What Happens in the Body While Running?

How the heart, lungs, and muscles turn food and oxygen into movement

Running may seem like one of the simplest forms of movement. You put on sportswear, lace up your running shoes, and head out. Moving your legs and breathing rhythmically feels completely natural, almost self-evident. Yet behind the scenes something much more intricate is taking place. With every step, the body responds with a series of precisely coordinated processes: the lungs increase the exchange of oxygen, which passes into the blood; the heart pumps this oxygen-rich blood more rapidly throughout the body; the muscles use the oxygen to produce energy; and the nervous system coordinates millions of signals. What we experience as simple bodily motion is in fact a highly complex physiological process.

In this essay, we will explain what happens inside the body during running and how these changes are reflected in the numbers tracked by a smartwatch: heart rate, energy expenditure, or the estimated VO₂max. Once a runner understands the background of these measurements, a clearer picture emerges of why training strengthens the body and how regular practice gradually improves performance.

How the Body Produces Energy for Running

The moment we start running, countless tiny machines inside our muscle fibers switch on at once. Each of them functions like a microscopic motor that needs energy to do its work. Without energy, the motor stops—along with the contraction of muscle fibers that makes bodily movement possible.

The sole “fuel” for these motors is the molecule ATP (adenosine triphosphate). The simplest way to imagine it is as a very small battery. The name “triphosphate” means that the molecule carries three phosphate groups. When one of them breaks off and releases energy, what remains is only a “diphosphate” with two phosphates. ADP (adenosine diphosphate) represents the empty battery.

There are many such “batteries” in the body, but still only enough for a few seconds of intense physical activity. If muscles relied solely on this initial stock of energy, bodily movement would quickly come to a halt. That’s why a system exists for continuously recharging these energy carriers. When the molecular “battery” runs down and ATP turns into ADP, special cellular processes immediately begin refilling it back into ATP. Without this “recycling,” running would last only a moment; with constant recharging, however, we can keep running far longer.

The most important contributors to this steady “recharging” are the mitochondria (often called the cell’s power plants). Inside them, a multitude of reactions transform energy from food—primarily carbohydrates and fats—into ATP. Mitochondria continuously generate ATP from nutrients so that muscle fibers never run out of energy.

Thus every step in running is not just a mechanical movement, but the outcome of a carefully coordinated process: muscle fibers consume ATP molecules, mitochondria recharge them, and the body works constantly to keep the supply of nutrients and oxygen sufficient. It is precisely this ability to renew energy on the go that enables humans to run even very long distances, depending on how efficiently and harmoniously these tiny machines in the body perform their work.

Different Energy Sources for Different Running Challenges

Inside the body, various energy systems are constantly at work to ensure that muscles have fuel in the form of ATP molecules. These systems never operate entirely in isolation; all are always active, but the share of their contribution shifts depending on the duration and intensity of running. Through training, the body gradually adjusts the thresholds that determine when one system takes priority over another, thereby improving endurance.

In the very first moments when a runner launches forward from rest, the fastest yet most limited energy source kicks in. This is the phosphocreatine system, also known as the ATP-PCr system. It operates without oxygen—hence it is called anaerobic—and functions like a backup generator that switches on under maximum strain. Muscles store a special molecule called phosphocreatine (PCr). Its role is simple: when an ATP “battery” runs down and only ADP remains, phosphocreatine instantly donates its phosphate group, converting ADP back into ATP. In this way the battery is quickly recharged, and the muscle can continue working at very high intensity without movement stopping.

This system is extremely fast and delivers an explosive burst of power—the very thing a runner needs for those first decisive steps when pushing off. It is most evident in 100- and 200-meter sprints. But it also has a limitation: the store of phosphocreatine is very small. After less than ten seconds of intense activity, it is depleted, and the body must turn to other, more sustainable energy sources to keep running.

When the runner continues at a fast but submaximal pace, the next energy source comes into play more strongly: anaerobic glycolysis. This is the process by which muscle cells break down glucose. As a simple sugar, glucose provides a readily available fuel that the body can use almost immediately, which is why glycolysis allows for quick and efficient production of new ATP.

In the past, it was thought that anaerobic glycolysis switches on because oxygen supply cannot keep up with demand at high intensity. More recent findings suggest that the reason is not necessarily a lack of oxygen but rather what is called metabolic inertia. The processes in the mitochondria, where energy is generated with the help of oxygen, need some time to reach full potential. During this transitional phase, anaerobic glycolysis steps in rapidly to provide energy until the aerobic system takes over the main load.

