Fig. 2

Frequency signals of ground reaction force (full line) and gastrocnemius acceleration (dotted line) during running. The blue area is equivalent to the natural muscle frequencies (Wakeling et al. [19]), which coincide with impact frequencies during different running velocities (from Trama et al. [26] and Boyer et al. [20]). Shifts in natural frequency are not sufficiently important to be outside the GRF frequency content