The Physiology of Endurance
VO2 Max, Lactate Threshold, and What Limits Human Endurance
- 8 chapters
- 45m
- Fitness Training
- Free · no sign-up
This guide explains exercise physiology basics and how VO2 max determines your aerobic capacity. It covers cardiovascular fitness, endurance training methods, physical fitness principles, interval training techniques, high-intensity interval training protocols, and altitude training effects. Each chapter builds on the previous one to explain how your body adapts to different training stimuli.
Whether you're a runner, cyclist, or triathlete looking to understand what limits your performance, this audiobook provides the physiological foundation for better training decisions.
Listen
-
Read this chapter
Overview
Exercise physiology is the study of how the body responds to physical activity, and it falls under the category of allied health professions. Professionals in this field, known as exercise physiologists, use their education and expertise to help manage injuries and chronic conditions through lifestyle changes and targeted exercise. They examine how muscles, the heart, and the nervous system adapt during and after exercise, and how these changes affect overall strength and endurance. The effects of training are understood as the body’s adaptive reaction to increased metabolic demands. These experts also investigate how physical activity can influence disease progression, offering insights into both healing and prevention.
History
In 1922, British physiologist Archibald Hill introduced the ideas of maximal oxygen uptake and oxygen debt. He shared the Nobel Prize in Physiology or Medicine that year with German physician Otto Meyerhof for their work on muscle energy metabolism. Scientists soon began measuring oxygen use during exercise, building on this foundation. Henry Taylor at the University of Minnesota made early contributions, while Per-Olof Åstrand and Bengt Saltin in Scandinavia, the Harvard Fatigue Laboratory, German universities, and the Copenhagen Muscle Research Centre all added to the field in the 1950s and 60s. In some places, accredited exercise physiologists—university-trained professionals who prescribe specific exercise interventions—are now part of primary health care.
Energy expenditure
The human body can sustain high energy expenditure for hours, as shown by someone who cycled 8,204 kilometers over 50 days at 26.4 kilometers per hour, burning 1,145 megajoules of energy with an average power output of 173.8 watts. Skeletal muscle uses glucose at a rate of 90 milligrams per minute during continuous activity like knee extension, producing about 24 watts of mechanical energy—though the conversion is only 22 to 26% efficient, meaning roughly 76 watts of heat are also generated. Resting muscle consumes just 0.63 watts per kilogram, a difference of 160 times less than when active. During short bursts, such as jumping from a squat, an adult male can generate up to 314 watts per kilogram; some nonhuman animals like bonobos and small lizards can produce twice that. Compared to the typical resting metabolic rate of 45 to 85 watts in adults, physical exertion dominates energy use, especially during prolonged activity. Total daily energy expenditure depends on how much someone moves, and it correlates strongly with gender, age, weight, heart rate, and VO2 max.
Rapid energy sources
When your body needs energy for short, intense bursts of activity, it turns to anaerobic metabolism happening right in the muscle cells' cytosol, not aerobic respiration which uses oxygen and occurs in the mitochondria. The fastest energy source is the phosphocreatine system, powered by the enzyme creatine kinase, combining phosphocreatine and ADP to make ATP and creatine. This supply runs out quickly—between ten and thirty seconds—because the body can’t replenish phosphocreatine without oxygen. Another rapid system is fast glycolysis, which uses intracellular glycogen, broken down by glycogen phosphorylase into glucose, then turned into pyruvate and finally lactic acid under anaerobic conditions. This process builds up hydrogen ions, causing acidosis and limiting how long it can keep going—about two minutes before fatigue sets in.
Plasma glucose
During exercise, blood glucose levels are kept steady when the rate of glucose entering the bloodstream equals the rate it's removed. This balance usually holds during moderate activity, but intense or long-lasting exercise can tip the scale, causing glucose levels to drop and fatigue to set in. The liver plays a key role, releasing glucose from stored glycogen through glycogenolysis and making new glucose via gluconeogenesis. Unlike muscle cells, liver cells can release glucose because they contain the enzyme glycogen phosphatase. Glucose disposal mainly happens when working muscles absorb it, increasing uptake even as insulin falls. Hormones like glucagon, epinephrine, and growth hormone boost liver glucose output and help spare glucose by promoting fat burning. For people with diabetes, exercise helps lower blood sugar independently of insulin and boosts insulin sensitivity for up to a day after. Weight loss through activity and diet can improve insulin response significantly, sometimes enough to normalize glucose control, though weight regain often brings symptoms back.