But Quick Fuel Comes at a Price

When glucose is broken down without sufficient oxygen (under anaerobic conditions), lactate is also produced. For decades it was widely believed that lactate was the main culprit behind the burning sensation and fatigue in muscles. Today we know this is not entirely true: lactate is actually a valuable intermediate product that other cells can use as an additional fuel, while also helping shuttle hydrogen ions (H⁺) from muscle cells into the bloodstream. Rather than being just a “waste product,” it is a bridge that transfers energy to where it is most needed. Some lactate can also travel to the liver, where it is converted back into glucose through the so-called Cori cycle, circulating again as an extra energy source.

Anaerobic glycolysis can effectively power a runner at high intensities, typically for efforts lasting from about thirty seconds to two minutes. This time window corresponds to medium-length sprints or demanding intervals, when the body has not yet had time to fully engage slower but more sustainable aerobic mechanisms.

When running extends into a true endurance challenge, the oxidative system takes over as the main source of energy. It requires a continuous supply of oxygen and is therefore the most efficient energy provider during prolonged running.

Mitochondria in muscle cells act like tiny power plants. From food—mainly sugars and fats—they first generate a common “fuel” called acetyl-CoA: sugars reach it through glycolysis and pyruvate, fats through the sequential cleavage of carbon chains (β-oxidation). This fuel enters the citric acid cycle (also known as the Krebs cycle), where its carbon atoms are finally converted into carbon dioxide, while electrons are loaded onto special energy carriers (NADH and FADH₂)—as if small batteries were being charged.

These “batteries” then deliver their electrons to the respiratory chain in the inner mitochondrial membrane. There, the energy is transformed into a “pressure” of protons on one side of the membrane, while oxygen at the end of the chain accepts the electrons and is turned into water. As the protons flow back through the enzyme ATP synthase, it acts like a tiny turbine, rebuilding ATP from ADP.

Whereas anaerobic glycolysis generates only 2 ATP molecules from one molecule of glucose, the oxidative system produces as many as 30 to 32—about fifteen times more. That is why it can provide energy for virtually unlimited periods, as long as fuel reserves and oxygen remain available.

The Aerobic Advantage

If the first energy pathways are like short-term backup generators that quickly fail, the aerobic system is the main and most reliable energy source for long-distance running, providing a continuous supply of energy as long as oxygen and fuel reserves are available. This is the reason a runner can keep going for hours at a time, covering even dozens of kilometers.

The aerobic system switches on more slowly, but it has a crucial advantage: endurance and adaptability. At lower running intensities it draws most of its energy from fats, of which the body has large stores. When the pace increases and muscles need more energy in less time, carbohydrates take on a larger share. This very ability to shift between fuel sources allows the body to remain efficient under widely varying conditions—from an easy jog to a demanding endurance race.

During a run, the ATP–PCr system provides the explosive start; then, at high intensity for a short time, anaerobic glycolysis dominates; and for longer, endurance efforts, the oxidative system takes the lead. Each energy system thus has its role: from powerful takeoffs, through brief accelerations, to marathon endurance. Only their coordinated operation makes it possible for a person to run both fast and long.

Muscles and the liver also store glycogen, a concentrated form of glucose that serves as a readily accessible energy reserve. When intensity rises, the body breaks glycogen down into glucose, which quickly enters glycolysis to produce additional ATP. The amount of glycogen stored in muscles largely determines how long we can maintain high intensity.

The drop in performance known as “hitting the wall” is not necessarily the result of a complete depletion of glycogen stores. Newer findings show that fatigue occurs earlier due to a combination of factors: local glycogen depletion in the most heavily loaded muscle fibers, the influence of declining liver glycogen on the brain (central fatigue), and protective mechanisms by which the brain prevents complete exhaustion.

With regular endurance training, the body increases its glycogen storage capacity while also becoming more economical: at the same pace, it draws more energy from fats, preserving glycogen reserves for longer. In this way, training directly improves endurance by extending the time we can sustain demanding running speeds.

Understanding the Lactate Threshold

After the first minutes of running, when different energy systems are working at full tilt, a process begins in the body that for a long time was considered the main barrier to endurance. In the previous section we encountered glycolysis, the fast way of generating energy that produces the molecule lactate. For many years, lactate was thought to be responsible for the burning sensation in muscles and for exhaustion. Today we know that this is not the case. Lactate is not a waste product but a useful source of energy that the body continuously produces and uses. The easiest way to picture it is as a circulating fuel: when it forms in one muscle cell, the body can transport it to other cells—or even to the heart—where it is consumed as extra energy. In this way lactate acts as a portable form of fuel that the body constantly generates and at the same time uses.

