loobylou,
Damage to muscle, especially a deep injury, can be a significant problem. Often, the muscle will heal with scar tissue, which of course, does not contract and stretch like regular muscle.
Sometimes, with all the bleeding into the muscle after the injury, it can be a set up for the development of heterotopic ossification (HO). Or the growth of bone within the muscle tissues. This can be a real problem. Your surgeons should be on the lookout for HO. Trying to prevent it is better than trying to treat it.
You need to try to keep the muscles stretched out, gently. Stretching will probably be more of a problem than strengthening. It can take quite a while to get over such an injury. Hang in there, good luck.