Runners adjust leg stiffness for their first step on a new running surface

doi:10.1016/S0021-9290(99)00078-0

Journal of Biomechanics

Volume 32, Issue 8, August 1999, Pages 787-794

https://doi.org/10.1016/S0021-9290(99)00078-0 Get rights and content

Abstract

Human runners adjust the stiffness of their stance leg to accommodate surface stiffness during steady state running. This adjustment allows runners to maintain similar center of mass movement (e.g., ground contact time and stride frequency) regardless of surface stiffness. When runners encounter abrupt transitions in the running surface, they must either make a rapid adjustment or allow the change in the surface stiffness to disrupt their running mechanics. Our goal was to determine how quickly runners adjust leg stiffness when they encounter an abrupt but expected change in surface stiffness that they have encountered previously. Six human subjects ran at 3 m s⁻¹ on a rubber track with two types of rubber surfaces: a compliant “soft” surface (k_surf=21.3 kN m⁻¹) and a non-compliant “hard” surface (k_surf=533 kN m⁻¹). We found that runners completely adjusted leg stiffness for their first step on the new surface after the transition. For example, runners decreased leg stiffness by 29% between the last step on the soft surface and the first step on the hard surface (from 10.7 kN m⁻¹ to 7.6 kN m⁻¹, respectively). As a result, the vertical displacement of the center of mass during stance (∼7 cm) did not change at the transition despite a reduction in surface compression from 6 cm to less than 0.25 cm. By rapidly adjusting leg stiffness, each runner made a smooth transition between surfaces so that the path of the center of mass was unaffected by the change in surface stiffness.

Introduction

A simple spring-mass system models the basic mechanics of running remarkably well. The model consists of a single Hookean spring representing a runner’s leg and a point mass equivalent to body mass (Blickhan, 1989; McMahon and Cheng, 1990). This simple model has proven effective in describing the mechanics of both animals and robots as they run, hop, or trot (Blickhan and Full, 1993; Farley et al., 1993; He et al., 1991; Raibert, 1986, Raibert, 1990; Raibert et al., 1986, Raibert et al., 1993). At faster speeds, animals and robots use the same leg stiffness but increase the angle swept by the leg during stance (Farley et al., 1993; He et al., 1991; Raibert et al., 1993). Increasing the angle swept by the leg at faster speeds decreases the vertical displacement of the center of mass during stance and shortens the time that the foot remains on the ground.

Although leg stiffness is independent of running speed, humans adjust leg stiffness when they run on different surfaces (Ferris et al., 1998). For surfaces of lower stiffness, runners decrease leg spring compression by increasing leg stiffness. This adjustment offsets the increased surface compression and keeps the path of a runner’s center of mass the same regardless of surface stiffness. Because many biomechanical parameters depend on the combined series stiffness of the runner and surface, adjusting leg stiffness allows humans to run in a similar manner on different surface stiffnesses (Ferris et al., 1998). Stride frequency, ground contact time, and peak ground reaction force are all independent of surface stiffness (Ferris et al., 1998). All of these observations are for steady-state running on a continuous surface. It is not known how quickly runners can adjust to an abrupt change in surface stiffness.

We hypothesized that runners would adjust leg stiffness for their first step on a new surface of a different stiffness when they knowingly ran from one surface to another. We based this hypothesis on data from running quail that suggest running birds do not maintain an invariant leg stiffness when they step on small areas of different mechanical stiffnesses (Clark, 1988). In order to compare our experimental findings with the results that would be expected if runners did not adjust leg stiffness for the new surface stiffness, we constructed a computer simulation of a spring-mass model on a compliant surface. We used the computer simulation to assess how the global running mechanics (e.g., path of the center of mass, contact time, running speed) would be affected by the transition in surface stiffness without a concomitant leg stiffness adjustment.

Section snippets

Methods

Six healthy female subjects (body mass 52.7±3.1 kg, leg length 88±3.5 cm; mean ± s.d.) between 21 and 25 years of age participated in this study. The university committee for the protection of human subjects approved the experimental protocol and subjects provided informed consent. We recorded ground reaction force (1000 Hz) and high-speed video (200 Hz) as subjects ran at 3 m s⁻¹ on a 17 meter rubber track with two force platforms (AMTI, Inc.) mounted beneath the middle of the track. We cut sections

Computer simulations

The computer simulations revealed that running mechanics would be substantially altered if runners did not adjust leg stiffness (Table 1). An unadjusted leg stiffness resulted in an asymmetrical center of mass trajectory and a net change in velocity (Fig. 3). For example, when leg stiffness was not adjusted to accommodate the hard surface, the center of mass reached a higher height at the end of stance than at the beginning. Some of the initial kinetic energy was converted to gravitational

Discussion

Humans and other animals run from one surface to another with amazing grace and agility in the natural world. Our past research has shown that runners maintain similar center of mass movements on surfaces with different stiffnesses by adjusting leg stiffness to accommodate the surface stiffness (Ferris et al., 1998). In most real-life situations, runners see changes in the terrain ahead of them and have knowledge of the mechanical properties of the surfaces from previous experience. Thus,

Acknowledgements

This research was supported by a graduate fellowship from NASA to D.P.F. (NGT-51416) and a grant from the National Institutes of Health to C.T.F. (R29 AR44008).

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Citation Excerpt :
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Runners adjust leg stiffness for their first step on a new running surface

Abstract

Introduction

Section snippets

Methods

Computer simulations

Discussion

Acknowledgements

Journal of Biomechanics

Journal of Biomechanics

Journal of Biomechanics

Journal of Biomechanics

Progress in Neurobiology

Journal of Biomechanics

Trends in Neuroscience

Corrective reactions to stumbling in manneuronal co-ordination of bilateral leg muscle activity during gait

Journal of Physiology (London)

Similarity in multilegged locomotionbouncing like a monopode

Journal of Comparative Physiology

Force platforms as ergometers

Journal of Applied Physiology

Mechanics and control of the limb of Bobwhite quail running and landing on substrates of unpredictable mechanical stiffness. Ph.D. Thesis

BiostatisticsA Foundation for Analysis in the Health Sciences

Contribution of spinal stretch reflexes to the activity of leg muscles in running

Neuronal mechanisms of human locomotion

Journal of Neurophysiology