Your legs gradually awaken to the movement of another ride. The early moments of this 25-miler are difficult, but after only a couple of minutes, your breathing increases in depth and you begin feeling fast and efficient, like a well-tuned cycling machine. Your body is extracting energy from the most abundant of sources - the air itself. You feel like you could ride this fast forever.
The process of using energy begins with adenosine triphosphate (ATP), a compound that scientists refer to as the currency for energy exchange in the body. Muscles cannot contract without it. In fact, most life-sustaining functions depend on it.
ATP consists of an adenosine molecule linked to three phosphate molecules by high-energy chemical bonds. To liberate energy for muscular contraction, one of the phosphate molecules is released. You may not realize it as you ride, but this process is crucial. Your body's ability to satiate your muscles' need for ATP determines your performance.
Working muscles can get ATP from any of three sources, depending on the situation. When you attack a hill, for instance, you need energy released quickly. However, a long flat stretch calls for steady production of ATP lasting many minutes or hours. Your body senses the difference and taps the appropriate source.
SOURCE 1: ATP-PC
A limited amount of ATP exists in the muscle cell. It's found close to a moderately high-energy compound called phosphocreatine (PC). This naturally occurring and immediately available ATP source is known as the ATP-PC (adenosine triphosphate-aphosphocreatine) system. It's anaerobic, which means it doesn't need oxygen to function, and though it's the most powerful energy supply in the body, it can be exhausted by 5-7 seconds of intense cycling.
When this happens, the phosphocreatine splits into phosphate and creatine, creating fragments that can reform into ATP. This extends the system's usefulness, but it still can't provide energy for more than 30 seconds. When you need a short burst of speed, this is the energy you tap. After it's depleted, it's eventually replenished by ATP from the other energy sources.
SOURCE 2: GLYCOLYSIS
The other type of anaerobic system is glycolysis, which is also known as the lactic acid system. While not as powerful as its predecessor, it lasts slightly longer. ON a climb or extended sprint, this is the energy source your body draws from. Glycolysis uses muscle glycogen (carbohydrate), then blood glucose, then finally liver glycogen as fuel to produce ATP.
When carbohydrate is broken down, energy is released, creating ATP, with lactic acid left as residue. This happens within the cell, but not directly at the muscle contraction site.
Since this process requires more than 10 chemical reactions, its rate of ATP production is slower than the ATP-PC system. For up to 5 mnutes, a great number of ATP molecules can be formed. But ultimately, excessive accumulation of lactic acids undermines this sytem by inhibiting muscular contraction. This burning sensation in your quadriceps as you attack a steep incline is an excellent example of the accumulation of lactic acid. Fortunately, you can train your body to delay the negative effects of lactic acid buildup through interval training.
SOURCE 3: THE AEROBIC SYSTEM
Of the three energy sources, only the aerobic system can provide energy almost indefinately. Like the lactic acid system, it uses muscle glycogen, blood glucose, then liver glycogen as fuel. It also taps slow-processing fat as an energy source. But the main difference is that this system requires oxygen to produce ATP molecules. It also relies on more than 20 chemical reactions and a shuttle to move ATP from specialized production sites (called mitochondria) within the muscle cell to the contraction area. For these reasons, the aerobic system is a slower producer of ATP.
But it can produce the greatest amount. Intimately linked to the cardiorespiratory system, it depends on an ongoing supply of oxygen molecultes. They're inhaled into the lungs for transfer to the blood and consequently delivered to the muscles of the heart. Vigorous training can improve delivery by enhancing the amount of blood expelled with each heartbeat, as well as the ability of muscles to extract the oxygen upon arrival.
Generally, any effort in excess of 5 minutes derives the majority of its energy from the aerobic system. A prolonged ride puts the cyclist in a so-called "steady state," which means the oxygen consumed for ATP production is equal to the ATP necessary for muscular contractions. Your ability to maintain a steady state depends on a combination of your genetic potential and your conditioning.
Transition between anaerobic and steady-state cycling commonly occurs early in a ride. Until steady state is achieved, the two anaerobic systems supply most of the ATP. By starting slowly, you can avoid unnecessary reducing ATP-PC or accumulating too much lactic acid. The more you train, the faster you can achieve steady state. In just 2 minutes of riding, your oxygen system can reach its goal of being the predominant supplier of ATP needs.
Robert M. Ott, Ph.D.