Fatigue Resistance in Ultrarunning: Why Some Runners Keep Their Legs Late and Others Fade

Fatigue resistance, often called durability in exercise physiology, matters to ultrarunners because most races are decided not by your fresh legs, but by how much of your ability is still there after three, six, or ten hours of running.

Most runners understand fitness in the fresh state. They know their 10K pace, their threshold effort, maybe even their lab-tested VO2 max, which is the maximal rate at which the body can use oxygen during exercise. Those measures matter. But ultrarunning asks a different question. What happens to your pace, mechanics, fuel use, and decision-making after prolonged work? Two runners can look similar in the first hour and become completely different athletes by hour six.

That difference is what coaches and researchers are increasingly trying to describe with the word durability. In practical terms, durability means the ability to preserve physiological function and performance as exercise duration increases. A durable runner does not necessarily start faster. They simply lose less. Their easy pace stays easy longer. Their climbing stride holds together. Their heart rate does not rise as sharply at the same output. Their quads keep producing force. Their fueling plan still works because the body is not falling apart around it.

This is not one single trait. It is an umbrella concept that sits on top of several systems that fatigue at different rates and for different reasons.

One piece is metabolic. As long exercise continues, muscle glycogen, which is stored carbohydrate in the muscle, gradually declines. Glycogen is not the only fuel during an ultra, but it is a critical one, especially when terrain changes, climbs steepen, or you need to respond to surges. As glycogen falls, the body relies more heavily on fat oxidation, meaning the use of fat as fuel. Fat is an enormous energy store, but it is slower to turn into usable energy at high outputs. That is one reason late-race pace often drops even when an athlete is still moving steadily and still eating. The body can continue, but not at the same speed.

Another piece is neuromuscular. Neuromuscular fatigue refers to a reduction in the nervous system’s ability to activate muscles fully, or in the muscles’ ability to respond with force. Downhills, uneven terrain, and long hours of repeated footstrikes create microscopic damage and disrupt how force is produced and coordinated. A fresh runner may have enough aerobic capacity to run a climb smoothly, but late in a race the limiter can become local muscular function. The lungs are fine, the heart is working, but the legs cannot express the same output.

A third piece is cardiovascular. During prolonged exercise, especially in heat, heart rate often rises over time even if pace stays the same. This is called cardiovascular drift, or cardiac drift. Part of it comes from rising body temperature and fluid shifts. Part comes from the body having to work harder to maintain blood flow to both skin and muscle. If heart rate must climb to support the same pace, then the cost of holding that pace goes up. Over hours, that matters.

A fourth piece is biomechanical. Running economy is the oxygen cost of moving at a given speed. If you need more oxygen to run the same pace, your economy has worsened. Researchers have shown that economy can deteriorate during prolonged exercise. The cause is not one thing. It can reflect muscle damage, altered stiffness in the tendon-muscle system, reduced force from fatigued muscle fibers, and subtle changes in coordination. Late in an ultra, a runner may still be fit enough in theory to run a certain pace, but the movement itself has become more expensive.

This is why fatigue resistance deserves its own attention. Traditional performance metrics measured fresh do not fully capture what happens after prolonged work. In recent work in exercise physiology, durability has been described as the change in physiological profiling characteristics over time during prolonged exercise. In simple language, it asks how much your key numbers fall apart as the work accumulates. That could mean your lactate threshold pace drops, your heart rate rises at the same pace, your economy worsens, or your maximal sprint power disappears after several hours.

For ultrarunners, this idea matches experience. The race is not a static test. It is a moving target. Your capacity at mile 5 is not your capacity at mile 45.

So what determines fatigue resistance?

Training volume is a major factor. A larger aerobic base supports many of the systems that help preserve performance over time. More capillaries, which are the tiny blood vessels supplying working muscle, improve oxygen delivery. More mitochondria, which are the structures in cells that generate usable energy, improve the muscles’ ability to sustain aerobic metabolism. Better fuel handling improves the ability to spare glycogen at a given pace. None of this is glamorous, but it changes the durability equation. If a pace requires less relative effort, it can stay available longer.

Long runs matter for the same reason, but not because they are magical. They are specific exposure to the problem. Long runs teach the body and brain what prolonged running feels like. They stress fuel management, muscle recruitment, connective tissue tolerance, and pacing discipline. They also reveal where an athlete is fragile. Some runners are metabolically limited, meaning they run out of available carbohydrate too quickly. Some are structurally limited, meaning the quads or calves become the bottleneck. Some are limited by heat management and hydration. The long run makes those failure modes visible.

There is also evidence that both lower-intensity and higher-intensity endurance training can improve durability. That makes sense physiologically. Lower-intensity volume supports mitochondrial adaptations, capillary density, and fat oxidation. Higher-intensity work can raise the ceiling, improve lactate clearance, and increase the speed or power that an athlete can sustain before carbohydrate demand becomes overwhelming. If your threshold pace improves, then a moderate ultrarunning pace sits farther below it. The race intensity becomes cheaper.

