The purpose of this study was to reveal the minimum required muscle force for a sit-to-stand task. Combining experimental procedures and computational processing, movements of various sit-to-stand patterns were obtained. Muscle forces and activations during a movement were calculated with an inverse dynamics method and a static numerical optimization method. The required muscle force for each movement was calculated with peak muscle activation, muscle physiological cross sectional area and specific tension. The robustness of the results was quantitatively evaluated with sensitivity analyses. From the results, a distinct threshold was found for the total required muscle force of the hip and knee extensors. Specifically, two findings were revealed: (1) the total force of hip and knee extensors is appropriate as the index of minimum required muscle force for a sit-to-stand task and (2) the minimum required total force is within the range of 35.3–49.2N/kg. A muscle is not mechanically independent from other muscles, since each muscle has some synergetic or antagonistic muscles. This means that the mechanical threshold of one muscle varies with the force exertion abilities of other muscles and cannot be evaluated independently. At the same time, some kinds of mechanical threshold necessarily exist in the sit-to-stand task, since a muscle force is an only force to drive the body and people cannot stand up from a chair without muscles. These indicate that the existence of the distinct threshold in the result of the total required muscle force is reasonable.