Determinants of preferred ground clearance during swing phase of human walking

Abstract

During each step of human walking, the swing foot passes close to the ground with a small but (usually) non-zero clearance. The foot can occasionally scuff against the ground, with some risk of stumbling or tripping. The risk might be mitigated simply by lifting the foot higher, but presumably at increased effort, of unknown amount. Perhaps the normally preferred ground clearance is a trade-off between competing costs, one for lifting the foot higher and one for scuffing it. We tested this by measuring the metabolic energy cost of lifting and scuffing the foot, treating these apparently dissimilar behaviors as part of a single continuum, where scuffing is a form of negative foot lift. We measured young, healthy adults (N=9) lifting or scuffing the foot by various amounts mid-swing during treadmill walking, and observed substantial costs, each well capable of doubling the net metabolic rate for normal walking (gross cost minus that for standing). In relative terms, the cost for scuffing increased over twice as steeply as that for lifting. That relative difference means that the expected value of cost, which takes into account movement variability, occurs at a non-zero mean clearance, approximately matching the preferred clearance we observed. Energy cost alone is only a lower bound on the overall disadvantages of inadvertent ground contact, but it is sufficient to show how human behavior may be determined not only by the separate costs of different trade-offs but also by movement variability, which can influence the average cost actually experienced in practice.

Keywords: Biomechanics; Energetic cost; Foot–ground clearance; Locomotion; Metabolic power.

Conflict of interest statement

The authors declare no competing or financial interests.

© 2016. Published by The Company of Biologists Ltd.

Figures

Fig. 1.

Proposed cost of ground clearance, in terms of metabolic power, including separate contributions for scuffing the foot on the ground and for lifting the foot higher. Here, clearance is treated as if there were a single range of positive to negative foot lift, the latter causing foot scuff. The expected cost is the sum of these contributions, mediated by variability of foot motion, described by a probability distribution function (inset) about a mean foot lift. If the cost of scuffing is steeper than the cost of lifting near the origin, the average foot lift with least cost should be positive, thus favoring non-zero average foot lift.

Fig. 2.

Experimental set-up. Subjects (N=8) walked with varying levels of foot lift and scuff force on a split-belt treadmill at 1.25 m s−1. During scuffing conditions (left), subjects were asked to produce a drag force against the ground (aft ground reaction force, GRF, plotted in the positive direction) during swing phase walking, indicated by two target thresholds, one for each foot. In the foot lift conditions (right), they were asked to clear a target threshold for the height of the lateral toe marker. Visual feedback of both real-time data and thresholds was projected onto a screen visible to the subject.

Fig. 3.

Measures of foot scuffing and lifting observed in experimental conditions. (A) Foot scuff GRFs. Fore–aft GRFs versus time in gait cycle (% of full stride starting from heel-strike) from a representative subject indicate greater drag (aft) force achieved from low to high scuff threshold levels (rectangle). (B) Foot lift swing trajectories. Fore–aft and vertical trajectory of the lateral toe marker from a representative subject for various lift height thresholds from the treadmill. (C,D) Mean ground clearance levels across subjects measured through (C) scuff impulse (N=7) and (D) lift height (N=8). All levels of ground clearance were significantly different from normal (*P<0.05). Left-hand axes are dimensionless, using body mass, leg length and gravitational acceleration as base units; SI units are given in the right-hand axes. Bars denote means across subjects and error bars denote s.d.

Fig. 4.

Net metabolic cost as a function of measured scuff impulse and lift height. Net metabolic rate increased with greater scuff impulse (N=7) at a rate of −69.0 W N−1 s−1 (R2=0.75, P<0.05) and with greater lift height (N=8) at 2517 W m−2 (R2=0.77, P<0.05). The distribution of minimum toe clearance during swing indicates movement variability during normal walking. Different colors denote each subjects' data (squares for scuffing, circles for foot lift). Net metabolic rate for normal walking is also indicated (dashed line), defined as gross metabolic rate minus quiet standing rate. Metabolic rate, scuff impulse and lift height are shown in dimensionless units, using body mass, leg length and gravitational acceleration as base units; equivalent SI units are also indicated.

Fig. 5.

Force and power measures versus time within gait cycle for varying levels of ground clearance. (A) Vertical and (B) fore–aft GRF, (C) center of mass (COM) power and (D) summed joint power from the sum of ankle, knee and hip power from one leg. More qualitative changes are observed in lift conditions than in scuff conditions, compared with the normal condition. Vertical axes are shown in both dimensionless (left axes) and SI form (right axes); horizontal axes are shown as a fraction of gait cycle (% of stride) beginning with heel-strike, with a corresponding time scale for each condition shown in A. Each trace is a filtered average across subjects; see Fig. 3A for representative trials.

Fig. 6.

Joint angle, moment and power for foot scuff and foot lift conditions. (A) Foot scuff and (B) foot lift trajectories versus time for ankle, knee and hip, with gait cycle starting at heel-strike. Left-hand axes for moment and power are in dimensionless units and right-hand axes are SI units. Ext, extension; Flx, flexion.

Fig. 7.

Mechanical work rate against scuff impulse and lift height. (A) Mean COM work rate per stride and (B) mean summed joint work rate per stride for ankle, knee and hip. Lift height had a greater impact on work rate than scuff impulse. Greater lift contributed towards significant increases in positive and negative COM work rate and joint work rate. However, scuff impulse only affected negative COM work rate and positive joint work rate, both to a lesser extent than for lift height. Different colors denote each subjects' data (squares for scuffing, circles for foot lift). Trend significance is indicated by solid lines (P<0.05) and non-significance by dashed lines.