During postural responses to perturbation, the directions of forces at the ground change at different stance distances in cats (Macpherson and Fung 1994). When the fore- and hindlimbs are closer together, a wider range of ground reaction force directions are used; at longer stance distances, forces are constrained to act along a single diagonal axis. We hypothesize that the set of feasible forces the hindlimb can produce is more elongated in long stance due to changes in biomechanics rather than neural control strategy. To investigate the force producing capacities of the leg we used a 3D musculoskeletal model to compute the feasible force set (FFS) and physiological force set (PFS) of the hindlimb over a range of postures. Based on a model of the hindlimb that includes the hip, knee, ankle, and 13 muscles (Korkmaz et al, 2004 SfN abstract), we generated equations describing the Jacobian transformation from muscle force to endpoint force. We generated a set of joint angle combinations which spans the physi...