After a sprint or a particularly intense set of squats, we’ve all felt that burning sensation in our muscles. Many of us have probably been told that this is due to “lactic acid buildup”.
We’ve all heard that statement before. It almost feels like common knowledge. The burning sensation during exercise is blamed on lactic acid, while the soreness we feel the next day is often attributed to the same culprit lingering within our muscles.
Unfortunately, biology is rarely that straightforward.
In reality, the lactate produced during exercise serves a remarkably important purpose in helping our muscles continue generating energy. In fact, by the time delayed onset muscle soreness (DOMS) develops one or two days after a workout, the lactate has already been removed from our muscles.
Therefore, if lactate isn’t responsible for the soreness that we experience after exercise, why does our body produce it in the first place?
To answer that question, we once again have to return to the tiny mitochondria within our cells and revisit the chemistry behind ATP production.
As we discussed previously, our muscles rely on ATP as their immediate source of biochemical energy. During moderate exercise, ATP can be regenerated efficiently through oxidative phosphorylation within the mitochondria. However, during very intense exercise, our muscles begin consuming ATP so rapidly that an alternative strategy becomes necessary.
From Oxygen to Sweat: The Biochemistry and Thermodynamics of Exercise
It is a common sight to see people slumped over or being one with the floor at the end of an exhausting workout. The inertia becomes too much to overcome. It’s difficult to stay up straight. If you’ve been in that position before, there are days when you would very much prefer to remain in that horizontal position rather than get up and get washed up. I…
In that discussion, we explored how oxygen is consumed by the electron transport chain to produce ATP, water and heat, and why our bodies need to dissipate that heat through sweating.
During a particularly intense workout, however, our muscles can consume ATP faster than it can be regenerated through oxidative phosphorylation alone. The body therefore increases its reliance on glycolysis, which is another pathway that is capable of producing ATP much more rapidly, although less efficiently.
However, glycolysis eventually encounters a problem. As glucose is broken down, it is converted into pyruvate while simultaneously reducing nicotinamide adenine dinucleotide (NAD⁺) into its reduced form, NADH. Unless NAD⁺ is regenerated, glycolysis eventually grinds to a halt because there is no longer sufficient NAD⁺ available to accept electrons.
This is precisely where lactate comes into the picture.
Rather than allowing glycolysis to stop, the enzyme lactate dehydrogenase converts pyruvate into lactate. In doing so, NADH is oxidised back into NAD⁺, replenishing the supply required for glycolysis to continue producing ATP.
In other words, lactate production is not a design flaw. It is a biochemical adaptation that allows ATP production to continue during periods of exceptionally high energy demand.
Unfortunately, lactate has acquired a rather poor reputation over the years. Many people have been taught that it is responsible for the muscle soreness we experience after a workout.
The problem is that the timeline simply doesn’t add up. The burning sensation experienced during intense exercise occurs during the workout. Delayed onset muscle soreness (DOMS), on the other hand, usually develops 12 to 48 hours later, often peaking around 24 to 72 hours after exercise.
By that time, the lactate produced during exercise has long since been cleared from the muscles. It cannot reasonably be blamed for soreness that develops a day or two later.
Once exercise intensity decreases, much of the lactate produced by our muscles is transported through the bloodstream to other tissues. The heart can readily oxidise lactate as a source of energy, while the liver can convert it back into glucose through a process known as the Cori cycle. This glucose can then be stored as glycogen or used again for future energy production.
As we’ve seen throughout this series, the human body is rarely as simple as it first appears. Whether it is fat being converted into carbon dioxide, sweat cooling us through evaporation, or lactate sustaining ATP production during intense exercise, many of the processes that seem mysterious on the surface become beautifully logical once we understand the underlying biochemistry.
The next time your muscles begin to burn during a workout, don’t blame the lactate. It may well be the very reason you’re still able to keep going, albeit with much more huffing and puffing while you’re at it!
Do feel free to share this article and hit the “subscribe” button to get more updates about the science concepts in nutrition and health, all deconstructed nicely for your convenient perusal!