Scientists have found an explanation for runners who struggle to increase their pace, cyclists who can’t pedal any faster and swimmers who can’t speed up their strokes. Researchers from the University of Exeter and Kansas State University have discovered the dramatic changes that occur in our muscles when we push ourselves during exercise.

We all 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% of our maximal capacity.

Published in the American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, this research shows why, when we go beyond this level, we have to slow down or stop altogether. This is the first time that scientists have looked at processes taking place inside the muscles when we exceed the critical power.

The study showed that when we exceed our critical power, the normally-stable pH level in our muscles, is quickly pushed to levels typical of exhaustion. Moreover, the level of phosphocreatine in the muscles, a high-energy compound which serves as an energy reserve, is quickly depleted when exercise intensity exceeds the critical power.

Professor Andy Jones of the University of Exeter, lead author on the paper, said: “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.”

The research team used a magnetic resonance scanner to assess changes in metabolites in the leg muscles of six male volunteers who exercised just below and just above the critical power.

The research offers a physical explanation for the experiences of exercisers of all levels of ability. Professor Jones concludes: “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. Exercising above the critical power cannot be sustained for long because it is associated with changes in the muscle which lead to fatigue.”