Figure 9.

Simulation of the changes in muscle recruitment for different off-diagonal components of the stiffness matrix (for main study: negative–x force direction task). Muscle force as a function of K_xy. The elements K_xx and K_yy are fixed at 330 Nm⁻¹ and 460 Nm⁻¹ respectively. In order to increase the magnitude of K_xy, most muscles decrease their activity, resulting in lower global cost (Fig. 8). However, the mono-articular muscles around the shoulder (dashed lines) increase their activity, resulting in local fatigue and putting a limit on the negativity of K_xy.