Fig. 1. Schematic illustrating how residual force enhancement (rFE) could be generated by a parallel elastic component (PEC) that increases in stiffness when the muscle is activated. The solid line is the force–length relationship of the PEC in a relaxed muscle. The broken line is the force-length relationship of the PEC when the muscle is activated with the PEC at the length L_A. The stiffness of the PEC, but not the force, increases upon activation. If an isometric contraction is performed when the PEC is at L_B, it exerts a force F₁. However, if the muscle is activated when the PEC is at L_A and is then stretched until the PEC reaches L_B, the PEC force will be F₂. The rFE is F₂–F₁.