Human hoppers compensate for simultaneous changes in surface compression and damping

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Abstract

On a range of elastic and damped surfaces, human hoppers and runners adjust leg mechanics to maintain similar spring-like mechanics of the leg and surface combination. In a previous study of adaptations to damped surfaces, we changed surface damping and stiffness simultaneously to maintain constant surface compression. The current study investigated whether hoppers maintain spring-like mechanics of the leg–surface combination when surface damping alone changes (elastic and 1000–4800 N s m−1). We found that hoppers adjusted leg mechanics to maintain similar spring-like mechanics of the leg–surface combination and center of mass dynamics on all surfaces. Over the range of surface damping, vertical stiffness of the leg–surface combination increased by only 12% and center of mass displacement decreased by only 6% despite up to 55% less compression of more heavily damped surfaces. In contrast, a simulation predicted a 44% decrease in vertical displacement with no adjustment to leg mechanics. To compensate for the smaller and slower compression of more heavily damped surfaces, the stance legs compressed by up to 4.1±0.2cm further and reached peak compression sooner. To replace energy lost by damped surfaces, hoppers performed additional leg work by extending the legs during takeoff by up to 3.1±0.2cm further than they compressed during landing. We conclude that humans simultaneously adjust leg compression magnitude and timing, as well as mechanical work output, to conserve center of mass dynamics on damped surfaces. Runners may use similar strategies on natural energy-dissipating surfaces such as sand, mud and snow.

Introduction

Quickly moving legged animals can gracefully traverse a variety of natural terrain. Specifically, hopping and running humans adjust leg mechanics to compensate for changes in surface properties and maintain similar center of mass dynamics. On elastic surfaces, humans increase the stiffness of their spring-like stance legs to compensate for softer surfaces, thereby maintaining similar bouncing center of mass dynamics regardless of surface stiffness (Ferris and Farley, 1997; Ferris et al., 1999, Ferris et al., 1998; Kerdok et al., 2002).

Humans hopping on damped surfaces also maintain bouncing center of mass dynamics. To maintain steady hopping on a damped surface, the stance legs cannot behave like springs because they must produce mechanical work to replace the energy dissipated by the surface. We recently examined the leg mechanics of hopping on surfaces with a range of stiffness and damping combinations, but constant peak surface compression (Moritz and Farley, 2003). We found that on more heavily damped surfaces, hoppers perform more work with their stance legs to replace the energy dissipated by the surface and adjust leg compression timing to offset the slower surface compression and rebound. Because we maintained a constant surface compression regardless of surface damping, hoppers could maintain similar center of mass dynamics on a wide range of damped surfaces without adjusting the magnitude of leg compression.

If surface damping increases with no decrease in surface stiffness, hoppers may have to adjust the magnitude of leg compression and extension to compensate for reduced surface compression and thereby conserve similar center of mass dynamics regardless of surface damping. Indeed, a simulation of running predicts that high levels of surface damping lead to less surface compression (Nigg and Anton, 1995). Surfaces with simultaneous changes in both surface compression and damping are common in the natural world, as animals traverse sand, dirt, mud and snow.

The goal of this study was to determine whether humans adjust leg mechanics to compensate for simultaneous changes in surface compression magnitude and timing as well as energy dissipation. We hypothesized that hoppers would maintain similar center of mass dynamics regardless of surface damping by adjusting the magnitude and timing of leg compression as well as mechanical work output. ‘Leg’ refers to all segments between the body's center of mass and the ground. We tested this hypothesis by measuring ground reaction force and surface position while humans hopped in place on surfaces with a fixed stiffness but a range of damping. We chose to study hopping in place as it is an excellent analog to forward running (Farley et al., 1991), and it is technically more feasible to construct an adjustable damped surface for hopping in place than for running.

Section snippets

Materials and methods

Eight male subjects (body mass 76.2±1.7kg, height 176±5cm, age 28±2; mean±SD) hopped in place on a surface with adjustable stiffness and damping. All subjects gave informed consent, and the University of Colorado and California Human Research Committees approved the protocol.

The lightweight hopping surface (effective mass 3.7 kg; Fig. 1) was supported by steel springs (Century Springs, Los Angeles, CA, USA) and a bi-directional hydraulic damper (Taylor Devices, New York, NY, USA). The apparatus

Results

Hoppers maintained similar center of mass dynamics on all surfaces despite large changes in surface damping and surface compression. The elastic surface compressed by 6.7±0.1cm while the most damped surface compressed by only 3.0±0.1cm (P<0.001; Fig. 2A). Hoppers compensated by increasing leg compression by 4.1±0.2cm between the elastic surface and the most damped surface (P=0.001; Fig. 2B). In contrast, simulation results predicted a much smaller change in leg compression than observed in the

Discussion

As predicted by our hypothesis, hoppers maintain similar center of mass dynamics as surface damping increases by simultaneously changing the magnitude and timing of maximum leg compression and leg mechanical work output. By making this complex adjustment to leg mechanics, hoppers maintain spring-like center of mass dynamics despite large changes in both surface compression and energy dissipation as surface damping increases (see Fig. 5). These findings and earlier studies suggest that

Acknowledgements

The authors thank Spencer Green for his assistance and the University of Colorado Locomotion Laboratory for comments on the manuscript. This work was supported by NIH Grant R29 AR-44008 to CTF and an American Society of Biomechanics Grant-in-aid to CTM.

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