The floor bridge is an excellent exercise to strengthen the muscles of the backside. By locking the leg into the body it prevents the lower back from arching, which is a common fault when performing this exercise.
Lie on your back on the floor.
Bring one leg into your chest and hold it there.
Keep the other leg bent with the foot flat on the floor.
Activating the abdominals, squeeze your bum and lift your backside off the floor between 3in and 6in.
Hold at the top of the movement and control the return back to the floor.
If you find this exercise easy, it sounds like your hip muscles are working well. Increase the challenge by holding at the top of the movement for five seconds on each repetition. Alternatively, try the exercise with the feet on a stability ball.
If this is proving too difficult, reduce the intensity by placing both your feet on the floor and doing a bridge from that position. Take care to keep your belly button drawn in and not to let your lower back arch.