The crunch strengthens the trunk flexor muscles. Strengthening the trunk and hip muscles helps to improve gait, which in turn improves the muscle balance around the knee.
In case you cannot see the video, here is a still sequence -
|Lie on your back with your knees bent to support you. Hold a ball between your hands high above your head with the arms straight.|
|Keeping the ball as high as you can, bend the trunk forwards, lifting the head off the pillow.|
|Hold the position for a few seconds.|
|Slowly relax. Repeat 5 times.|