Bridge with alternating leg extension ("Bridge" Core/hip stabilization, straightening leg (alternate); 01)

Lie on your back with your knees bent and your feet flat on the floor.
Tighten your buttock muscles and lift your hips up into the bridge position.
Lift one foot off the floor then straighten your knee until it is aligned with your other thigh.
Hold this position.
Lower the leg back on the floor and repeat with the other leg.
Make sure you keep your hips up and level throughout the movement.