Fig. 1. Spring-mass model with retraction. Swing-leg retraction in running, as indicated by the photographs of Muybridge (1955; reproduced with permission from Dover Publications), is modeled assuming a constant rotational velocity of the leg (retraction speed _R), starting at the apex of the flight phase at retraction angle _R. Depending on the duration of the flight phase, the landing angle of the leg (angle of attack ₀) is a result of the model dynamics and has no predefined constant value in contrast to the previous model of Seyfarth et al. (2002). The axial leg operation during the stance phase is approximated by a linear spring of constant stiffness k_LEG.