Why Exhaustion Follows Exertion

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Why Exhaustion Follows Exertion

http://www.medicalnewstoday.com/articles/92559.php

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 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 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. "