I would do this exercise with a longer hold to ensure that the glutes and hamstrings are doing most of the work and not the low back.
- Lie on back, low calves on swiss ball, arms fairly close to sides.
- Lift up body as if it were a stick.
- Give the buttocks a bit of a prod (keep your elbows on the floor or you will wobble off the ball). Are they satisfying buns of steel?
- If not, feel your low back. It should feel long and the muscles on either side of the spine not too tense.
- So as the spinal muscles lengthen, the tail should tuck under and the bottom become noticeably tighter.
- It feels as if you sink into your backside even though your body remains like a stick.
- Hold for up to 10 secs.
- Do it again.
- Do 6 – 10 times.
- If this is easy, do it with only one leg on the ball. Make sure your hamstings and bottom are doing most of the work and not your back or your arms.
Do enjoy your knees having a good bojning.
This exercise leads onto another which is much harder.
If your bottom remains obstinately slack, I refer you to the section 10 of the page How To Exercise, ‘How to get a tight bottom’.