When we runners think of fatigue, we tend to think only about our muscles. But the reality is that the brain, like muscle, can also experience physical fatigue.
To make things simple, fatigue can generally be classed as either PERIPHERAL FATIGUE, relating to our skeletal muscles, or CENTRAL FATIGUE, relating to our central nervous system (though we’ll narrow it down to the brain here).
PERIPHERAL FATIGUE occurs when our brain sends our muscles all the right signals to contract, but something at a muscular level prevents or makes it more difficult for these signals to be translated into the same fast, forceful contractions that the brain signalled for. Possible sources of peripheral fatigue include: depleted energy stores, accumulation of metabolites and mechanical stress.
CENTRAL FATIGUE on the other hand occurs when exercise exertion (lack of sleep and mental exertion can also play a part) acutely changes the brain’s chemical balance. The brain has certain chemical mediators that allow it to communicate with muscles effectively. However, one can consider these chemical mediators to have a certain operating bandwidth which, if exceeded, causes them to slowly deplete and force other chemicals out of balance (to oversimplify). When this chemical imbalance occurs, it doesn’t only influence our state of mind, but also physically impacts our brain’s ability to recruit muscle.
Muscle recruitment (in layman’s terms) is the role the brain performs in ‘choosing’ how many muscle fibers of a muscle are activated/stimulated to work. Naturally the brain recruits as few fibers as possible to minimise energy expenditure. When centrally fatigued, muscle recruitment decreases and fewer muscle fibers are activated to work – leading to our muscles performing with less force and slower contraction velocity despite experiencing the same exertion.
So, there are 2 different types of fatigue; but what does this mean in practice?
In short: this means that fatigue exists beyond one’s legs feeling sore. PERIPHERAL and CENTRALLY FATIGUE tend to occur together, but not at an even 1:1 ratio. As a general rule of thumb, the higher the intensity of the workout, the greater the chance of CENTRAL FATIGUE being present longer than PERIPHERAL FATIGUE. The opposite is true as well. Compare how you might feel after a set of short hillreps as opposed to a long run – both cause fatigue, but in a slightly different way.
Knowing this is one good reason why you should polarise your types of workouts within a period of time. For instance, if you plan to run hard 200m intervals on Monday, your legs may feel great come Thursday (assuming peripheral fatigue has disappeared), but your body could still be slightly CENTRALLY FATIGUED – so rather than running a CENTRALLY demanding workout like you did on Monday, you could perform a threshold run which is more PERIPHERALLY challenging and your session quality won’t be as affected as it would running another short-speed session.
If you still don’t know what central fatigue is, try this workout:
Time yourself doing a typical sprinter’s workout: 8 x 10sec hill sprints, running at maximal effort for every rep while taking as much rest as you need between each rep to feel fully recovered (usually 2.5-4min).
What most runners (non-seasoned sprinters) will find is that no matter how much rest they take between each rep, they can only maintain their fastest pace (or distance run in 10sec) until the 4-5th rep, after which all their remaining reps gradually get slower despite their persistence. So what is happening here?
During this session, PERIPHERAL FATIGUE will probably be fairly low but as the workout progresses, CENTRAL FATIGUE begins to feel quite noticeable. Your legs won’t feel tired in the way that you’re used to, but they won’t feel like they have the same acceleration and power as they did at the start. Yet, if you had to try running a hard 10km the next day, you’d probably find you could perform quite well– some might not notice any difference in performance. However, if you had to try repeating this same
session, it’s very likely your times would all fall short as your level of centrally fatigue would remain high.
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