Oxygen
When you push your body hard through exercise or labor, it needs more oxygen, and your heart rate, breathing rate, and depth of breathing all jump to meet that demand. Oxygen use during activity is measured by the Fick Equation: VO2 equals cardiac output times the difference in oxygen between arterial and venous blood. That means how much oxygen you use depends on how much blood your heart pumps and how well your muscles pull oxygen from that blood. But it's not just about the heart—factors like lung function also matter. Conditions such as diffusion limitation or ventilation/perfusion mismatch can block oxygen from getting into the blood. The blood's ability to carry oxygen is another key factor, which is why some endurance athletes use methods like blood doping or erythropoietin to raise their red blood cell count. Also important is how blood flows during exercise—redirecting from organs to muscles—and how capillaries are distributed within the muscle itself.
Dehydration
Dehydration harms aerobic endurance by raising body temperature, heart rate, and perceived exertion, with even modest 2% losses impairing performance in hot conditions. Pre-exercise dehydration effects vary by cause; diuretics or sauna use reduce plasma volume more than prior exercise. While dehydration clearly lowers aerobic endurance, its impact on muscle strength remains unclear. During intense exercise, sweat cools the body, with male marathon runners losing 0.83 liters per hour in cool weather and up to 1.2 liters in warmth—females lose roughly 68-73% less. Heavy exercisers may sweat two and a half times more fluid than urine production. Cycling two hours in 35°C heat with minimal intake causes 3-5% body mass drop, reduced blood volume, rising temperature, and increased heart rate. These effects are largely reversed by replacing 50-80% of lost sweat.
Brain
The human brain consumes about 20% of the body’s energy even at rest, relying entirely on oxygen to fuel its activity. It receives roughly 15% of the heart's output, which means any interruption in oxygen supply can quickly lead to loss of consciousness within seconds, with brain activity ceasing entirely in around 23 seconds. During intense physical effort, especially for someone who walks upright, the brain must work harder to manage balance and movement, increasing its energy needs. Because of this, maintaining steady blood flow and fuel to the brain during exercise is essential. Exercise physiologists help treat a variety of conditions affecting the nervous system, including disorders that impact motor control and cognition.
-
Read this chapter
Overview
VO2 max measures the highest rate your body can use oxygen during intense exercise, with terms V̇O2 max showing volume of oxygen consumed per kilogram of body weight. It differs from V̇O2 peak, which occurs during submaximal workouts and is usually lower than true VO2 max. This measurement reflects how well your heart, lungs, and muscles work together during endurance activities. Athletes like distance runners, cyclists, and cross-country skiers can reach values over 90 mL/(kg·min), while some animals, such as Alaskan huskies, exceed 200 mL/(kg·min). In training, VO2 max helps set exercise intensity levels; for example, 65% of VO2 max is seen as a sustainable threshold, more precise than heart rate but harder to measure.
Normalization per body mass
VO2 max can be shown as either an absolute amount, like liters of oxygen per minute, or as a relative amount, such as milliliters per kilogram of body mass per minute. The relative measure is often used when comparing athletes. But because VO2 max doesn’t increase in a straight line with body size—whether among individuals of the same species or across different species—direct comparisons between people or animals of different sizes need special statistical methods, like analysis of covariance, to be accurate.
Measurement
Measuring VO2 max means pushing your body hard enough to really test its aerobic limits. In a typical test, you exercise on a treadmill or cycle ergometer while researchers track how much air you breathe in and out, as well as the oxygen and carbon dioxide levels. This kind of test is called a cardiopulmonary exercise test, or CPX. For untrained people, cycling tends to give results about 10% to 20% lower than running, but trained cyclists often perform equally well or better on a bike. The classic definition of VO2 max comes from Hill and Lupton in 1923, and it’s reached when oxygen use stays steady even as workload increases—though that plateau doesn’t always show up clearly, which can lead to different results depending on the testing method.
