Why Do Muscles Fatigue?

One piece of advice I give all athletes is not to get carried away at the start of a race. There is a speed limit that you should not cross when running the first couple of kilometres. Cross that barrier by 1 or 2 seconds and it will cost you 30 or 40 seconds at the finish.

In my years at University studying Exercise Physiology I read many text books and articles on what happens when a muscle gets fatigued. Back in the 1970s and 80s there was this idea that as muscles tired, lactic acid was produced and as a result they were forced to swim upstream through a thickening mass of lactate that just got thicker as the muscles got even more tired. Most runners will be familiar with the feeling. At first it is manageable, but the fatigue gradually gets worse to a point where, no matter how hard you try, your muscles just don’t move as fast as you would like.

In the 1990s the theory was still that fatigue occurred in the muscles being used and as the demand became too great, they failed. However, if that was the case, why could athletes speed up at the end of a race? How was that final spurt possible? An exercise scientist called Tim Noakes introduced a new approach to fatigue. In an experiment Noakes attached sensors and had athletes lift weights with their legs until they could lift no longer. He then ran an electrical current through the sensors and noted that the fatigued muscles were still able to contract. The participants legs seemed drained of energy (due to their exertions) but Noakes proved that the muscles still had the ability to give more effort.

Another Professor in Exercise Physiology, Samuele Marcora, found that when perception of effort was high, the likelihood of experiencing fatigue was also high. Athletes who thought they had just run outstandingly could then not repeat the effort. By contrast if they thought it was only an average effort, they could repeat it again and again. Elite cyclists that stopped after complete exhaustion could carry on after a very short rest and exert even more power. Muscle fatigue, then, was not the whole limiting factor.

Both Noakes and Marcora conclude that fatigue occurs in the brain. By shutting down the muscles, the body is protecting itself from harm, injury or failure. Studies since then have concentrated on how to override this urge to slow. Quite often it comes down to purpose and motivation. Why are you in the race and how badly you want to attain that goal? A purpose fosters motivation which in turn allows us to endure a greater effort and as a result a better performance.

Over the years I have witnessed many great athletic performances and training. Some athletes have walked away from sessions and never returned. The training and effort required was too hard. Others have failed in championships but have returned with an even greater resolve. Their purpose and motivation for competing was greater.

While fatigue in any race is inevitable, being able maintain the best possible speed is a major part of being able to run fast times or to win races. But remember you can’t maintain a speed that you haven’t attained! If you are an athlete trying to break 10 minutes for 3000m (80s per lap) then don’t try and run the first lap in 70s. That is the fastest way to fatigue and failure. Your brain will react and slow you down. You need to override this process by starting conservatively and therefore delaying the muscle shut down. Practising at your goal race pace is essential in convincing your brain that it is possible. Even Eliud Kipchoge commented before his sub 2-hour marathon attempt that it was essential that the first few kilometres were run at the correct pace. Trying to run 2 or 3 seconds faster would result in the finish being way slower than wanted.

If the professionals are following this advice then it seems a good bet for the rest of us. Rather than struggle to the finish line aim to finish the final kilometres with a burst.