What is Fatigue?
Fatigue resistance is the critical component of endurance exercise. Fatigue can be defined as strength loss (i.e. a decrease in maximal voluntary contraction [MVC]), which is known to be dependent on the type of exercise you are doing. Important variables include the intensity and duration of the activity, as well as the muscle groups involved and the type of muscle contraction (Figure 1.)
Traditionally, to measure the level of muscular fatigue a test of maximal voluntary contraction (MVCs) and maximal electrical/magnetic stimulations is used. These tests provide an insight into whether “the brain” (central nervous system) is failing to contract the muscle, or if the muscle itself (peripheral) is fatigued and unable to produce force.
However, such measurements do not necessarily predict how muscle function may influence ultra-endurance running because the muscle is always able to produce MVCs at forces that far exceed the forces require to run.
In other words, the nature of the relationship between fatigue as measured using maximal contractions/stimulation and submaximal performance limitation/regulation is questionable.
Introducing The Flush Model
Dr Guillaume Millet (Gui) suggests a holistic model that can be applied to all endurance activities and is specifically adapted to ultra-endurance running: the Flush Model (Figure 2.).
The Flush Model model has the following four components:
The ball-cock (or buoy)
The filling rate
The water evacuated through the waste pipe; emptying rate
A security reserve
Understanding the Four Components
The Ball-Cock represents your rating of perceived exertion (RPE). Ideally, at the start of a race, your RPE is zero. However, if you’ve had a bad nights sleep you may be starting at 1 or 2.
The Filling Rate
The filling rate represents the rate at which you are building fatigue or at least the perception of fatigue. If you were undertaking a leisurely stroll, you would have a very slow filling rate because there is very little contributing to your RPE.
However, if you were racing a marathon in 40⁰C (104°F) heat with a stomach ache your filling rate would be rapid, and your RPE would start high and increase shortly after the start.
3. Emptying Rate
The emptying rate reflects your ability to attenuate your fatigue. That is your mental and physical capacity to reduce your RPE throughout the race. In the above hot marathon example, you could douse yourself in iced water to reduce your core temperature or go to the toilet to relieve your sore stomach.