Fick equation
VO2 max can be calculated using something called the Fick equation, which looks at how much oxygen your body uses during intense exercise. It’s expressed as V̇O2 equals cardiac output times the difference between oxygen in the arteries and veins. This difference is known as the arteriovenous oxygen difference. The formula helps measure oxygen consumption during maximal effort, but it's not very reliable for measuring cardiac output in critically ill patients, even when they're at rest. Still, using breath-based methods to estimate cardiac output does seem to work better.
The heart rate ratio method
An estimate of VO2 max can be made using maximum and resting heart rates, based on a formula from Uth et al. (2004). It’s calculated by dividing maximum heart rate by resting heart rate and multiplying that ratio by 15.3 milliliters per kilogram per minute. This equation was developed using data from well-trained men aged 21 to 51, so it may not apply to other groups. Researchers warned that the formula works best when maximum heart rate is actually measured rather than estimated. Later studies, like Voutilainen et al. (2020), adjusted this constant for different populations. For example, in around 40-year-old men without health issues or smoking habits, the factor should be 14 instead of 15.3. Each decade of age, as well as changes in weight or smoking status, affects the coefficient. So a 60-year-old obese smoker would use a factor of 10.
Cooper test
Kenneth H. Cooper developed a method for the United States Air Force during the late 1960s that became known as the Cooper test. Participants run for twelve minutes, and the distance covered determines their estimated VO2 max. One equation calculates this value in milliliters of oxygen per kilogram of body weight per minute using meters, while another uses miles to arrive at the same result. Both formulas allow scientists to predict aerobic capacity based on how far someone can run in that time.
Rockport fitness walking test
To estimate VO2 max, you can use a timed one-mile walk on a track, noting the time in decimal minutes—like 20.58 for 20 minutes and 35 seconds. You’ll also need your sex, age, body weight in pounds, and your heart rate at the end of the walk. For males, there's a constant value of 6.3150 added into the calculation; for females, that number is zero. The formula uses all these inputs to predict your maximum oxygen uptake. The correlation coefficient for this method is 0.88, meaning it’s fairly reliable.
Non-athletes
The average untrained healthy male has a V̇O2 max of around 35 to 40 milliliters of oxygen per kilogram of body weight per minute. For the average untrained healthy female, that number falls between 27 and 31 milliliters per kilogram per minute. These values can increase with training and tend to decline as people age, though how much someone can improve varies quite a bit from person to person.
-
Read this chapter
Overview
Cardiovascular fitness is part of overall physical health, and it’s about how well your heart and blood vessels deliver oxygen to your muscles during activity. This kind of fitness comes from regular endurance training and depends on several key factors like how much blood your heart pumps each minute, how open your blood vessels are, and your body’s ability to use oxygen, known as VO2 max. Being fit in this way lowers your risk of heart disease, helps your brain function better, and can even help you live longer. A study in the American Journal of Epidemiology showed that people with higher cardiovascular fitness had a reduced risk of dying from any cause, including cancer.
Physiological basis of fitness
Cardiovascular fitness hinges on how well your circulatory system delivers oxygen to your tissues during aerobic activity. As this capacity improves, your metabolism rises, muscles work more efficiently, haemoglobin increases, and your blood's buffering ability grows. Venous return and stroke volume also improve, allowing your heart to adapt quickly to changing demands. All of these changes boost muscular endurance and indirectly support strength and flexibility. To build aerobic capacity, you must engage in activities that raise your heart rate over time—like cardio workouts. A 2005 Cochrane review confirmed that physical activity effectively enhances cardiovascular fitness. The American Heart Association recommends at least 150 minutes of moderate exercise or 75 minutes of vigorous exercise weekly. A study in the European Journal of Preventive Cardiology found that high-intensity workouts lead to greater improvements than moderate ones.
Assessing cardiovascular fitness
Cardiovascular fitness can be measured in several ways, with one of the most important being maximal oxygen uptake, or V̇O2max. This refers to the highest amount of oxygen your body can use during intense exercise. Scientists also look at certain biomarkers to track how well your cardiovascular system is improving. These include markers related to blood fats, inflammation, how your body handles glucose, and blood clotting. Monitoring these levels helps show progress as someone builds better heart and lung function through training.
The role of exercise in cardiovascular fitness
Regular exercise plays a key role in building cardiovascular fitness, and according to the American Heart Association, people should aim for at least 150 minutes of moderate-intensity aerobic activity or 75 minutes of vigorous-intensity exercise each week to see real improvements. This level of physical activity not only helps strengthen the heart but also lowers the risk of developing cardiovascular disease. Whether you're walking briskly or pushing yourself during a high-intensity workout, making time for aerobic movement is essential if you want to boost your heart's performance and protect your overall health.
