Figure 5.

Effects of muscle physiology on activation illustrated by GAS activation and normalized fiber length during the stance phase. Note the nonlinearity of the activation curve generated by the linear force feedback control law (Section 4.1). While the foot is flat on the ground during mid-stance, GAS activation does not significantly change the fiber length, and force feedback leads to an activation build-up. As heel loses contact during late-stance, the fiber rapidly shortens and reduces f_l and f_v (Figure 3). As the generated force decreases, the same force feedback leads to a drop-off in activation.