Adequate hip extension is critical to normal function as well as most athletic activities. Hip extension means the movement of the upper leg relative to the pelvis in a posterior (backwards) direction. Normal hip extension is ~10 degrees from parallel when lying on your stomach.
If you do not have adequate hip extension, try this simple stretch. Lay on your stomach, and put a large towel roll under your knee. Make sure your hips are even; in other words, do not rotate upwards to the side that the towel is on. You should feel a gentle stretch in the front of your hip. Maintain this position for 30-60 seconds.