Cardiovascular changes attributed to aerobic exercise
When you exercise aerobically, your heart pumps more blood and your body uses more oxygen, following the Fick equation: VO2 equals cardiac output times the difference in oxygen between arterial and venous blood. Cardiac output depends on stroke volume—how much blood your heart pushes out with each beat—and heart rate. A common way to estimate maximum heart rate is 220 minus your age. Stroke volume increases because the heart fills more completely and contracts stronger, though if your heart rate gets too high, it can reduce how much blood is pumped out. Regular aerobic exercise strengthens the heart over time, improving both pumping power and oxygen use. As we age, arteries stiffen, contributing to dementia and kidney disease, but exercise can reverse aging—just six months of training can undo four years of arterial decline. Even people with cardiovascular disease benefit from exercise, showing how important it is for heart health. Research by Barry et al. found that those with low fitness levels had twice the risk of dying early compared to fit individuals, no matter their weight or other factors.
Prescribing exercise: type, dosing, and adverse effects
For people with heart conditions, moderate-intensity continuous exercise remains the standard, though high-intensity interval training can provide better heart and lung benefits. The Physical Activity Federal Guidelines recommend 150 minutes of moderate or 75 minutes of vigorous aerobic activity weekly, but more than half of all adults fall short of these levels. Even small amounts—like less than six miles of running a week—can greatly reduce risk of death from any cause or heart disease. Adding resistance training helps by improving muscle strength, lowering cardiovascular risks, boosting insulin sensitivity, and reducing atherosclerosis. It's recommended to do resistance exercise twice a week for at least 15 to 20 minutes, especially for older adults and those with heart failure. However, too much endurance training can damage the heart, raising injury markers, enlarging heart chambers, and weakening the right ventricle. Over time, this may lead to harmful changes, scarring, and higher chance of irregular heartbeats—especially atrial fibrillation. The ideal amount seems to be less than 30 miles of running or 46 miles of walking per week; going beyond that might reduce heart benefits. Still, the biggest public health issue remains that most people don't get enough exercise at all.
-
Read this chapter
Overview
Endurance training means exercising with the goal of building stamina, and it focuses on improving the aerobic system rather than the anaerobic one. While people often think of endurance in terms of heart health and muscle strength, the concept is actually more complex. Endurance can be split into general and specific types, and it’s closely connected to how well someone performs skills and techniques. A fit athlete is someone who can carry out their movements consistently and efficiently without using too much energy. To measure this kind of fitness, trainers look at heart rate, power output during cycling, or running pace.
Endurance in sports
Endurance training plays a key role in many sports, from distance running—starting at 800 meters and going all the way up to marathons and ultra-marathons—through cycling, especially road racing, and competitive swimming. These three disciplines come together in triathlon. Rowing and cross country skiing also rely heavily on endurance fitness. Even sports that aren’t purely endurance-based still benefit from it; racket sports, football, rugby, martial arts, basketball, and cricket all require some level of aerobic stamina. Beyond elite competition, people turn to endurance exercise for general health or to burn more calories and support weight loss goals.
Physiological effects
Endurance training works through a process called supercompensation, where your muscles adapt and grow stronger after being stressed. Over time, long-term training causes changes in both the heart and the muscles. Your heart rate goes down while its ability to pump blood increases, and your body’s capacity to use oxygen improves. Mitochondria, the energy factories in your cells, become more efficient, with enzymes like succinate dehydrogenase increasing by two and a half times. Muscle tissue also changes, with myoglobin — the protein that stores oxygen — rising by 75 to 80 percent in well-trained athletes. These are the key adaptations that make you more enduring.
Risks of excessive endurance training
Long-term, high-volume endurance training has been linked to serious health risks, according to recent studies. These dangers are most commonly seen in people who train for or compete in extreme endurance events. The main concerns involve changes to the heart and blood vessels, including abnormal heart rhythms. Overtraining can also lead to lower testosterone levels in athletes.
