Locomotor-respiratory coordination dynamics and metabolic cost during walking at different stride frequencies

Abstract

Human locomotion has been modeled as a force-driven harmonic oscillator (FDHO). The minimum forcing function in locomotion has been shown to occur at the resonant frequency of the FDHO and results in the suggestion that oxygen cost may be considered an optimality criterion for locomotion. The purposes of this study were twofold: first, to determine the relationship between stride frequency and shock attenuation, and second, to determine whether shock attenuation may also be considered an optimality criterion. Ten healthy young adult males served as subjects in this study. Each subject's preferred running speed and preferred stride frequency (PSF) were determined. In addition to the PSF, they ran at stride frequencies corresponding to −20%, −10%, +10%, and +20% of the PSF at the preferred running speed. Metabolic data as well as leg and head acceleration data were collected during a steady state run at each of the stride frequency conditions. The metabolic data produced a U-shaped curve hypothesized by the FDHO model. Spectral analysis on the leg and head acceleration data were used to develop transfer functions for each of the stride frequency conditions. Analysis of the transfer function indicated that there was a gain at the low frequencies and an attenuation at the higher frequencies. The transfer function at the higher frequencies indicated that the impact shock signal was attenuated as it passed through the body. However, the transfer functions appeared to vary according to the amount of shock input to the system with the result that the head accelerations remained constant. It would appear that impact (high frequency) shock attenuation increases with stride frequency and thus does not fit the FDHO model as an optimization criterion. At all stride frequencies, regardless of the impact shock, head accelerations were maintained at a constant level.