The strength community in the US is slowly welcoming conversation about aerobic activity for health, performance, and recovery purposes. This post seeks to add to the current information available about the benefits of aerobic training for strength athletes, most specifically in terms of recovery. It will first attempt to clarify misunderstandings about lactate, the single substrate, versus lactic acid. Secondly, it will explain how lactate’s involvement in lactic acid can be converted back into re-usable fuel by performing low intensity aerobic activity.
Glucose is the body’s fuel source, and when energy expenditure is low, say, when walking to catch a stationary bus, glucose is broken down into a substrate called pyruvate. At this point, if you are taking relaxed breaths, you have oxygen and your body continues to break down glucose into pyruvate. You catch your bus without much effort.
There are some instances in life, however, such as sprinting to catch a bus or a 5 Rep Max Back Squat, that might require ATP (energy) production faster than our bodies can deliver oxygen. In these instances, muscles generate energy anaerobically, meaning “without oxygen”. Without oxygen, the pyruvate breaks down to a substrate called lactate.
You can think of the body as a nuclear reactor that builds up waste in the form of acid. Under normal, low energy activities, the waste from the reactor is recycled, but during high energy demands, the waste of the reactor builds up very quickly.
It is important to distinguish between the lactate molecule, and the lactate molecule that acts as the conjugate base for the acid. Lactate is the substrate that actually acts to weaken the burning effects of the acid, which allows us to push longer. Lactate weakens (buffers) the acid by chemically binding to the acid; when the lactate molecule and acid combine, it becomes “lactic acid”.
Even though lactate's goal is to weaken the acidity in the muscle cell by binding to the acid itself, the amount of acid will reach a point where there is more waste than can be buffered by lactate. This is called our acid threshold, when pain receptors are stimulated that yell to the brain to slow down the exercise.
Where though, does low intensity aerobic activity fit into this discussion?
Low intensity aerobic activity, namely in the form of cool-downs after strength training, though also included in occasional sessions multiple hours before strength training, takes advantage of a process referred to as the Cori Cycle thats helps us recover faster and perform with more energy.
I will explain how low intensity aerobic activity enables the Cori Cycle in 3 simple steps:
In the presence of oxygen, lactate unbinds from the acid inside the muscle and it is drawn into the bloodstream where it binds to oxygen.
The oxygen bound lactate is transported through the blood to the liver.
In the liver, the lactate is ultimately converted back to glucose.