We all know that exercise is good for our health. We hear it often: moving more helps prevent disease, improves physical fitness and reduces the risks associated with inactivity. But the most interesting question is not only whether exercise is beneficial, but what actually happens inside the body when we move.
During intense exercise, skeletal muscle temporarily moves away from its internal balance, a state known as homeostasis. It needs to produce more energy, uses more oxygen, relies on calcium to coordinate contraction and relaxation, and increases metabolites associated with effort, such as lactate and protons. When this demand starts to limit the ability to maintain the same performance, fatigue appears.
At first glance, these changes may seem negative. However, this stress is not necessarily a threat. Muscle does not simply contract to produce movement; it also detects that the demand has increased and activates internal responses to adapt better to future efforts.
One example is reactive oxygen species, commonly known as ROS. For years, they have often been presented in a very simple way: free radicals are bad, antioxidants are good. But the reality is more complex and more interesting. When ROS production is excessive, sustained or poorly controlled, it can contribute to cellular damage. However, during well-dosed exercise, a moderate, localised and transient increase in ROS can act as an adaptive signal.
These signals can participate in pathways related to antioxidant defence, cellular repair, metabolism and mitochondrial function. For this reason, it does not always make sense to try to suppress every oxidative signal: some of them are part of the processes that help the cell adapt.
The key is the dose. A stimulus that is too low barely produces changes and does not generate a meaningful adaptation. On the other hand, an excessive stimulus, or one repeated without enough recovery, can lead to persistent fatigue, poor recovery or muscle damage. Between these two extremes lies the stimulus we are looking for: a challenge strong enough to trigger a response, but not so large that it prevents recovery.
This is why some authors have compared exercise to a biological polypill. Not because it replaces medical treatment or works for everything, but because the same behaviour can influence several systems at once, including muscle, blood vessels, metabolism, the brain, inflammation and bone health.
Our body constantly faces demands: physical effort, illness, ageing, lack of rest or metabolic stress. Well-dosed exercise does not remove these challenges, but it can improve the way we respond to them. Each session leaves a biological signal that, when repeated over time and combined with enough recovery, helps strengthen cellular defences and the body’s capacity to adapt.
Ángel Gallego Sellés is a researcher in sport sciences at the University of Las Palmas de Gran Canaria. His work explores how the human body responds to exercise, from cellular signals in skeletal muscle to their relationship with health, strength and physical performance.