From a kinetic standpoint, the knee is a hinge joint (just as the elbow is a hinge joint). They both are designed to flex past 90 degrees (knee flexion 0-130 degrees, elbow flexion 0-160 degrees).
If the knee has 130 degrees of flexibility, why, then, are we told to keep it at 90 degrees or less?
Logically, this would mean bicep curls should not go past 120 degrees, or that we should not touch our shoulders.
Let's look at this from an athletic point of view: whenever we jog, run, bike or climb stairs, our knees extend beyond our toes. This is a natural movement for the knee.
This position is not what causes injury. In most cases, what causes injury is a deceleration with a change of direction.
If I stop short with one knee over my toes and attempt to turn, the ACL experiences extra force from the pressure of the leg trying to slow movement. Then the knee attempts to turn as I change direction. If the knee turns either before or after the foot, the ACL can become damaged.
Ideally, the foot, knee and hip should rotate in unison to avoid injury.
In order to do proper lunges with more range of motion and less risk, follow this procedure:
- Take a large step forward with your front leg just slightly bent. (Do not step into a deep lunge position, as this will cause a deceleration. If your balance is off, the resulting change of direction can cause injury.)
- Make sure your balance is secure and your feet are firm on the ground.
- Drop down so your back knee is at 90 degrees. This will put your front knee over the toes.
- Straighten the legs and repeat to the other side.
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