Hamstring speed kicks in prone

Lie on your front on a mat.
Have a partner hold a large stability ball on the back of your thighs.
Tense your core and buttock muscles and keep your body still.
Flex your feet, pulling your toes up towards your shins.
Alternately kick your heels into the stability ball at a rapid tempo.
As you straighten one leg, kick the other into the stability ball.
Continue at a high pace