So a lot of us in the gym have heard lactic acid build up is causing fatigue. It’s lactic acids fault that I didn’t get that last rep, run for that last mile, or be able to push myself that much further. Well ….. It’s not lactic acid that is stopping you!
Recent studies on mammalian muscle show little direct effect of acidosis (lactate build up) on muscle function at physiological temperatures. A study conducted by H. Westerblad, D. Allen, J Lannergren, titled “Muscle Fatigue: Lactic Acid or Inorganic Phosphate the Major Cause?”, Department of Physiology and Institute of Biomedical Research, University of Sydney (2006). Proves to show lactic acid is not the fatiguing factor of skeletal muscle.
So lets get down and dirty, here is an explanation of where lactic acid comes from. The anaerobic breakdown of glycogen leads to an intracellular accumulation of inorganic acids. Out of all these acids, lactic acid is quantitatively the most important. Lactic acid being a strong acid then dissociates into lactate and H+. The increase in H+ (i.e., reduced pH or acidosis) is the classic cause of skeletal muscle fatigue.
So here you may be thinking… I knew it, lactic acid must be the reason you feel fatigued since quantitatively it’s the most important, and for many years this was thought to be correct, but wait!!!
Besides acidosis caused by lactic acid, anaerobic metabolism in skeletal muscle also involves hydrolysis of creatine phosphate (CrP) to creatine and inorganic phosphate (Pi). Now some of you may be saying to yourself well I only run long distance that is always aerobic so why are we talking about anaerobic metabolism. The important thing to note, is the fact that both aerobic and anaerobic energy systems are working at the same time during exercise, one more then other depending on the activity. For example even though long distance running may seem to be predominately aerobic which is correct, there are still times during the activity especially nearing then end where your aerobic system may not be able to meet the energy demands and therefore your anaerobic metabolism must sub in. Thus the anaerobic metabolic energy pathways are always being used, it simply depends on the type of physical activity to determine where this energy source comes into play.
Back to our discussion of Pi. New research shows Pi may depress muscular contractile function. On the basis of recent findings, increased Pi rather than acidosis appears to be the most important cause of fatigue during high-intensity exercise (both short and long duration). Okay wait a minute here, so which one is it? Lactic acid or in organic Pi that is causing my muscles to feel tired? Well that’s a great question, continue reading…
This figure illustrates where Pi may affect muscle function during fatigue. Increased Pi may act directly on the myofibrils (the parts of muscle tissue) and decrease cross-bridge force (muscle contractions) production through decreasing myofibrillar sensitivity to calcium which the muscle needs to contract. Therefore if you reduce a muscles ability to contract properly, your resulting outcome is less power output…. DUH… leading to muscle fatigue!
So what have we figured out so far.. Lactic acid is no longer the fatiguing factor when working out. So next time you are in the gym and you hear someone say, “This lactic acid build up is killing my lifts” you can go over to them and say, “actually it is the inorganic phosphates that are decreasing your myofibrils sensitivity to calcium ions that is reducing your muscular strength.” now if they look at you funny, perhaps with a stink eye don’t feel bad. You probably deserve it, most people in the gym wont know what the heck you are talking about. In all seriousness though, this may be information you want to hold out on for the right moment (ha-ha)
According to H. Westerblad, D. Allen, J Lannergren authors of the article mentioned earlier, further research is needed to investigate the causes of muscle fatigue. With this new discovery we have stepped further into fully understanding our internal physiological environments. Who knows, maybe in the future we will be able to understand how to prevent muscle fatigue all together. Wouldn’t that be crazy??
H. Westerblad, D. Allen, J Lannergren. Muscle Fatigue: Lactic Acid or Inorganic Phosphate the Major Cause?. Department of Physiology and Institute of Biomedical Research, University of Sydney 2006
About the Author
Coach Chuck Dertinger is an accomplished fitness and nutrition expert, holding a Masters Degree in Science with a concentration in Exercise Physiology. Chuck lives, breaths, and loves to promote healthy lifestyle choices. He went from skinny to brawny gaining over 50-lbs of muscle in a couple of years through the use of sound training and nutrition tactics. Feel free to add him on Facebook and ask any questions with relation to nutrition or exercise.