Rhythmic performance during a whole body movement: dynamic analysis of force-time curves.

The purpose of this study was to investigate rhythmic performance during two-legged hopping in place. In particular, it was tested whether (a) timing control is independent of force control, (b) a dynamic timer model explains rhythmic performance, and (c) it is a force related parameter that carries the timing information. Eleven participants performed two-legged hopping at their preferred hopping frequency (PHF) and at two hopping frequencies set by an external rhythmic stimulus as lower (LHF) and higher (HHF) than their PHF, respectively. A force plate was used to record the ground reaction force (GRF) time curves during two-legged hopping. The primary temporal and force related parameters determined from the GRF-time curves were the durations of the cycle of movement (t(cycle)), of the contact phase (t(contact)), of the flight phase (t(flight)), the magnitude of peak force (Fz(peak)) and the rate of peak force development (RFD). Control of t(cycle) was independent of force control as shown by the non-significant correlations between t(cycle) and the force parameters of the GRF-time curve. Lag 1 autocorrelations of t(cycle) were not significant in any of the HF, thereby a dynamic timer model is considered to explain the timing of t(cycle) during two-legged hopping. RFD varied more than any other GRF-time curve parameter, exhibited consistent significant strong correlations with the GRF-time curve parameters and significant negative lag 1 autocorrelations in PHF, thus, it was highlighted as the potent timing control parameter. Finally, we provide a practical application for the optimization of rhythmic performance.