A muscle stretching device essentially comprises a base and an outer cover pivoted to each other. In the base are provided a motion transmission device, a lifting mechanism, an inner cover. The lifting mechanism comprises gear shafts, pushing members disposed in the gears shafts, and screws screwed in the threaded holes of the pushing members. By such arrangements, the motion transmission device drives the gear shafts and the screws of the lifting mechanism to rotate synchronously, and the pushing members are restricted in the restricting holes of the inner cover and therefore move along the screws to control rise and fall of the outer cover.
