What is post-exercise fatigue?

Edit: For all those of you who got this by email, WordPress just completely dropped all the formatting, for no reason I can understand, (but it happens occasionally), and you got a giant wall of text. Sorry!

It’s with trepidation I approach this subject. I don’t have the medical background that seems essential in trying to understand all of it so bear with me and any potential mistakes I’ve made.

Years ago I discovered the best questions were the dumbest questions, the ones where you are almost embarrassed to ask, but when you do, you discover more than you hoped to find.

After the two recent posts on the value of long swims and the post swim fatigue caused, I asked myself just what was the fatigue we all experience for a week or longer after long training swims (six hours and greater). It was such an obvious question I felt stupid by framing it to myself. What I found, in as far as I can tell, is that this is an area that is still very much being researched and not all the factors are known. Quoting this abstract on physical fatigue, “physical exercise affects the biochemical equilibrium within the exercising muscle cells. Among others, inorganic phosphate, protons, lactate and free Mg2+ [magnesium] accumulate within these cells. They directly affect the mechanical machinery of the muscle cell”.

As you will see, we could consider this one side of fatigue, that of muscles and the causes of muscle fatigue.

We know that endurance exercise requires energy and for distance swimmers this means first using the glycogen stored in muscles, blood and liver, and after that’s consumed, later switching to ketosis and starting to use fat stores. So there is an initial fatigue or tiredness caused partly by energy depletion. But 24 hours later, the body’s glycogen stores are pretty much replenished (but not entirely, depending on food type High Glycemic Index food replenished stores faster, type of sugar has an effect also, maybe even that the Golden Window oft referred to, isn’t relevant, and various other factors).

We also know, I think, that carb-loading works, and various strategies for carb-loading are better than others. On long swims, depending on effort, type of sessions and previous training, we may experience muscle soreness. Generally, if you are trained enough, this isn’t too common a problem and muscle soreness is a sure obvious sign of over-work. Part of the fatigue and recovery process is for muscles which have been worked to the point of breakdown to recover and the micro lesions get repaired. This is how muscles get bigger and/or stronger. When the exertion is enough, this may result in DOMS, delayed onset muscle soreness, that can last for a few days. DOMS is a whole subject onto itself, and it’s not what we’re concerned about here, but similar long-lasting effects without the soreness.

Muscle work is done by a process called the Excitation–contraction coupling mechanism, whereby an electrical discharge at the muscle initiates chemical events at the cell surface, releasing intracellular calcium, which causes calcium sensitive proteins to contract using ATP (Adenosine Tri-Phosphate, produced from glycogen or fat) ultimately causing muscle action. Lower ATP is part of the post-swim energy depletion mentioned above. However for long-term fatigue, the problem is not a lack of phosphate, but an impairment of the excitation–contraction mechanism, and possible other causes. This article, which is based on some actual studies such as this and this, says that part of tiredness, the inability of the person to make the muscles work to what they had previously, is actually also related to changes in the brain and communication between the muscle itself and the intra-cortical area of the brain. It seems like, (if I am reading it all correctly), there is a negative feedback loop operating between the two, with responses from the muscles during a tiring activity signalling the cortex to reduce the force (contraction) that can be applied. That mean it’s not just the muscle’s inability to function but that there is a central nervous system (CNS) fatigue also (whereas the muscular aspect is metabolic fatigue) and it seems that the CNS fatigue is the one that takes longer to recover from, that makes us feel low after long swims. On one study I read, (I seem to have lost the link for that one), it was found that immediately after stopping due to perceived exhaustion (on a cycling stress test), the muscles were still capable of exerting three times the work necessary for the test.

As this study says, “Fatigue from SDE [Short Duration Exercise] may arise primarily from metabolic mechanisms, whereas fatigue from LDE [Long Duration Exercise] involves an additional slowly recovering nonmetabolic mechanism that may arise from impaired activation, beyond the cell membrane, at the level of excitation contraction coupling”. Symptoms of CNS fatigue include lack of motivation, poor mood, impaired cognitive ability and incorrect perceptions of exertion levels –  where we think we’re exercising/swimming harder than we actually are. Sound familiar? The body needs rest and we need to avoid injuring ourselves. Fatigue cold (and has been) even described as a brain-derived emotion that regulates the exercise behavior to ensure the protection of whole body homeostasis. If we didn’t have fatigue feedback, we’d overuse muscles and probably injure ourselves but at the same time, endurance performance itself is limited by perception of effort as the primary reason for stopping. (More to come on this in another post,as so often happens when I start one of these science-based posts). Possible causes of fatigue, long-term and short-term:

  • DOMS
  • CNS fatigue (neurotransmission problems)
  • Insufficient hydration
  • Low insulin
  • Increased ammonia in blood
  • Disturbed hormone and electrolyte levels
  • Other nutritional (vitamin or trace element) deficiency
  • Low glycogen
  • Tryptophan depletion

This isn’t a comprehensive list, just what I’ve come across. I had to stop at some point. 🙂 I’ve found some impossible to understand (for me) speculation about potential mitochondria damage, and I’m sure there are other possibilities that are completely mainstream. This is all very well and interesting, you probably won’tsay, but what does it mean in terms of recovery? How can we shorten recovery or do it better or differently. Is there anything that helps? I think we’ll stop here, more study is called for, maybe we’ll return to this at some point. 🙂 There are no smilies in scientific papers.


5 thoughts on “What is post-exercise fatigue?

  1. Pingback: Post Exercise Muscle Fatigue | Get in Shape with Diet and Exercise

  2. Pingback: Top 10 Great Tips to Overcome Muscle Fatigue

  3. Donal, the CNS element of fatigue has as its chief proponent Dr. Tim Noakes of South Africa. Google “Central Governor Theory” for a full rundown. It’s been a long time since coaches who pay close attention to sports science have focused solely on metabolic pathways as the limiters of performance.


    • Hey Mark, in fact I did read up on it (briefly) while trying to understand fatigue. My use of CNS as term, probably incorrectly, didn’t come from that but mainly from the research papers indicating a link between intracortical activity and inability to apply force. I found it really hard to find good reading on the subject so the post kind of peter’s out into the list of various factors I could find. Where have I gone wrong?


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