Methods and training plans
Endurance training involves several methods, including periodization, intervals, hard easy, long slow distance, and high-intensity interval training. The periodization method, credited to Tudor Bompa, organizes training into blocks that typically last between four and twelve weeks. Historically, strength training wasn’t considered suitable for endurance athletes because it was believed to interfere with adaptations from endurance work and could lead to increased body mass that might hurt performance. But recent research shows that adding short-term (eight-week) strength training alongside endurance training can improve long-distance running performance. Literature describes various forms of endurance exercise.
Devices to assess endurance fitness
Heart rate monitors are one of the simpler ways to track fitness gains in endurance athletes. When heart rate goes down during running or cycling at a steady speed, it shows improvement. In cycling, though, wind can mess with speed, so many riders now use power meters built into their bikes. These meters measure actual power output over time, making it easier to compare progress. During the 2008 Olympics, Michael Phelps used repeated lactate threshold tests to help his coaches adjust his training. That allowed him to recover between swimming events that were only minutes apart. While blood glucose monitoring is routine for diabetics, similar lactate testing is still mostly used by elite athletes and their professional coaches, even though less expensive devices are now available.
-
Read this chapter
Overview
Physical fitness is more than just being able to move well—it's about health, strength, and the ability to handle whatever life throws at you, whether that’s work, play, or daily tasks. In the past, before the Industrial Revolution, fitness meant simply having enough energy to get through the day without feeling tired. Today, it’s much more than that. It includes being able to perform efficiently in both work and leisure, staying healthy, fighting disease, boosting your immune system, and even reacting quickly in emergencies. Achieving this kind of fitness comes from eating well, exercising regularly, getting enough rest, and having a plan to recover properly after hard efforts.
Overview
Fitness, defined as the quality or state of being fit and healthy, saw a tenfold rise in usage in western language around 1950, possibly linked to the Industrial Revolution and World War II. Today, fitness describes either a person’s ability to perform specific tasks or their overall adaptability. It’s often tied to aerobic or anaerobic capacity—endurance or strength—and a well-rounded program improves all areas rather than just cardio or weight training. A good fitness plan considers age, health needs like bone strength, and includes mental, emotional, and social well-being. Physical activity helps prevent disease, supports recovery, and boosts mental health by reducing anxiety and depression. Research shows that muscles act as endocrine organs during exercise, releasing myokines that aid tissue repair, reduce inflammation, and lower the risk of chronic conditions.
Activity guidelines
The 2018 Physical Activity Guidelines for Americans, issued by the U.S. Department of Health and Human Services, offer science-based advice for people aged three and up to boost their health through regular movement. These guidelines say adults should aim to move more and sit less to enhance mental, emotional, and physical well-being. For significant health gains, adults are advised to complete 150 to 300 minutes of moderate-intensity activity or 75 to 150 minutes of vigorous-intensity aerobic activity weekly, or a mix of both, spread across the week. Importantly, the guidelines no longer require these activities in bouts of at least 10 minutes, as new research shows even shorter periods help. Doing more than 300 minutes of moderate-intensity exercise each week can bring added benefits. Adults should also engage in muscle-strengthening activities involving all major muscle groups at least two days a week. The United Kingdom's 2011 guidelines echo similar ideas, emphasizing that only moderate- to vigorous-intensity activity—ones that raise heart rate and cause sweating—provide real health gains. They recommend at least 150 minutes of such activity weekly, with more being better. These UK guidelines also warn that sitting too much is harmful, and no amount of exercise can fully counteract the risks of prolonged inactivity.
Recovery
Recovery is a core part of physical fitness, happening in moments or over time, through active or passive means. Immediate recovery follows exertion—like a runner regaining breath after a step or a boxer recovering after a punch—and helps prevent fatigue and loss of balance, showing fitness. Interval training uses this idea too, alternating intense effort with lighter activity or rest, allowing more high-intensity work overall. Short-term recovery involves lighter sessions between harder ones, such as running on Monday, biking on Wednesday, then running again on Friday, known as the hard/easy rule. Long-term recovery happens over days, weeks, or months, letting muscles heal and grow stronger, with sleep and diet playing key roles. As fitness improves, the body becomes more efficient at long-term recovery.
