Motor units are recruited in a task-dependent fashion during locomotion.

Muscle fibres have a range of contractile properties from fast to slow. Traditional understanding of muscle fibre recruitment suggests that the slower fibres within a mixed muscle are used for all contractions including those at rapid speeds. However, mechanical arguments predict that some locomotor tasks are best performed by solely the faster fibres. Motor recruitment patterns can be indicated by the spectral properties of the myoelectric signals. High- and low-frequency myoelectric spectra that have similar spectral power indicate the activity of faster and slower motor units, respectively. In this study, the myoelectric signals in humans were measured from nine muscles of the leg during walking and running at 1.5, 3 and 4.5 m s(-1). The myoelectric spectra for 20 points in each stride were calculated using wavelet techniques, and the spectral properties quantified using principal component analysis. Bursts of muscle activity were characterized by hysteresis in the myoelectric frequencies, with different frequencies occurring at different times, indicating time-varying shifts in the motor recruitment patterns. This hysteresis occurred at all locomotor speeds tested. It is likely that the different types of motor unit are recruited in a task-dependent fashion during locomotion.