In studies on skeletal muscle, an in vitro force measurement has been widely used to evaluate its function. However, it is recently suggested that in some cases, the results obtained by such measurement do not necessarily reflect the force in vivo, because the measurement has some disadvantages. For example, the muscles are contracted under different conditions from in vivo and there is no blood flow. To resolve this issue, we have developed an experimental system, in which muscles are contracted in vivo and the organelle function is subsequently estimated by an in vitro force measurement using a mechanically skinned fiber technique. This experimental system makes it possible to examine not only the muscle force in vivo but also the mechanisms of changes in the force at organelle levels. In this review, we depict the advantages and disadvantages of the in vitro and in vivo measurements of force and then discuss the effectiveness of our experimental system.