Thigh muscle weakness is common with ACL injuries both before and after surgical repair. Studies show that muscle strength does improve with time but it may take up to 18 months before the quadriceps strength approaches normal.
Normal may be defined as at least as strong as the other side (if the other side remains uninjured). Normal also suggests a balance between the quadriceps and hamstrings muscles. The hamstrings are more likely to return to near normal strength than the quadriceps in the first year after injury.
There is evidence to show that patients with ACL deficiency have some minor changes in the way the muscles fire and contract. Motor messages are not as fine-tuned as they once were. It may be necessary to complete a rehab program to retune the muscles.
When and how the muscles contract is a process called motor control. ACL injured knees may need to reprogram the muscle activation strategy to function normally. This rehab step is often missing but very important. Helping the joint regain its ability to handle shear force during knee loading will increase its stability and prevent reinjury. This is true after both ACL injury and after ACL repair.