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.