Sit on the gym ball, keep your knees bent at 90 degrees.

Keeping your arms at shoulder level. Place them straight out in front of you. This will help keep your balance.

Straighten one leg out, whilst keeping the other leg bent and foot on the floor.

Bring the straight leg back to the starting position in a slow and controlled manner.

Repeat on opposite side.

Repeat as prescribed.

Will help:

Posture

Back pain

Knee rehab