There are many different ailments that can limit the range of motion in the knee. But the major one is probably an ACL repair/replacement. It seems intuitive that after a knee surgery the problem would be bending the knee. But in many cases, it's the opposite. The knee will bend but there are no stabilizing muscles to hold the knee steady when it's straight.
Over time, the knee then loses its ability to straighten and is stuck in a semi-bent position. This happens after kneecap arthroscopy, total knee replacements, meniscus repair, and, as mentioned above, the ACL repair or replacement.
Why is knee extension important?
Proper extension of the knee is what allows for stability in all standing activities and daily living. If you're not able to straighten your leg and hold it there, your knees buckle and you'll fall over.
You also need to extend the knee to walk. When you walk, your knee bends, your foot hits the floor and then you straighten your leg to push yourself forward. This propulsion drives your gait. Walking without proper knee extension creates an extremely inefficient gait pattern. This inefficiency leads to instability in the rest of the chain causing ankle and hip instability.
Last but certainly not least, knee extension is important for the balance of rest and use. Getting into proper extension triggers a sequence of muscle activation and deactivation for everyday tasks. Basically, when you mess with one part of the body, the rest will be affected.
So what are you supposed to do about it?
One of my favorite exercises for people who are coming out of ACL repair/replacement is this knee extension mobilization.
Lay your body down on a stable surface: a bed or a table works well. Lay in such a way that your legs are hanging off of the surface from about the knee and below. Use your "good leg" to press your toes into the heel of your "bad leg". Contract the target leg into the top foot and relax. After relaxing, push and stretch down. Watch the video for more information!
Compare your target leg to the other side. Check the springiness and see if it's easier to extend the knee without the assistance of the other leg. That's how you know it's working.
Of course, be sure to check with your physician/surgeon/physical therapist before adding a new exercise to your routine, especially if you're post-op!
Dr. Justin C. Lin