Adding in the use of a Swiss ball amplifies the balance element of a workout, forcing your body to switch on all your stabilising muscles to maintain the position.

Hamstring curls on Swiss Ball - Women's Health & Fitness

Hamstring curls using Swiss ball

Lying on back, place feet onto ball and toes pulled back towards you. Place hands out to the side, palms facing down.  

Lift hips off floor, activate core and roll the ball slowly in until your knees are above your hips.

Roll back to starting position.

Tip: Keep hips up, position ball under calves – place ball under feet to increase difficulty