I am coding to calculate the wrench (forces and torques in 6DOF) at each joint using a link model during a sit-to-stand task. The model consists of rigid segments. For each segment, the equation of equilibrium is solved. Starting the hands and head, the wrench is calculated for the wrists and neck, then elbows, shoulders and lumbosacral joint. The next segment to solve the equilibrium equation for is the pelvis, which is connected to the thighs through the hip joints. The problem here is that unlike the previous segments, here we have more unknowns than equations (redundancy). In other words, the sum of the weights and inertial forces as propagated from the upper segments are being shared by the two hip joints in some way that I don't know. I don't think it would be acceptable to simply assume that the subjects rely equally on both legs. Do we have any criteria to solve this? Is there any variable here to be optimized for? Thank you!