The tendons at the back of your knee..

[YODA]

Try the squat thrusts by bringing just one leg up at a time - you'll still get the benefit to your knees & you'll take a whole lot of pressure off your lumbar spine!

 

 

[Joecooke007]

I cannot express my views any better then the ones that have already been expressed. I have also benifited from this knowledge.