The passive knee extension exercise increases your ability to fully extend...

or straighten... your leg.

Here's how to do it.

Lie down and raise your leg slightly... by supporting your heel on the edge of a couch, or with a pillow or rolled-up towel.

Completely relax your raised leg and let gravity help straighten your knee as much as possible.

Relax in this position for at least 30 seconds.

Then move your leg to a more comfortable position for about a minute.

Repeat this stretch 2 to 4 times, working up to holding the position for 5 to 6 minutes at a time.

If you still need to work on extending your knee after you've been doing this exercise for a few weeks, try placing a weight just above the knee, to encourage it to straighten a little more.

A sock full of rice can work as a weight.

When you're ready, you can also try a variation of this exercise on your belly.

Lie on your stomach.

Place a rolled-up towel just above your knee.

Hold this position for 30 seconds, and then rest in a comfortable position for a minute.

Then repeat the stretch 2 to 4 times, working up to holding the position for 5 to 6 minutes at a time.

If it's okay for your knee, you may want to use a weight here too...

on your ankle... to encourage the knee to extend a little more.

Some tension may be expected...

but stop doing an exercise if you have increased pain.