What is muscle fatigue?

At its core, fatigue can be see as the failure to maintain the required force-generating capacity in muscles during voluntary effort.

When it comes to Peripheral Fatigue specifically—which is the one that occurs at the muscular system—, it can also refer to an increase in muscular force (at first), because via fatiguing mechanisms, high threshold motor units get recruited to meet the demands of force production required by the CNS.

With additional repetitions, however, force production diminishes, finally reaching what’s known as “task failure”.

But what actually causes muscles to fatigue?

According to research, two primary theories emerge to explain this phenomenon: “the depletion hypothesis” and “the accumulation hypothesis”.

The depletion hypothesis suggests that muscles begin to tire as they progressively run out of their primary energy reservoirs – ATP, phosphocreatine (PCr), and glycogen. These energy molecules are the primary fuel for muscle contractions, and their depletion signifies a diminishing energy supply, eventually leading to reduced muscle performance.

On the other hand, the accumulation hypothesis focuses the metabolic byproducts that accumulate within our muscle cells, particularly hydrogen ions (H+) and inorganic phosphate (Pi). When our muscles work hard, especially during high-volume exercises, they produce these metabolites, which, if in significant amounts, can interfere with muscle contraction and function. Furthermore, intense muscular activities can lead to restricted blood flow, exacerbating the accumulation of these metabolites (this is especially seen with BFR training, for example!)

Importantly however, peripheral mechanisms of fatigue are not enough to explain all there is to know about fatigue. In fact, as we’ve already seen in previous posts the role of central fatigue plays an interesting part as well.