“Are we feeling the lactic acid build-up in our legs and feeling the burn everyone!?”  A younger and more ignorant me used to say to my cycling classes and patients.  I, like most fitness professionals, was under the impression that lactate build-up in muscles from intense exercise caused the awful burn (acidosis) to occur.  How does this lactate build-up occur though?

Lactate is a product of glycolysis.  Glycolysis is an energy system used by the body to break down glucose (stored as glycogen in the liver) into pyruvate and lactate.  Think of pyruvate and lactate as teeny-tiny molecules that the mitochondria use to produce ATP (energy).  The amount of pyruvate or lactate produced depends upon how much oxygen is present in the cells at the time of glycolysis.

In exercise where lots of oxygen is present (aerobic) your body will produce more pyruvate and less lactate.  When you continue to push yourself into the higher power zones and above your lactate threshold (LT), the oxygen level in the cells will decrease and you will go into anaerobic energy production, thus producing increased lactate and decreased pyruvate.  Continue to push harder and the levels of lactate produced will continue to increase while pyruvate levels decrease.  For this reason,  exercise physiologists used to attribute lactate to fatigue due to the increasing amount of it present during intense activities.  Plus, a study from 1976 conducted on frog legs found the contractile force of the frogs decreased as lactate levels increased (1).   I am not sure how the connection was made between human legs and frog legs, but this is the point where everyone started saying lactate causes the muscle burn and fatigue associated with intense exercise.

Jump forward to 2008, and the scientific community began to question the old adage that lactate was the bad guy.  The new studies were all pointing to an increase of hydrogen (which is a by-product of mitochondria using lactate as fuel) causing a decrease of blood pH and resulting in acidosis (2).  Let’s put this all together…

• Stay below your LT and your mitochondria will use more pyruvate to produce ATP and little lactate will accumulate.  You will feel little to no muscle burn and your body will have enough oxygen present to stay in the aerobic energy system.  Whatever lactate is produced can be used efficiently.
• Push harder and above your LT, your body will start to produce more lactate and less pyruvate.  Less oxygen will be present in the cells and your body will switch to the anaerobic energy system.
• As your body begins to produce and use lactate for ATP, hydrogen ions will begin to accumulate in the muscle.
• Hydrogen ions present in the muscle will lower the blood pH (increased acidity).
• Lower blood pH will result in acidosis (burning muscles).
• Keep pushing, and this cycle will continue until the burn becomes unbearable and fatigue sets in, ceasing exercise and allowing your body to recover.

So, what is lacate?  An energy source for your muscles!

Does lacate cause your muscles to burn?  No!  An increase of hydrogen ions decreases the pH of your blood which causes the burn.

References:

1. Lactate and contractile force in frog muscle during development of fatigue and recovery

RH Fitts, JO Holloszy

American Journal of Physiology — Legacy Content Aug 1976, 231 (2)430-433

2. Skeletal Muscle Fatigue: Cellular Mechanisms

D. G. Allen, G. D. Lamb, H. Westerblad

Physiological Reviews Jan 2008, 88 (1) 287-332; DOI:10.1152/physrev.00015.2007