Lie on the floor with the knees bent at 90 degrees and the forefoot (toes) off the floor. Press the hips up into the air until they align with the knees and shoulders in a straight line. Next, begin to march the legs by reaching one knee up into the sky at a time. Make sure that the hips remain level with the floor. Do not allow one hip to drop towards the floor while marching; this is a sign of instability in the hip socket. Also, keep a neutral spine and do not arch in the lower back.