Abstract: An improved muscle strengthening apparatus is disclosed which causes “eccentric contraction” of an antagonist muscle of a limb through electrical stimulation so as to apply so-called “closed-kinetic-chain exercises” for the muscles involved to thereby increase the strength thereof. Flexing or extending motion of a knee joint is sensed by a sensor to indicate contraction of the agonist muscle. Upon contraction of the agonist muscle, electrodes placed over the antagonist muscle are energized to cause the eccentric contraction thereof. The invention provides a portable home gymnasium in that it is relatively small, light-weight, cost effective and easy to transport and also relatively simple in construction.