Elsevier

Journal of Biomechanics

Volume 43, Issue 3, 10 February 2010, Pages 506-511
Journal of Biomechanics

Leg stiffness adjustment for a range of hopping frequencies in humans

https://doi.org/10.1016/j.jbiomech.2009.09.040Get rights and content

Abstract

The purpose of the present study was to determine how humans adjust leg stiffness over a range of hopping frequencies. Ten male subjects performed in place hopping on two legs, at three frequencies (1.5, 2.2, and 3.0 Hz). Leg stiffness, joint stiffness and touchdown joint angles were calculated from kinetic and/or kinematics data. Electromyographic activity (EMG) was recorded from six leg muscles. Leg stiffness increased with an increase in hopping frequency. Hip and knee stiffnesses were significantly greater at 3.0 Hz than at 1.5 Hz. There was no significant difference in ankle stiffness among the three hopping frequencies. Although there were significant differences in EMG activity among the three hopping frequencies, the largest was the 1.5 Hz, followed by the 2.2 Hz and then 3.0 Hz. The subjects landed with a straighter leg (both hip and knee were extended more) with increased hopping frequency. These results suggest that over the range of hopping frequencies we evaluated, humans adjust leg stiffness by altering hip and knee stiffness. This is accomplished by extending the touchdown joint angles rather than by altering neural activity.

Introduction

The spring-like leg behavior of running, hopping, and jumping is a general feature of the mammalian gait. To describe this type of gait, the whole body is often modeled with a “spring-mass model” which consists of a body mass supported by a spring (Farley and Ferris, 1998; Farley et al., 1993; Blickhan, 1989; Butler et al., 2003). In this model, stiffness of the leg spring (“leg stiffness”), defined as the ratio of maximal ground reaction force to maximum leg compression at the middle of the stance phase, has been shown to change depending on the demand.

It has been demonstrated that leg stiffness becomes higher with an increase in hopping frequency (Dalleau et al., 2004; Farley et al., 1991; Ferris and Farley, 1997; Granata et al., 2002; Rapoport et al., 2003; Padua et al., 2005). Although these findings suggest that humans have a sophisticated system of leg stiffness control, detailed mechanisms of the frequency-dependent leg stiffness modulation are not well understood. The aim of the present study was to determine how humans adjust leg stiffness over a range of hopping frequencies.

Leg stiffness depends on the stiffness of the torsional joint spring (joint stiffness, defined as the ratio of maximal joint moment to maximum joint flexion at the middle of the stance phase). Previous studies suggest that leg stiffness during hopping largely depends on ankle stiffness (Farley and Morgenroth, 1999). Ankle stiffness is regulated by pre-activity (muscle activity before ground contact) and muscle activity including the short-latency stretch reflex response of the triceps surae at landing (Hobara et al., 2007). Moreover, several studies indicate that joint stiffness is influenced by antagonistic co-contraction (Hortobagyi and DeVita, 2000). In addition, joint stiffness is also influenced by changes in the touchdown joint angle (Farley et al., 1998). In the present study we hypothesized that increases in leg stiffness with increasing hopping frequency are due to changes in ankle stiffness, which is associated with the pre-activity and stretch-reflex responses of the triceps surae and/or co-contraction levels.

Section snippets

Participants

Ten healthy male subjects participated in the study. Their physical characteristics were: age 22.9±2.9 years, height 174.0±5.4 cm, and body mass 65.1±6.1 kg (mean±SD). Informed consent approved by the Human Ethics Committee, Faculty of Sport Sciences, Waseda University, was obtained from all subjects before the experiment.

Task and procedure

Barefoot subjects were asked to hop in place with their hands on their hips. Hopping was performed on a force plate (60×120 cm, Power Max-1500, Bertec Inc., Japan); the vertical

Hopping frequency, contact time and flight time

Ground contact time and aerial time under three hopping conditions are shown in Table 1. Ground contact time was the shortest in the 3.0 Hz, followed by the 2.2 Hz and then 1.5 Hz. Similarly, aerial time was the shortest in 3.0 Hz, followed by the 2.2 Hz and then 1.5 Hz.

Leg stiffness

Fig. 1 shows a typical example of the relationship between GRF and COM displacement in single cycles of hopping at 1.5, 2.1 and 3.0 Hz, recorded from one subject. The leg was compressed from the touchdown, and GRF increased with COM

Discussion

The purpose of this study was to determine how humans adjust the leg stiffness over a range of hopping frequencies. Our data clearly showed that leg stiffness increased with an increase in hopping frequency (Fig. 2). The results correspond well with those of previous studies in that leg stiffness increased with an increase in hopping frequency (Dalleau et al., 2004; Farley et al., 1991; Ferris and Farley, 1997; Granata et al., 2002; Rapoport et al., 2003; Padua et al., 2005). The increases in

Conflict of interest

None of the authors have any conflicts of interest associated with this study.

Acknowledgements

The authors thank Dr. Larry Crawshaw for his careful review of earlier drafts. The authors also thank members of the Sport Neuroscience laboratory, Faculty of Sport Sciences, Waseda University for useful comments on the manuscript.

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