An apparatus for the development of body parts and muscles effecting the movement of a user's body parts. The apparatus includes a frame on which is pivotally mounted a force applying member against which a body part may be placed and urged for purposes of developing said body part and muscles. The user is positioned on the front side of the frame during an exercise program. The apparatus also includes a weight pivotally mounted on the rear side of the frame and a force resolving spiral means rigidly connected to the force applying member for rotation therewith whereby the pull on the weight mass is continuously varied over the full range of rotation of said force applying member to produce a varying resistance force in direct opposition to the force applied to the force applying member by a body part and thereby to stress and develop said body part and muscles. The resolving spiral is in the form of one or more pulleys which continuously resolves the force of the weight mass to thereby provide optimum stress for developing the muscles. The effort of continuously exposing the body part undergoing an exercise to a varying force over the full range of movement will cause the muscle effecting movement of said body part to be subjected to optimum development conditions.