This study is intended to evaluate the effects of rehabilitation (exercise) and electrical stimulation on ligament strength and tightness after injury. Ligament injuries are extremely common in the practice of sports medicine and their treatment is to a large degree, empirical. Documentation of the effectiveness of treatment regimens at increasing ligament strength and tightness after injury is very desirable. The literature documents that immobilization has an adverse effect and mobilization a beneficial effect on uninjured ligaments and on the healing of injured ligaments. There is little data on the effectiveness of exercise, over and above mobilization ad libitum, in the healing of ligament injuries. In addition, it is felt that mobilization may have its beneficial effect through stimulation of the cellular elements by the production of stress generated electrical potentials (streaming potentials), and it may be possible to simulate such potentials artificially. The investigators propose to evaluate the effectiveness of exercise on a well established rat medial collateral ligament injury model. Rats with MCL injury will be exercised by being forced to swim on a daily basis. The degree of exercise which produces the most beneficial effects on tensile strength and MCL laxity will be documented. A similar evaluation of a model with an unstable knee (not a simple MCL injury) will be evaluated to see whether exercise is also beneficial in this more severe injury. The unstable knee model will also be treated with immobilization and electrical stimulation in order to determine whether this combination can produce ligament strength without the increased laxity seen with mobilization. The same rat model will be used to evaluate electrical stimulation and exercise to determine whether they produce increased collagen synthesis and collagen or affect collagen alignment at the injury site. This is in order to determine the mechanism by