Different exercises place different amounts of strain on the repaired ligament. As the tissue repairs itself, a small amount of stress and strain can help the healing process. Too much strain at the beginning of rehab can damage the repair.
Although studies in this area are limited, research has shown that exercise is important to healing. Exercises with your weight on the knee (foot on the ground) are called "weight-bearing." Exercises with the foot off the ground are "nonweight-bearing." Weight-bearing exercises seem to have a better result in the end and are most often recommended.