With two different postures, USP (Upright Standing Posture) and BTP (Bent/Turned Trunk Posture) maximal force, maximal endurance time, and the necessary recovery period were ascertained using four subjects. The theoretical endurance time (TmaxRo) was calculated and compared with the empirically measured endurance time (Tmaxemp). The slight difference between empirical and calculated data for USP results from neglecting tissue forces and using idealized biomechanical models during the calculation of muscle forces. The longer empirical endurance time for BTP compared with the theoretical value probably results from the use of a number of muscles, in the back, especially those working in parallel. The models used do not correspond totally with the actual conditions, but are simplified. It is necessary to use a correction factor for calculating endurance time and recovery period for BTP.