Recreational bicycle riders probably should not try to tuck their heads down in the form of racers. A study from Trinity College in Dublin, Ireland shows that "aerodynamic position" (with the head down near the handlebars) causes bicycle riders to tire earlier when they ride fast (European Journal of Applied Physiology, March 2010 and January 2006). Researchers compared how quickly cyclists tired during high-intensity cycling at constant speed in upright and supine postures. During the fatigue tests, riders performed a 10-second all-out effort followed by riding at a fast speed for 50 seconds. They repeated the all-out, 10-second bursts every minute until they couldn't go fast any more. Riding supine caused a drop in power and fatigue earlier than riding upright.
Riding bent over can reduce lung capacity in cyclists who have not trained in an aero position and adapted to it. The limiting factor in how fast a person can ride is the time it takes to move oxygen from the air you breathe into your muscles. If your lung volume is diminished, you take in less oxygen and tire earlier.
Then why do virtually all bicycle racers try to ride lower and lower? Because air resistance slows you down and the lower and narrower you ride, the less air pushes against your body. When you pedal on level ground with no wind blowing, 60 percent of your energy is directed to overcome air resistance against your body. Ed Pavelka, a world-class endurance bicycle racer, says: "The fastest speeds in cycling are obtained on aero bikes with the handlebar well below the height of the saddle. Fatigue is caused by the duration and intensity of effort, and reducing the work you have to do against air resistance is more important than anything else."
If you are not a bicycle racer, you will probably be more comfortable and ride longer if you don't try to get as low as possible. To receive Ed Pavelka's free weekly newsletter, with great information for racers and recreational riders, go to http://www.roadbikerider.com/newsletter.htm