That point is easy to miss. Fatigue resistance is not built only by shuffling for hours. It is built by making the aerobic system more robust and by making race pace a smaller fraction of your maximum sustainable output. For an ultrarunner, tempo work, hill work, and threshold sessions can all contribute if they are placed appropriately and supported by enough easy volume.

Strength training is part of the picture too, especially in hilly or technical ultras. A stronger runner generally uses a smaller percentage of their available force with each step. If every footstrike costs less relative muscular effort, the legs tend to hold up longer. Strength work can also improve tendon stiffness and force transmission, which can support running economy. It does not replace running-specific training, but it can reduce the rate at which form and force deteriorate.

Fueling deserves direct discussion because many runners use fatigue resistance as a catch-all term for underfueling. Sometimes that is accurate, sometimes not. If you take in too little carbohydrate during a long run or race, especially once intensity rises or the terrain gets harder, glycogen depletion becomes a much bigger problem. Late-race fade then looks like poor durability when in fact part of the issue was that the system ran low on fast fuel. For many ultras, carbohydrate intake in the range of roughly 60 to 90 grams per hour is common, and some athletes can tolerate more with training. The right number depends on intensity, gut training, product choice, and event duration. The key point is that physiological durability and nutritional execution interact. You cannot train your way out of a fueling error on race day.

Heat has a similar effect. A runner who looks highly durable in cool weather can appear fragile in warm conditions because heat amplifies cardiovascular strain, fluid loss, and perceived effort. Plasma volume, which is the liquid part of blood, tends to expand with heat acclimation. That helps support stroke volume, meaning the amount of blood pumped by the heart per beat, and can reduce the strain of exercising in hot conditions. Heat adaptation therefore does not just make heat more tolerable. It can improve the ability to preserve output in the heat, which is one form of fatigue resistance.

This helps explain a common training experience. A runner may complete a four-hour long run at a stable heart rate and even pace in February, then struggle to keep the same relationship between effort and pace in June. That is not always a loss of fitness. Sometimes it is the environment changing the durability demand.

The same is true of terrain. Downhill-heavy races often produce dramatic late-race muscle damage, especially in runners who have not prepared eccentrically. Eccentric muscle actions occur when a muscle lengthens while producing force, such as the quadriceps controlling descent on downhills. Those contractions are particularly damaging when the athlete is unaccustomed to them. A runner may have plenty of aerobic fitness and still implode because the legs were not prepared for the specific muscular demand.

Researchers are still working on how best to measure durability. There is not yet one universally accepted field test. Some studies compare physiological markers before and after several hours of steady exercise. Some look at how thresholds or economy shift over time. Some use decoupling between heart rate and pace or power, which means the relationship between internal effort and external output starts to drift. All of these methods are trying to capture the same basic reality, which is that prolonged exercise changes the athlete.

For the runner training without a lab, the useful question is simpler. What degrades first when duration accumulates?

Does heart rate climb while pace falls, suggesting increasing cardiovascular and thermal strain? Do your legs become unresponsive on climbs, suggesting local muscular fatigue? Does your form collapse on descents, suggesting eccentric weakness and neuromuscular breakdown? Do you feel fine until fueling starts to slip, then unravel quickly, suggesting a major metabolic contribution? Those observations are not as clean as a lab protocol, but they are actionable.

That actionability is what matters most. If you know what kind of fade you are dealing with, your training can become more precise.

If the issue is general aerobic durability, the answer is usually more consistent volume and a progression of long runs that are hard enough to be specific, but not so hard that they wreck the following week. If the issue is that race pace sits too close to threshold, then improving threshold and overall aerobic power can make the same pace more sustainable. If the limiter is muscular, then hill work, downhill practice, and strength training may matter as much as another hour of easy running. If the limiter appears in heat, heat acclimation and revised pacing may be more important than trying to force the same splits. If the issue shows up when fueling gets off schedule, gut training and carbohydrate planning belong in the center of the plan, not as an afterthought.

This is also why the best ultrarunners often look almost conservative early. They are not just saving energy in a vague sense. They are choosing an intensity that their current durability can support after all the predictable losses of the day are accounted for. They know that pace is not only about what feels manageable now. It is about what remains manageable after glycogen falls, temperature rises, muscles stiffen, and mechanics get a little more expensive.

For a motivated amateur runner, the most useful shift is to stop treating late-race fade as a mystery or a character flaw. It is physiology. Specific systems are degrading under prolonged stress. The goal of training is not to become immune to fatigue. That will not happen. The goal is to become the athlete whose key systems deteriorate more slowly, whose movement stays economical longer, and whose fueling and pacing preserve more of what is available. That is fatigue resistance, and in ultrarunning it is often the trait that matters most when the race finally begins.

Written by Wade Wegner. Train Ultra is a private AI coach that reads every workout you post to Strava. Try it free.