When training with a fitball make sure it is appropriate to your height. When you when sit on the ball, your knees and hips should be at 90 degrees or slightly higher.
Fitballl Size
Over 180cm (Over 6ft) 75cm
170cm – 180cm (5ft.6 – 6ft) 65cm
160cm – 170cm (5ft.2 – 5ft.6) 60cm
Under 160cm (under 5ft.2) 53-55cm
Set up
Lie on a your fitball with the back slightly extended, the shoulders higher than the hips and lower back pressed firmly against the ball. Bend knees to form 90 degree angle between upper and lower legs. Position the legs so knees are placed just wider than shoulders. Cross arms over chest or over your head.
Technique
Contract abdominals and pull rib cage toward the pelvis until the abdominals are fully contracted. Keep lower back pressed against the ball. Hold, continue to contract the abdominals and breathe out remaining air. Slowly lower trunk to original positions, maintaining abdominal tension. Repeat.