Have you ever experienced muscle fatigue? You know the feeling where your muscles start to burn and no matter how hard you try, you simply cannot summon any more strength?

It is often assumed that the burning sensation associated with muscle fatigue is due to lactic acid building up in the muscles. But is that the full story? In this time machine post, we examine the science behind muscle fatigue to find out what’s really going on.

Sure, high intensity exercise can lead to the production of lactate and hydrogen ions (H+), which theoretically could cause a burning sensation since H+ is acidic. But, studies of mammalian muscles have shown that this acid buildup does not play a direct role in muscle fatigue, at least not at normal body temperatures (Westerblad et al., 2002).

Enter inorganic phosphate. When we exercise, muscles break down creatine phosphate into both creatine and inorganic phosphate. It’s easy to see how this process could lead to an accumulation of inorganic phosphate during high endurance exercise. Research has shown that increases in inorganic phosphate reduces the force muscles can produce, partly by decreasing calcium levels in the muscle cells. Since calcium is necessary for muscle contractions, this provides strong evidence linking increased inorganic phosphate to muscle fatigue (Westerblad et al., 2002; Sunderberg and Fitts, 2020).

Not all animals experience muscle fatigue. Take pronghorn antelope, for example, these animals are able to maintain a rapid pace while running over long distances. Similarly, Arctic terns spend a lot of time flying. Sled dogs are also rock stars when it comes to being able to travel long distances, even in harsh weather, before they tire out.

Sources:

H Westerblad, DG Allen, J Lannergren. Muscle fatigue: Lactic acid or inorganic phosphate the major cause? Physiology. 17(1): 17-21, 2002. doi: 10.1152/physiologyonline.2002.17.1.17

CW Sundberg, RH Fitts. Bioenergetics of skeletal muscle fatigue. Curr Opin Physiol. 10: 118-127, 2020. doi: 10.1016/j.cophys.2019.05.004

• Ocean oddity: how teleost fish stay hydrated in saltwater
• Red, White, and Bison: Celebrating Freedom and National Bison Month

Categories: Exercise, Extreme Animals, Nature's Solutions, Time Machine

Tags: American Physiological Society, diet, fitness, health, muscle fatigue, nutrition, physiology, wellness