Fig. 1.

Illustration of the early phase of the torque- and EMG-time curves of the knee extensor muscles in two subjects during an explosive voluntary isometric contraction (continuous line) and in response to electrical stimulation (8 pulses at 300 Hz; discontinuous line). The rate of torque development during the two types of contraction is similar for subject 2 (b), whereas it is substantially smaller during voluntary activation for subject 1 (a). Interestingly, EMG activity (expressed as % of EMG during MVC) of the vastus lateralis is much greater at the onset of muscle activation in subject 2 (d) compared to subject 1 (c). These data indicate that the rate of torque development is mainly limited by muscle activation (neural factors) in subject 1 whereas it is more likely constrained by muscular factors in subject 2. Arrows and vertical lines indicate, respectively, the onset of torque development and the force attained 40 ms after the onset of the contraction. Figure reproduced with permission from de Ruiter et al. (2004)