The high cost of swing leg circumduction during human walking

Humans tend to walk economically, with preferred step width and length corresponding to an energetic optimum. In the case of step width, it is costlier to walk with either wider or narrower steps than normally preferred. Wider steps require more mechanical work to redirect the body's motion laterally with each step, but the cost for narrower steps remains unexplained. Here we show that narrow steps are costly because they require the swing leg to be circumducted around the stance leg. Healthy adults (N=8) were tested walking with varying levels of circumduction, induced through lightweight, physical obstructions ("Fins") attached medially to the lower legs, during treadmill walking at fixed speed (1.25ms^-1) and step width. The net rate of metabolic energy expenditure increased approximately with the square of circumduction amplitude, by about 50% for an amplitude (measured at mid-swing) of about 18cm. Subjects also generated greater stance leg torque and more arm motion to counter the circumduction, among other compensatory motions that may contribute to energy expenditure. The costs of producing and countering lateral leg motion partially explains the poorer economy of some gait pathologies where circumduction may occur, for example stiff-knee gait. And for healthy individuals, it explains how the energetically optimal average step width, along with the additional variability inherent with multiple steps, should be narrow enough to avoid excessive redirection of the body, yet wide enough to avoid costly circumduction. Humans appear to prefer a step width that compromises between the competing energetic costs for either wider or narrower steps.