Human hoppers compensate for simultaneous changes in surface compression and damping
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 , height , age ; 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 while the most damped surface compressed by only (; Fig. 2A). Hoppers compensated by increasing leg compression by between the elastic surface and the most damped surface (; 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.
References (18)
- et al.
Leg stiffness primarily depends on ankle stiffness during human hopping
Journal of Biomechanics
(1999) - et al.
Runners adjust leg stiffness for their first step on a new running surface
Journal of Biomechanics
(1999) - et al.
Biomechanics and muscle coordination of human walking. Part I: introduction to concepts, power transfer, dynamics and simulations
Gait Posture
(2002) - et al.
Drop jumping. II. The influence of dropping height on the biomechanics of drop jumping
Medicine and Science in Sports and Exercise
(1987) Force platforms as ergometers
Journal of Applied Physiology
(1975)Forces and energy changes in the leg during walking
American Journal of Physiology
(1939)- et al.
Hopping frequency in humans: a test of how springs set stride frequency in bouncing gaits
Journal of Applied Physiology
(1991) - et al.
Mechanism of leg stiffness adjustment for hopping on surfaces of different stiffnesses
Journal of Applied Physiology
(1998) - et al.
Interaction of leg stiffness and surface stiffness during human hopping
Journal of Applied Physiology
(1997)
Cited by (21)
Leg stiffness measures depend on computational method
2014, Journal of BiomechanicsCitation Excerpt :Applied to a hopping task, the force in Fig. 1 is the external vertical (upwards) force from the ground support, whereas the displacement is the vertical (downwards) movement of the CoM during the ground contact. There has been a rapid increase in the number of applied research studies documenting stiffness values for the lower-extremity (Hobara et al., 2012; Jacobs et al., 1996; Lloyd et al., 2012; Maquirriain, 2012; Moritz and Farley, 2006; Pruyn et al., 2012), with researchers suggesting that sufficient levels of stiffness are required to optimize the utilization of the stretch-shortening cycle (Belli and Bosco, 1992; Kubo et al., 1999) and minimize the risk of musculoskeletal injury or re-injury (Maquirriain, 2012; Watsford et al., 2010). More specifically, high leg stiffness has been associated to a heightened risk of bony injuries (Granata et al., 2002; Williams et al., 2004), whereas low leg stiffness gives an increased susceptibility to soft tissue injuries (Butler et al., 2003; McMahon et al., 2012).
Methodological considerations of task and shoe wear on joint energetics during landing
2012, Journal of Electromyography and KinesiologyCitation Excerpt :A study comparing habitual barefoot and shod runners revealed that barefoot runners adopted a strategy of forefoot striking (more plantarflexed ankle) while shod runners adopted a rearfoot striking strategy (Lieberman et al., 2010). Although the demands of running and landing differ, control of center of mass positioning is needed in landing (McNitt-Gray et al., 2001) as well as running (Moritz and Farley, 2006). Center of mass vertical position during functional tasks is largely influenced by lower extremity flexion angle (Farley and Morgenroth, 1999).
Does a crouched leg posture enhance running stability and robustness?
2011, Journal of Theoretical BiologyIntralimb compensation strategy depends on the nature of joint perturbation in human hopping
2008, Journal of BiomechanicsCitation Excerpt :We gain some insight into how the leg might respond to increased mechanical demand at a joint through studies of hopping on energy-dissipating surfaces. Moritz, Farley and colleagues showed that ankle, knee and hip joints all responded to preserve global spring-mass dynamics while hopping on an energy-damping surface through increased power output from all of the joints rather than a single-joint response (Moritz and Farley, 2003, 2006; Moritz et al., 2004). Since energy was dissipated from the entire leg, it remains unknown whether this reflected coordinated action of the joints or whether each joint responded independently to local conditions.
Neuromechanical adaptations of foot function when hopping on a damped surface
2022, Journal of Applied Physiology