In sport
In sports, one of the most crucial skills an athlete can master is the ability to recover quickly and consciously during competition. This rapid recovery must be woven into their specific sport techniques to ensure peak performance. A tennis player, for instance, needs to regain control after each stroke, while a rugby player must do the same after every tackle. Top-level athletes usually stand out because they can recover faster and more efficiently than others. Integrating this skill with their game-specific movements leads to better overall results. The key is not just physical strength but also mental readiness to reset between efforts. This controlled recovery allows them to maintain high performance levels throughout a match or training session. It's a critical edge that separates good athletes from great ones.
High intensity interval training
High-intensity interval training, or HIIT, involves alternating short bursts of intense exercise with periods of rest or low-effort activity. These workouts can be as brief as ten minutes, making them a time-efficient option for people who struggle to find time for longer sessions. Research shows that HIIT delivers greater cardiac benefits than moderate or light exercise, pushing the body to work harder to restore oxygen levels. It's also highly effective for fat loss, especially in the abdominal area, and burns more calories during and after the session compared to steady-state workouts. Because of its intensity and efficiency, HIIT offers a powerful alternative for those looking to maximize their fitness gains in less time.
Controlling blood pressure
Physical fitness plays a key role in managing blood pressure, and regular exercise strengthens the heart, which is central to controlling both systolic and diastolic readings. When you're active, your blood pressure rises, but as your body gets fitter, this process becomes more efficient. The more you exercise, the easier it is for your cardiovascular system to handle those changes. Over time, this leads to a healthier heart profile and lower overall blood pressure. Through consistent physical activity, your body learns to respond better to increases in pressure, which reduces the strain on your arteries. As a result, your blood pressure stays more stable and controlled.
Inflammation
Studies have shown that doing more physical activity is linked to less inflammation in the body. When you exercise, your body reacts with a short-term inflammatory response, but over time it leads to a reduction in inflammation. This effect happens whether or not your body weight changes. Still, scientists don’t fully understand how physical activity connects to inflammation.
-
Read this chapter
Overview
Interval training is a method that mixes high-intensity exercise with periods of rest or lower activity, pushing the body close to its anaerobic limit during work phases and allowing recovery in between. This kind of workout strengthens the heart, boosts aerobic capacity, and helps people exercise longer or at higher intensities. It’s used across many sports, especially running, and can include different styles like pyramid intervals, hill repeats, or staircase runs. Soichi Sakamoto, who coached the University of Hawaii, was an early supporter of this approach for swimmers, and James Counsilman, a coach with a Doctorate in Exercise Physiology, also advanced its use with his team.
Fartlek training
Fartlek training, developed in Sweden, blends interval training with regular distance running. The name means "speed play," and it involves running at a steady pace with irregular bursts of harder effort. A typical session might start with a 5- to 10-minute warm-up, followed by 2 km at a hard but controlled speed. Then comes 5 minutes of rapid walking for recovery, followed by sprints lasting 50 to 60 seconds mixed in with easy running. The workout includes a full-speed uphill climb for 200 meters, another minute of rapid walking, and then the whole routine repeats until the session reaches at least 45 minutes. This method builds both aerobic and anaerobic fitness while adapting to specific sport demands, much like other interval training styles.
Walk-back sprinting
Walk-back sprinting is a kind of interval training runners use to build speed and finish strong. In this workout, you sprint a short distance—between 100 and 800 meters—then walk back to where you started for recovery. You repeat this cycle several times. To make it harder, you can time your sprints so that each one starts at set intervals, like every three minutes. That way, you get just enough rest between efforts. The goal is to improve your race speed and give yourself a powerful kick at the end.
High-intensity interval training
High-intensity interval training works by pushing you harder during shorter, more intense bursts, so you can reduce how much total time you spend exercising while still getting the same benefits. These high-effort intervals are broken up by periods of easier activity or rest. Sprint training is a bit different—it uses timed efforts that push your body even further than typical high-intensity work. Some people use the acronym DIRT to keep track of the key parts: D stands for the distance of each speed burst, I is the rest time between bursts, R means how many times you repeat the effort, and T is how long each interval lasts.
Effectiveness
Interval training can boost calorie burn and aerobic capacity faster than steady exercise, with studies showing that four-minute high-intensity sets improve VO2 max more than moderate continuous training in overweight people. Young and healthy individuals see similar benefits to traditional endurance training but in less time. Though research is limited, it suggests older adults and those with heart disease may also benefit. Athletes experience improvements in lactate threshold and VO2 max, along with enhanced fat burning and vascular function. This kind of training may help manage diabetes, obesity, and cardiovascular issues by increasing insulin sensitivity and reducing body fat. It can be more effective for fat loss than moderate exercise, thanks to its metabolic impact.
