This is one of my favourite pieces of equipment in the gym, if you have not seen one before it is called the bosu ball.
Balance yourself with your bum slightly of centre of the bosu ball, focus on having a locked core.
Alternate between bringing your opposite knee towards your chest.
Aim to complete:
3 x 10 each reps
This exercise is not for everyone, if you have lower back problems please stop, of contact me for other options!