Abstract: An apparatus for enhancing muscle strength is designed based on a curve of relation between joint angle and producible muscle strength, and includes a resistance-varying device that provides progressive resistances to give a trainee's body area being trained the most suitable load when the joint angle at the trained area changes. The resistance-varying device is embodied through serially connected four-bar linkages, two-bar linkages, or tension cords pivotally connected to one lateral side of weights of the apparatus with pins. Strength exerted by the trainee at the trained area is transmitted via a steel cord to pull a top weight upward and sequentially stretch the linkages or tension cords open, so that subsequent weights are pulled upward one by one until the last linkage or tension cord is fully stretched. A method using the apparatus to enhance muscle strength meets the human engineering and avoids unwanted injury of trained muscles.