Forgotten Knee Mechanics

Knee mechanics are more complex than you think. As much as they appear to be a hinge joint that flexes and extends, most people don’t realise that the knee rotates. 

Look at your sore knee and look at your good knee. If you straighten all the way out, can your good knee go further? 

Full extension into a lockout position is a normal and natural movement of the knee and is an energy conservation position for your quadricep muscles. This is called “terminal extension”. For terminal extension to occur the tibia needs to externally rotate in relation to the femur. Rotation is often lost as a result of arthritis, joint stiffness or even more simply a muscle spasm in a small muscle in the posterior knee called popliteus. The role of popliteus is to control the rotation of knee. Popliteus lengthens to allow terminal extension and contracts to unlock a terminal knee allowing the knee to bend. 

Decreased knee rotation can present in a number of ways:
Knee Mechanics
  • Pain in the medial knee joint 
  • Patellofemoral joint pain 
  • Posterior knee pain 
  • Occasional collapsing or buckling knee that is frightening but may not be painful.
  • Loss of knee range in either direction (usually a small change in knee extension and a more noticeable change in flexion, though issues with full knee extension can be more problematic). 

The reality of this issue is sometimes the knee is too far gone. Prolonged poor knee mechanics leads to joint degeneration (hence the large volume of total knee replacements every year). However, if you address it early and prevent excessive joint wearing, this is a very treatable condition. 

A physiotherapist can help by:
  • Releasing the popliteus muscle and help educate you on how to do so safely yourself. A necessary measure as the muscle is in a particularly sensitive area. 
  • Mobilising your knee to encourage synovial fluid production and passive knee rotation in a stiff/arthritic knee
  • Demonstrate active range of motion exercises to encourage better active knee rotation
  • Provide you with resistance exercises to strengthen and learn to better activate popliteus
  • Educate you on how to better identify when you knee is starting to stiffen again and how to use the strategies listed above to manage your knee as this does often recur

Warning: Although a subtle rotation of the knee joint is normal, you should NOT attempt to twist and your knee aggressively or force rotation underload as most knee injuries in sport occur due to a twisting mechanism. Please ask your therapist and try to better understand the role and timing of knee rotation if your think this can help you.  

A terminal knee extension myth

A common misconception is that you should avoid locking your knees. This is good advice that has been misinterpreted. You are allowed to lock out and in fact this is normal. Where locking out your knees should be avoided is during heavy gym exercises such as leg press where the knee is at risk of injury into the hyperextended position (we have all heard the horror stories).