World-record times in running, cycling and swimming have been decreasing since records have been kept. Is there a natural end to all this record breaking in endurance events? Will endurance athletes cease to get better without the help of scientists or doctors? Where, within the human body, are performance limitations set?
Obviously, one factor that stops endurance athletes from becoming even faster is running out of energy. But how and why do muscles run out of energy? Muscles use an energy source called ATP (adenosine triphosphate) and obtain this energy in a number of ways. It's when the production of energy can't keep up with a muscle's output that fatigue sets in. How energy is produced depends on whether oxygen is available. For short, intense activity lasting between five and 10 seconds, such as sprinting, energy is produced without oxygen. The downfall is that, like a car using petrol and producing exhaust emissions, the muscles also produce a waste product called lactic acid and if this builds up in the muscle, then performance is affected.
Energy production also occurs in the cells and uses mainly fat as a source of fuel. But is it the inability of our bodies to get enough oxygen that actually slows us down, or are our muscles the true limiting factor?
Between the process of breathing in oxygen and the muscles actually using the energy, science has identified four links that can limit us: first, the muscles; second, the lungs; third, the heart and circulatory system; and finally, the cells that produce the energy.
Research has shown muscles aren't to blame. The lungs aren't usually the problem either. Generally, even when lungs are inhaling and exhaling 25 more times during exercise than at rest, they're still only working at two-thirds capacity.
What about the heart? With training, the heart does become stronger and larger, causing the heart to pump more blood with each beat. But if you've reached a level of cardiovascular conditioning that many of the world's top endurance athletes have, your heart's already working close to its limit. So many scientists agree the heart does act as a limiting factor in endurance performance. The final, possible, limiting factor is the muscle cells themselves. Researchers in Switzerland have discovered that cells can only consume and produce energy as fast as their peak metabolic rate. Thus there could also be a limiting factor to endurance performance.
