Bridge on football with knee extension ("Bridge" Core/hip stabilization, straightening leg (alternate), feet on ball)

Lie on your back with your knees bent and a football under your feet.
Tuck in your navel and your pelvic floor and draw your pelvis inwards and up.
Hold this position and try to extend one knee at a time.
Make sure you keep your pelvis level.
Hold this position for the required time and then return to the start position in a slow and controlled movement.