The invention concerns a device in particular for use in the field of sport practice and body-building, for a person's muscular exercise. Said device enables the development of quadriceps and of the gluteal muscles and comprises a support surface for the feet and means for applying a force of resistance to the movements of the person's legs to shift from a bent position to an extending position by unfolding the knees. The invention is characterised in that the force applying means comprise a harness pressing on the upper side of the thighs. The invention is applicable to the manufacture of body-building machine.