Glycolysis is always switched on—the only question is what happens to its product, pyruvate. When physical activity is moderate, pyruvate smoothly enters the mitochondria, where it is broken down further in the presence of oxygen. But when a runner speeds up and the demand for ATP exceeds the current capacity of the mitochondria, some pyruvate remains outside and is converted into lactate. This is not a sign of system failure but a precise response of the body: glycolysis can continue producing ATP rapidly even when the aerobic “power plant” cannot keep up. Lactate here is not just a by-product but an important molecule that allows the process to go on and simultaneously serves as an additional fuel for other cells.

The lactate threshold is therefore not an enemy but a key turning point in the runner’s experience. The easiest way to imagine it is as a speed limit we know we cannot sustain for long. Below the threshold, the body removes all lactate as it appears and even uses it as extra fuel. Running below this threshold feels easy: breathing is still relaxed, conversation flows effortlessly. But as the pace increases, there comes a moment when talking becomes difficult, breathing deepens, and the muscles begin to “glow.” This is the sign that we have reached the lactate threshold—the point at which lactate accumulates faster than the body can process it.

Why then do we feel pain and fatigue if lactate is not to blame? Alongside its formation—and especially during the rapid breakdown of ATP in muscle fibers—hydrogen ions (H⁺) are produced, lowering the pH. These ions cause the acidity that disrupts normal muscle contraction, which we perceive as burning pain and rapidly rising fatigue. It is important to note that H⁺ are always formed, but at high intensity they are produced so quickly that the body cannot remove or neutralize them fast enough. The best way to picture this is like garbage collection: waste (H⁺) is always being generated, but if too much piles up at once, the trucks (buffering systems) cannot carry it away in time. In this process, lactate is not the problematic waste—it is actually part of the solution, since it binds some of the H⁺ and helps transport them into the blood, where they can be more easily processed. The drop in pH at high intensity is therefore primarily a result of the rapid hydrolysis of ATP (the chemical breakdown of the “energy battery” that releases energy), while lactate acts more like a “buffer” and at the same time an additional fuel.

Nevertheless, a raised lactate threshold is one of the key markers of progress. With regular, targeted training this threshold shifts to higher running speeds. The body learns to work more economically: muscles become better at consuming lactate, oxygen transport improves, and energy use becomes more efficient. The result? A runner can run faster at the same heart rate without excessive accumulation of hydrogen ions. That means less acidity, less fatigue, and greater endurance.

Modern smartwatches and apps can even estimate, based on heart rate and running pace, where a runner’s lactate threshold lies. This allows for smarter training: running fast enough to challenge the body but not so fast that fatigue stops us too soon. Training around the threshold is in fact one of the most effective ways for runners to improve gradually and push their limits.

It is also worth noting that scientific literature distinguishes between two different lactate-related thresholds. The first (the aerobic threshold) is the point at which lactate in the blood begins to rise above baseline but the body can still successfully remove and use it. This is the highest intensity at which you are still running in the easy zone. The second (the anaerobic threshold) is the level at which lactate starts to accumulate faster than the body can clear it. This is the threshold that marks the transition into the zone of high intensity. Understanding these two thresholds helps runners pace themselves more effectively and train right at the edge where improvement is greatest.

The Number That Reveals Running Potential

With regular training, a runner notices that at the same pace their heart rate drops, breathing becomes calmer, and the feeling of effort decreases. All of this is reflected in a single key indicator recorded by modern smartwatches, somewhat complicatedly called VO₂max. The simplest way to picture it is like the engine size of a car: the bigger it is, the more fuel and air it can take in and the more power it produces. Similarly, a higher VO₂max means the body can use more oxygen and generate more energy.

Physiologically, VO₂max is the maximum amount of oxygen the body can consume in one minute during intense exercise. It is usually expressed in milliliters of oxygen per kilogram of body weight per minute (ml/kg/min), which allows comparisons between individuals of different sizes. It is one of the most reliable indicators of aerobic fitness and endurance.

Behind this single number stands a whole orchestra of bodily adaptations. The heart is the main engine: with regular training the left ventricle, which pumps blood into the body, enlarges. Each beat therefore becomes stronger, so the heart can pump more blood with each stroke and thus beat more slowly at rest. This is known as an increased stroke volume. The lungs act like precise gas exchangers: training improves their ability to transfer oxygen into the blood and remove carbon dioxide. At the same time, on the level of the muscles, the network of tiny capillaries expands, allowing for better oxygen delivery to cells. In healthy recreational runners, performance is usually more limited by cardiovascular delivery and peripheral oxygen use than by lung diffusion. Taken together, this means the body can deliver more oxygen to working muscles—and sustain their energy supply for longer.

