Scientists have shed light on why exertion leads to exhaustion, by discovering the remarkable changes that occur in our muscles when we push ourselves during exercise.
All of us have a sustainable level of exercise intensity, known as the 'critical power'. This level can increase, as we get fitter, but will always involve us working at around 75-80 percent of our maximal capacity.
Now, researchers from the University of Exeter and Kansas State University have explained why we have to decelerate or stop altogether when we go beyond this level.
According to the researchers, this is the first study that has looked at processes taking place inside the muscles when we exceed the critical power.
The team demonstrated that when we surpass our critical power, the normally-stable pH level in our muscles, is quickly pushed to levels typical of exhaustion.
In addition, the level of phosphocreatine in the muscles, a high-energy compound that serves as an energy reserve, is quickly depleted when exercise intensity exceeds the critical power, they found.
"The concept of 'critical power' is well known by sportspeople, but until now we have not known why our bodies react so dramatically when we exceed it. We were astonished by the speed and scale of change in the muscles," Professor Andy Jones of the University of Exeter, and lead author on the paper, said.
For the study, the research team used a magnetic resonance scanner to calculate changes in metabolites in the leg muscles of six male volunteers who exercised just below and just above the critical power.
The authors say that their research offers a physical explanation for the experiences of exercisers of all levels of ability.
"The results indicate that the critical power represents the highest exercise intensity that is sustainable aerobically. This means that it is likely to be an important intensity for maximising training gains," Prof Jones said.
"Exercising above the critical power cannot be sustained for long because it is associated with changes in the muscle which lead to fatigue," he added.
The study is published in the American Journal of Physiology: Regulatory, Integrative and Comparative Physiology.