By Coach Scott Johnston
Fatigue in one form or another is a nearly constant companion of any endurance athlete. Learning how to regulate, control and absorb fatigue is the secret to successful training. Most people see fatigue in a blanket sort of way…all fatigue is the same…you feel tired. What is the big deal? But the successful endurance athlete becomes a connoisseur of fatigue. Learning to differentiate the subtle nuances of fatigue, getting on a first name basis with the all the members of the fatigue family, will allow you to know when to embrace it and when to step back before it embraces you.
Fatigue began to get serious study during WW2 as scientists strove to understand the stresses that combat soldiers were dealing with and how that stress affected them. The seminal studies were done at Harvard. Before that Dr. Hans Selye was a pioneer in developing our understanding of how organisms adapt to stress. It is adaptation to stress that underlies how the training effect leads to improved athletic performance.
Physical exertions lead to immediate, short term, medium term and sometimes to undesirable, long term states of fatigue. One of the most immediate signs of fatigue or impending fatigue is shortness of breath. Another is heaviness in the limbs often with a burning sensation. The Central Governor Model of Human Performance proposed by Dr. Tim Noakes says that these early signs (along with many other insensible signals) are the body’s feedback mechanism to the Central Governor, a part of our reptilian brain, that the body is moving away from homeostasis and adjustments need to be made before a critical condition is reached. The model says that if this critical condition is not dealt with the body could be at risk of damage. The Central Governor chooses from an arsenal of responses to the sort of fatigue that the feedback is alerting it to. One of the earliest recourses to this immediate sign of fatigue is to increase ventilation and heart rate.
There are myriad causes for fatigue, many of which are poorly understood. They can range from what used to be considered a pernicious culprit, lactic acid, to a slowing of the trans-membrane pumping of potassium and to glycogen depletion. Let’s dispense with that old bogyman lactic acid right way. Lactic acid does not really exist to any large extent in the working muscles. When it is formed as part of the glycolytic metabolic process it immediately dissociates into lactate and a hydrogen ion (H+). The lactate is a valuable metabolite and will ultimately be used for more ATP synthesis. The H+ on the other hand can lead to problems. It is an acid and as such can change to cell’s pH with results that trigger a chain of effects we call fatigue.
I won’t begin to try to catalog all the ways that fatigue manifests itself. The purpose of training is two fold: Firstly to accustom your body to handling greater and greater physical exertion before fatigue in any form rears its ugly head. Secondly, to improve your ability to maintain high levels of work when immediate or short-term fatigue begins to impact your performance. In short: by subjecting your body to chronic bouts of exercise-induced fatigue you hope to improve your ability to do more work for longer. This improvement comes about by numerous (and in many cases poorly understood) signaling pathways that result in particular gene expression. This is a perfect case of nurture’s influence on nature. Your response to training does have a large nature component as well. Some individuals are rapid responders to various types of training because they have more of certain types of the genes that signal training adaptations.
Noakes’ model is very good at explaining the complex interplay between the physical symptoms we experience and the reduction in performance associated with fatigue. In every understood type of fatigue the single common feature they all share is that the brain reduces the motor nerve signals thus reducing the power output which thereby aids in restoring the body’s homeostasis. So, the signaling pathways that encourage the body’s adaptation are intertwined with the signaling pathways that start alarm bells ringing in the Central Governor. You need to experience certain levels of fatigue in order to induce a response by the body to adapt and not have the alarm bells go off so easily the next time that training stimulus is supplied. It is proposed by Noakes that one of the principle effects to training is that the Central Governor resets itself to a new and higher level when repeatedly exposed to training stimuli. In effect it realizes that the threat to the body is not as severe as previously thought and allows your body to flirt a little more closely with disaster before limiting power output (ie, fatigue causing you to slow down). he danger zone moves higher and your Central Governor becomes more and more comfortable with greater and greater levels of exertion.
The kinds of fatigue most athletes experience can be lumped into groups. The fatigue induced by a challenging alpine route that takes 3 days car to car is quite different than the kind you’ll feel when bounding 10 seconds up a steep hill for the 6th time in a set of 10 reps and is different still from the kind you feel when running fast 400 meter repeats on the track or the major pump you feel on that red point attempt. The reason they are different is that each of these tasks involves different energy systems, different muscle fibers and different motor pathways.
The submaximal effort involved in a multi-hour effort results mainly in glycogen depletion in the working Slow Twitch muscle cells. This depletion in itself is a powerful trigger for the signaling pathway to adaptation towards endurance. It takes a long duration of work to deplete the glycogen stores in well-trained slow twitch fibers. When those fibers do become depleted their neighboring fibers join in the work and get the training effect as well. Too short of a run/hike/climb and these reserve fibers don’t get trained. So, duration and low to moderate intensity is essential for this kind of training to occur. Because well-conditioned ST fibers have a very high threshold for fatigue (they can complete thousands of cycles before becoming fatigued) it is possible and sometimes even desirable to undertake this sort of training when in a pre-fatigued state in order to enhance the training effect by causing an even larger pool of fibers to become depleted of glycogen. Daily bouts of low to moderate endurance training have the best effect on improving this system. Most often done at or below aerobic threshold for the individual.
The Maximal effort required in the short uphill sprints causes a strength training effect in the nervous system as well as the muscles. Your brain tries very hard to recruit more fibers and do it in a way that produces more power through a more efficient movement. This has great benefits for even the slowest of slow twitch athletes. It improves movement economy, which, like it sounds, is the measure of how much energy you must expend to move a certain way at a certain speed. In order to be able to maximally recruit the largest pool of fibers to do this work in the most powerful way you need to be fresh. This gives a big training stimulus to the FT fibers and to your nervous system. Several days of easy training are needed before this session is attempted again.
The hard anaerobic effort illustrated by the above example of the fast 400 meter runs or those hard burns on your project can be and often are the most demanding workouts an endurance athlete does. The intensity of these workouts will be done well above the individual lactate threshold (LT) and as such are unsustainable for more than a few seconds to a minute without rest.
Some fatigue needs to be expected and even encouraged. Deep fatigue needs to be treated more carefully and incurred less frequently. In general, deep fatigue, stiffness or soreness that lingers more than 2-3 days during normal training indicates an overreaching situation. This can be a good stimulus from time to time when it is followed by appropriate recovery time but should not be a constant in regular training. Occasionally you will feel the improvements in your endurance. On these days you can tell that even while you are carrying some (and maybe even a lot of) fatigue, you are, none the less, able to sustain a higher speed/power output than in pervious weeks. Recognize this a both a sign of the training effect but also a time to back off and let that fatigue dissipate.
It is easy to train hard. It is hard to train well.