The runner feels this directly. Because their “aerobic engine” is more efficient, they can run faster without immediately hitting the lactate threshold we discussed earlier. That is why monitoring VO₂max is not just a motivational number on a watch screen but evidence that the heart, lungs, and muscles are becoming more coordinated and powerful. It is important to remember, though, that modern smartwatches estimate VO₂max using algorithms, while the exact value can only be determined in a laboratory test. Estimates from watches are most reliable during longer, steady runs on flat terrain; hills, wind, altitude, and fatigue can distort the calculations. Still, the watch’s estimate is an excellent indicator of progress in running.

VO₂max values vary widely between people. In an average, untrained man around the age of thirty, it ranges from 40 to 45 ml/kg/min; in women, from 30 to 35 ml/kg/min. With regular, moderately intense training, these values can rise by 15 to 20 percent. At the other extreme, elite endurance athletes—marathoners, cross-country skiers, or cyclists—reach astonishing figures that exceed 80 ml/kg/min in men and 70 ml/kg/min in women. For perspective: top cyclists have estimated VO₂max values above 80, placing them among the very best in the world. Such achievements are not only a gift of genetics but also the result of decades of deliberate, specialized training.

It is also important to note that VO₂max gradually declines with age—after the age of 25, by an average of about 5–10 percent per decade. This is a natural part of aging, linked to changes in the heart and blood vessels. The good news is that regular physical activity greatly slows this decline. In other words: although age inevitably plays its part, training ensures that the aerobic engine can remain strong and reliable for a long time. Recreational marathoner Jeannie Rice, who runs marathons in just over three and a half hours, was measured at age 76 with a very high VO₂max (≈48 ml/kg/min; source)—the highest recorded for a woman over 75. That value is comparable to the VO₂max of a well-trained 25-year-old. Such examples show that with proper training, and likely favorable genetics, it is possible to maintain physical performance in later life at levels typical of people several decades younger.

Still, we must also keep in mind that two runners with the same VO₂max may run at different speeds due to differences in their running economy (how much oxygen an individual uses at a given speed) and their critical speed (the highest sustainable speed without progressive fatigue). VO₂max is therefore a very important measure of aerobic fitness, but not the only one. It reveals the size of our aerobic engine, but how fast and how far we can run also depends on how efficient that engine is and how we use it.

Practical Tips for Running Training

Understanding how the body works allows us to plan training more effectively. Instead of always running at the same pace, we can divide workouts into three key zones, each targeting different physiological systems. The largest portion of training should be dedicated to easy runs, when you can still chat comfortably. In this zone the body primarily uses fat for energy, while over time we build our basic endurance and strengthen the cardiovascular system.

When we want to improve speed, we include runs around the lactate threshold. These are longer segments at higher intensity, where talking becomes difficult and we begin to feel the onset of a burning sensation in the muscles. The goal of these runs is to shift the threshold upward, so that in the future we can maintain faster paces with less effort.

For developing maximum speed and power, interval runs are key. These are very short, high-intensity bursts that the body can sustain only for a few seconds to a few minutes. They help improve our maximal aerobic capacity, as reflected in VO₂max. Each of these three zones targets a different energy system, but together they form the complete picture—systems that the body continuously intertwines and coordinates during running.

Listen to Your Body with the Help of Technology

When we look into the physiology of running, it becomes clear that every step is not just a mechanical movement but part of a carefully coordinated process. The body knows how to switch between different energy sources, adapt to stress on the fly, and gradually strengthen through training—whether in a beginner or an experienced marathoner.

Modern technology is not just a timer or distance tracker but a tool that reveals what is happening inside the body. Metrics such as VO₂max or heart rate show the changes brought about by training: the heart grows stronger, the lungs become more efficient, and the muscles are better supplied with oxygen. If the heart rate is lower at the same pace, it means the body is doing the same work with less effort.

When runners understand these signals, they can more easily adapt their training—from fast intervals to slower long-distance runs. This approach brings not only progress but also greater safety and reduced risk of overuse injuries. The greatest advantage of understanding physiology, however, is that running becomes more than a sport: it turns into a healthy and lasting habit that supports the body over the long term. In the end, what matters in running is not only how fast we cover the kilometers, but also understanding why we are getting better with each passing day.

Source:

Kenney, William L., Jack H. Wilmore, and David L. Costill. Physiology of Sport and Exercise. 7th ed. Champaign, IL: Human Kinetics, 2020.

Translated from the Slovene original, available here: Kaj se dogaja v telesu med tekom?