Methodological approach. A) Musculoskeletal model was developed in SIMM and implemented in OpenSim (see Supplementary Material). B) Since several muscles cross multiple joints, we varied starting MTU length by changing the posture at the hip (h), knee (k), ankle (a), and the tarso metaphalangeal (t) joints and then swept the hip and ankle through a series of additional joint deviations. At each joint configuration, we simulated fixed-end muscle contraction at five levels of activation. At each level of activation, we implemented three different muscle models 1) muscles with a non-compliant tendon, 2) muscles with a compliant tendons, and 3) muscles with compliant tendons and a 15% activation dependent shift of the optimal fiber length. For each joint configuration and muscle model, we extracted the equilibrium muscle length and force capacity of each muscle during the fixed end contraction. With this data we evaluated how muscle length and force capacity changed with muscle activation level, passive muscle length, muscle–tendon unit compliance, and activation-dependent shifts of the F–L curve.