Lie face down on an exericse mat with your legs extended behind you and a dumbbell placed vertically by your feet. Grip the centre of the dumbbell firmly with both feet. This is your start position. Flex your feet towards your shin and then proceed to lift the dumbbell towards you by flexing at the knee and exhaling as you do so. Pause at the top, squeezing your hamstrings. With control, lower your legs back to the starting position, inhaling as you do so. Perform all reps and then repeat on the other leg.