Saturday, October 15, 2011

What the Heck is Lactic Acid and What is Responsible for the "Muscle Burn"?

While studying for my Exercise Physiology Exam, I came across lactic acid.  I remember my teacher telling us the "burn" in our muscles isn't actually from lactic acid, but from Hydrogen ions.  I couldn't remember why, exactly, and where lactic acid played a role.  So, I looked it up in my book.  It's pretty common language among athletes to blame the burning in their muscles from sprinting or racing, on lactic acid.  But actually....while it is somewhat true, it's not the real reason.  Let's dive into the underlying science behind it so you can actually understand why lactic acid accumulates in the blood and what, exactly, is responsible for the fatigue!!

I'm just going to quote my book here-

-"Various metabolic byproducts of metabolism have been implicated as factors causing, or contributing to, fatigue.  One example is Pi, which increases during intense short-term exercise as PCr and ATP are being broken down.  Additional metabolic by-products that have received the most attention in discussing fatigue are HEAT, LACTATE, and HYDROGEN IONS."
-Lactic acid is a byproduct of anaerobic glycolysis (what is broken down by anaerobic metabolism,  anaerobic means without oxygen, aka-working out above lactate threshold, aka- sprinting)
-Here is the important part:  "Although most people BELIEVE that lactic acid is responsible for fatigue in all types of exercise, lactic acid accumulates within the MUSCLE FIBER ONLY during relatively brief, highly intense muscular effort.  Marathon runners, for example, may have near-resting lactic acid levels at the end of the race, despite their fatigue.  Their fatigue is likely caused by INADEQUATE ENERGY SUPPLY, NOT EXCESS LACTIC ACID".  This is true for any endurance sport.--it's actually pretty hard to take in enough calories during exercise, despite your efforts, especially in a long race, hence, the fatigue (For me, when I race, it's usually at a higher intensity, and I don't know about you, but sometimes the last thing I want is food, but I know you have to get it down, little bit at a time, as long as you can get some energy in, that's good)

So, what is responsible for this type of fatigue (for short efforts), you say, if it is not what we all commonly think of, lactic acid? (Well, not all, but at least I, and a number of others I'm sure, have misinterpreted!)
Well, it's actually due to the accumulation of Hydrogen ions, wow!!

-"Short sprints in running, cycling, and swimming all lead to large accumulations of lactic acid"-What most of us believe to be true, yes.
-"But the presence of lactic acid should not be blamed for the feeling of fatigue in itself.  When not cleared, the lactic acid dissociates, converting to lactate and causing an accumulation of HYDROGEN IONS.  This Hydrogen ion accumulation causes MUSCLE ACIDIFICATION, resulting in a condition known as acidosis."
-"Activities of short duration and high intensity, such as sprint running and sprint swimming, depend HEAVILY on ANAEROBIC GLYCOLYSIS (remember, anaerobic=exercising without oxygen, which is why you can only do this for a few min, if that.) and produce LARGE AMOUNTS OF LACTATE and HYDROGEN IONS within the muscles" (remember, lactic acid is converted into lactate, which then is responsible for the accumulation of hydrogen ions).
-"Fortunately, the cells and body fluids possess buffers, such as bicarbonate (HCO3), that minimize the disrupting influence of the Hydrogen Ion.  Without these buffers, Hydrogen ion would lower the pH to about 1.5, killing the cells.  Because of the body's buffering capacity, the hydrogen ion concentration remains low even during the most severe exercise, allowing muscle pH to decrease from a resting value of 7.1 to no lower than 6.6 to 6.4 at exhaustion."--I learned in class that the better trained you are, the better you are able to buffer lactic acid.  Cool.
-However, those 6.6 and 6.4 levels are pretty low, and "most researchers agree that low muscle pH is the major limitor of performance and the primary cause of fatigue during maximal, all-out exercise lasting more than 20s to 30s."
-Now, "reestablishing the preexercise muscle pH after an exhaustive sprint bout requires about 30-35 min of recovery"--crazy!

How does this apply to athletes?  Well....

"Even when normal pH is restored, blood and muscle lactate levels can remain quite elevated.  However, experience has shown that an athlete CAN continue to exercise at relatively high intensities even with a muscle pH below 7.0 and a blood lactate level above 6 or 7 mmol/L, 4-5x the resting value.
     "Some coaches and sports physiologists have attempted to use blood lactate measurements to gauge the intensity and volume of training needed to produce an optimal training stimulus.  Such measurements provide an index of training intensity, but they might not reflect the anaerobic processes or the state of acidosis in the muscles.  Because lactate and hydrogen ion are generated in the muscles, both diffuse out of the cells.  They then are diluted in the body fluids and transported to others areas of the body to be metabolized.  Consequently, blood lactate concentrations depend on rates of production, diffusion, oxidation, and clearance.  A variety of factors can influence these processes, so measuring blood lactate is of questionable value for fine-tuning training."

There ya have it.  I hope this cleared up what exactly lactic acid is and what it does inside your body during exercise.  It's converted to hydrogen ions!

Wilmore, J., H., Costill, D., L., & Kenney, L., W.  (2008).  Physiology of Sport and Exercise.     
     Champaign, IL:  Human Kinetics.

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