and muscular work is needed to start it moving upwards on the new arc. This work would be avoided if we did not bob up and down, but each knee would have to bend and re-extend while its foot was on the ground, and work would be needed for that. Analysis of a simple theoretical model (Srinivasan & Ruina, 2006) concluded that work requirements would be minimized by keeping leg length constant, and so bobbing up and down. A more elaborate musculoskeletal model that predicted metabolic energy costs similarly showed a normal bobbing gait as optimal (Sellers et al. 2003). Massaad et al. (2007) measured the mechanical work and oxygen used in flat, normal and bouncy walking. Despite the indication of Srinivasan & Ruina's model, that normal walking would minimize work, Massaad et al. found no significant difference in work requirements between flat and normal walking. Oxygen consumption in flat walking, however, was up to double the rate for normal walking at the same speed, as also found by Ortega & Farley (2005). Thus, muscle efficiency was remarkably low, possibly due to the need for high forces in the quadriceps muscles when the knee was bent at mid-stance (the metabolic rate of an active muscle depends on the force it is exerting as well as on the rate at which it is doing work). This explanation has been offered for the high metabolic energy cost of the bent-legged bipedal walking style of chimpanzees (Carey & Crompton, 2005). In bouncy walking, both work and oxygen consumption were higher than for normal walking, and efficiency was close to normal. There are problems in research that depends on comparisons of metabolic and mechanical energy costs, between normal and unusual patterns of movement. Estimates of total muscular work are subject to uncertainties regarding co-contraction of antagonistic muscles, and regarding energy transfer between body segments. Unaccustomed gaits may be performed less skilfully than normal ones, increasing oxygen consumption, even when (as in this case) time has been allowed for practice. The conclusion of this study, nevertheless, seems clear; the bounciness of our walk may look like wasteful exuberance, but it does save energy.