Leg stiffness adjustment for a range of hopping frequencies in humans
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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2020, Gait and PostureCitation Excerpt :Leg stiffness may be described as linear stiffness, the change of elastic deformation, or the change of force divided by the elongation [Leg stiffness=ΔF/Δx]. Whereas joint stiffness may be defined as the change in moment divided by the change in angle [joint stiffness =ΔM/Δθ] [2], or defined as the ratio of maximal joint moment to maximum joint flexion angle [3–5]. Leg stiffness has been reported as the most suitable measure for evaluating the dynamic characteristics of the whole lower limb during walking or running [3,6,7].