What Happens In Muscle Fatigue When You Exercise? Is It Your Muscles That Are Tired or Your Brain?! Chiropractors in Southampton Look at Recent Research.


You are running or cycling and you become tired or fatigued – so much so that you just can’t keep going.

Ever wondered why?

Is it ‘lactic acid’ build up in the muscles? Is it just not enough muscle fibres? What IS happening to your body at that moment?

Recent research in the Journal of Applied Physiology 2015 15 Oct 1;119(7):840-50 by Johnson, Sharpe, Williams and Hannah looked into the potential reasons.

They took eight men and got them to cycle fast  for 4 minutes to the limit of their tolerance. Then they got them to do the same but after they had performed cycling movements with JUST their arms for 8 minute. Then they got them to do so a third time for 8 minutes without the arm cycling first.

They used EMG and electrical stimulation to measure how much fatigue was in the muscles themselves rather than the brain or spinal cord.

Findings showed that in the first group who had only cycled using their legs – after 6 minutes rest their heart rates were still elevated from the exercise and also the levels of lactic acid were elevated. However, their legs felt perfectly fresh and not fatigued.

The interesting finding is that they reached exhaustion point 38 % earlier when they had done the 8 minutes of arm cycling first BUT when EMG tested the leg muscles, there was no explanation for why they had to give up …. they gave up when their legs were still less fatigued that in the leg only test.

In the third test they didn’t reach exhaustion even though when the muscles were tested, their fatigue was the same as in the testing done after they had first cycled with their arms.

So what? SO they concluded, it is NOT fatigue of the MUSCLES that makes you stop when you feel that you just can’t go any further!!

It is the PERCEIVED effort that seem to be the reason they had to give up – they had exercised using their arms  which made them FEEL that they had done a lot of exercise and SHOULD be tired so when they came to cycle with their legs, they felt tired quicker and had to give up quicker – they THOUGHT they were more tired having exercised their arms than if they hadn’t before they cycled.

So is it the SENSATION that you have made a big effort that affects your ability to do exercise, not the reality of the situation – your muscles COULD keep going if your BRAIN thought that they could!

Exercise is thought to be potentially mediated by central fatigue and intolerable levels of sensory perception rather than a critical peripheral fatigue limit.

MIND over MATTER wins again eh???

Food for thought yet again!