Optimization problem parameter bounds.

The maximum isometric force (Fmax) values for each muscle were constrained to fall in a fairly wide window surrounding the scaled value from SIMM. The scaling factor for lslack and lopt was chosen to ensure that each muscle fascicle length stayed within physical operating ranges. Here the parameter w governs the width of the active force-length relation of the muscle; its value varies by muscle and all values are given in S2 Text. The bounds for Ksh and λref were taken from [16]. The bounds for the spring constant of the hip flexor ligament was chosen so that the ligament could provide anywhere from none to all of the required hip flexion moment near toe off. The engagement angle was chosen so that the tendon could engage with the hip no more than 10° flexed, as its physiological role is to prevent overextension.