Who, what & why?
Everyone reading this will be familiar with the sensation of gradually increasing effort to maintain the same pace whilst running or power output whilst cycling or rowing. But what exactly contributes to this sensation? And what can we do (safely) to reduce or delay it?
Two of the real experts in this field recently wrote a review paper on what we currently understand about exercise induced fatigue, primarily from a brain neurochemistry point of view.
What did they do?
They reviewed key scientific papers from the past 30 years to see how our understanding of this complex topic has evolved. It concluded with what we know now, and what more needs to be studied.
What did they find?
Before the late 1980s, the primary focus of fatigue research was the muscles themselves – which is a logical starting point since that is where the pain is felt! Muscle fatigue is certainly related to the availability of fuel, and removal of waste products, but in the late 1980s, it was discovered that the nerve inputs to the muscles was also being reduced, and emphasis shifted to the nervous system and central fatigue.
The initial focus was on serotonin – the same chemical that affects mood, and which is partly blocked by anti-depressants. An increase in serotonin produced negatively affects mood and drive – resulting in fatigue.
Ten years later, in the late 1990s, the fatigue delaying effects of both dopamine and adrenaline were added to this theory. This model has been the one most used to explore ways to reduce and delay fatigue using supplements.
How can we reduce the perception of fatigue?
Although we usually think of carbohydrates as pure muscle fuel, they also have an effect on the brain. Carbs decrease the amount of tryptophan – the precursor to serotonin, and thus also delaying fatigue. It’s the carbohydrates that supply a lot of the fatigue reduction properties of energy drinks. The carbs don’t even have to be swallowed to work – a mouth rinse is sufficient as there are carbohydrate receptors in the mouth that have been shown to activate areas of the brain involved in fatigue perception. This is why you may see many runners and footballers swilling and spitting out energy drinks. They are getting the benefit, and holding off fatigue without having it swill around in their stomach.
What about supplements?
A lot of the supplement findings were based on animal studies but have not been shown to work in the same way, or as effectively in humans. Some supplements have worked in humans, but only in conditions of high environmental temperatures (>30 C). This would suggest that humans have a safety switch (did someone say ‘Central Governor’?) that can be overridden in certain circumstances and that is not present in many animals.
And mental fatigue?
The review concludes by looking briefly at mental fatigue. Some interesting studies have shown that fatigue during exercise builds more rapidly following demanding mental work, and that caffeine can reduce this effect, but through mechanisms that are not yet clearly understood.
What does all this mean?
Fatigue during exercise results from a complex interaction between sensations from the working muscles and the central nervous system. The key neurotransmitters appear to be serotonin, which increases fatigue, and dopamine and adrenaline which delay and reduce fatigue.
For those wanting to reduce their own fatigue during exercise
Avoiding mental stress before your event is key. For example ensure travel and accommodation plans are well laid out to avoid stress when you arrive. Stick to a tried and trusted routine, including nutrition. Caffeine will always play an important role in fatigue perception. Again you need to know what dosage works best for you as an individual. Finally, carbohydrate ingestion and rinsing during exercise have the best proven effect on fatigue and are superior to taking expensive supplements.
By Simon Wegerif