The Burn in Movement: More Than “Lactic Acid”
What Is Actually Happening When You Feel the Burn?
That familiar burning sensation in Pilates is often explained as “lactic acid,” but the reality is far more complex. In this article, Luke Khoury explores what is actually happening within the muscle and nervous system when we experience the burn – and why understanding the sensation gives us a better insight into how the body responds to effort.
More Than Lactic Acid
If you’ve ever held a slow, controlled position in Pilates and felt that deep, rising burn in your muscles, you’ve likely been told some version of the same story: that’s lactic acid building up. It’s a simple explanation. It’s also not quite right.
The burn you feel during exercise is real, but it’s not caused by lactic acid “flooding” the muscle. In fact, lactate is better understood as a useful byproduct of metabolism rather than a waste product responsible for pain. The sensation you experience is more accurately the result of a complex interaction between muscle metabolism, local chemical changes, and how your nervous system interprets those signals.
To understand the burn, you first need to understand what the muscle is trying to do. Every contraction requires ATP, the energy currency of the cell. During controlled, sustained efforts like those in Pilates, especially isometric holds or high time-under-tension work, the demand for ATP remains elevated. Initially, the body uses readily available systems like phosphocreatine, but as the effort continues, it relies more heavily on glycolysis and oxidative metabolism to maintain output.
As glycolysis increases, lactate is produced alongside hydrogen ions. This is where the myth often begins. Lactate itself is not the problem. It can actually be reused as a fuel source by other tissues, including the heart and slower-twitch muscle fibres. The issue is the accumulation of hydrogen ions, which lowers the pH within the muscle. This more acidic environment can interfere with enzyme activity, calcium handling, and the efficiency of cross-bridge cycling, the fundamental mechanism that allows muscles to produce force.
At the same time, sustained contraction, particularly in positions that limit movement, can partially restrict blood flow. This reduces the muscle’s ability to clear these accumulating metabolites and deliver oxygen. The local environment becomes increasingly “stressed,” and the muscle’s internal chemistry begins to shift.
This is where your nervous system comes in. Specialised sensory receptors within the muscle, particularly group III and IV afferents, detect these chemical and mechanical changes. They send signals to the brain indicating that the local environment is becoming challenging to sustain. The brain interprets this input not just as information, but as sensation. The result is what you perceive as the burn.
From the perspective of the nervous system, the burn is not damage. It is a protective signal. It’s a way of saying, this is getting metabolically expensive, are you sure you want to continue? In response, the brain may adjust motor unit recruitment, redistribute load, or, if the demand continues, reduce output to protect the system.
This ties closely to principles outlined in Principles of Neural Science, where perception is understood as an active construction of the brain rather than a direct readout of tissue state. The burn is not simply occurring in the muscle; it is an experience generated by the brain based on incoming sensory information, past experience, and current context. Two people performing the same task may perceive the burn very differently, not because their muscles are behaving differently, but because their nervous systems interpret the signals differently.
In Pilates, this sensation is often amplified because of the way we train. Slow tempo, sustained holds, precise positioning, and reduced momentum all increase time under tension. There are fewer opportunities for the muscle to relax and restore blood flow, meaning metabolites accumulate more readily. At the same time, the demand for control keeps neural drive high. The combination of metabolic stress and ongoing neural effort creates the perfect environment for that familiar burning sensation.
Importantly, the presence of the burn does not necessarily indicate effectiveness, just as its absence does not indicate failure. It is simply one form of feedback. For some individuals, especially those new to training or working at the edge of their current capacity, it may appear quickly. For others, it may take longer or be less intense. Neither is inherently better.
Over time, with consistent exposure, the system adapts. Muscles improve their ability to buffer hydrogen ions, increase their oxidative capacity, and clear metabolites more efficiently. Blood flow improves, and the same task becomes less metabolically taxing. At the same time, the nervous system becomes more familiar with the sensation, often reducing its perceived intensity. What once felt overwhelming becomes manageable.
The key is not to chase the burn as a marker of success, nor to fear it as a sign of harm. It is a signal, one piece of information within a much larger system. When understood in context, it becomes less about pushing through discomfort for its own sake and more about recognising what the body is experiencing and responding with intent.
The burn is not lactic acid poisoning your muscles. It is your body communicating that the demand is high, the environment is changing, and the system is working hard to keep up.
The Burn Is Information
The next time you feel that familiar burn, it’s worth thinking beyond the simple idea of “lactic acid.” What you’re experiencing is a conversation between the muscles, the nervous system and the brain as the demands of movement increase.
The burn doesn’t automatically mean an exercise is more effective, nor does its absence mean you aren’t working hard enough. It is simply information – one piece of feedback within a much larger system.
Understanding that feedback changes the way we approach movement. Rather than chasing the sensation or fearing it, we can learn to recognise what the body is communicating and respond with greater awareness and intent.