-
Read this chapter
Overview
High-intensity interval training, or HIIT, alternates short bursts of maximum or near-maximum effort with brief recovery periods until exhaustion, relying heavily on anaerobic energy systems though some aerobic activity occurs. Workouts typically last under thirty minutes, with intervals ranging from twenty to seventy-five seconds depending on fitness level. While HIIT improves athletic capacity and glucose metabolism, it may not be as effective for treating obesity or building muscle mass compared to longer, moderate-intensity training. Sprint interval training, or SIT, is similar but uses "supramaximal" efforts—exercises exceeding VO2 peak—often in shorter bursts. Some researchers note HIIT demands high motivation and question whether most people can safely sustain its intensity. Tabata's regimen, which he called "supermaximal," exemplifies this approach, though he did not label it SIT.
Procedure
HIIT workouts usually start with a warm-up, then alternate between all-out effort and moderate recovery periods, often using a 2:1 work-to-rest ratio like 30 to 40 seconds of sprinting followed by 15 to 20 seconds of walking or jogging. The high-intensity bursts should be near maximum effort, while the recovery is about half that intensity. These sessions can last anywhere from four to 30 minutes and are ideal for people short on time. While most HIIT research has used cycling ergometers, running, rowing, stair climbing, and uphill walking also work. The exact routine varies depending on fitness level, with moderate efforts as slow as walking for some. A timer helps track the rounds and timing. Another version combines 30 minutes of cardio with 30 minutes of resistance training to keep heart rate elevated and boost both strength and endurance.
Peter Coe regimen
In the 1970s, Peter Coe, a coach who was influenced by German professor Woldemar Gerschler and Swedish physiologist Per-Olof Åstrand, used a specific kind of high-intensity training for his son Sebastian Coe. The sessions involved repeated fast 200-meter runs with just 30 seconds of recovery time between each sprint. This method was part of what helped shape Sebastian’s endurance and performance in athletics.
Tabata regimen
In 1996, Professor Izumi Tabata from Ritsumeikan University led a study with Olympic speedskaters that introduced what he called the IE1 protocol. It involved 20 seconds of all-out exercise at about 170% of VO2max, followed by 10 seconds of rest, repeated eight times in four minutes. The athletes did this four days a week, plus one day of steady-state training, over six weeks. Though they exercised far less time than those doing steady-state work five days a week, the HIIT group still improved their aerobic capacity comparably—going from 48 to 55 mL/(kg•min) versus the other group’s rise from 52 to 57 mL/(kg•min). The Tabata method also boosted anaerobic capacity, as it was “supermaximal,” placing it in the SIT category.
Gibala regimen
At McMaster University in Canada, Professor Martin Gibala and his researchers have spent years exploring high-intensity training methods. In a 2010 study with students, they tested what became known as “The Little Method,” which began with three minutes of warm-up, followed by 60 seconds at 95% of VO2max, then 75 seconds of rest, repeated eight to twelve times. Participants trained three times a week on a stationary bike and saw gains comparable to those doing steady-state exercise at 50–70% VO2max five days a week. A year later, in 2011, Gibala’s team introduced a gentler version for people who hadn’t exercised in over a year. This plan started with three minutes of warm-up, included ten 60-second bursts at 60% peak power with one minute of recovery between each, and ended with a five-minute cool-down.
Zuniga regimen
In 2011, Jorge Zuniga, an assistant professor of exercise science at Creighton University, wanted to figure out how to get the most oxygen use and workout time in the least amount of time. He discovered that doing 30-second bursts at 90% of his VO2 max power output, followed by 30 seconds of rest, allowed for the highest oxygen consumption and the longest workout at that intensity. Some alternatives he tested included 100% maximum power output on the same schedule, like the Coe regimen, or 90% power for three minutes, which is more traditional interval training. Zuniga’s protocol was later used successfully by students in Creighton’s Army ROTC program, with cadets doing it twice a week seeing better APFT scores than in previous years. The routine usually involves ten repetitions.
Vollaard regimen
Dr. Niels Vollaard of the University of Stirling discovered that health improvements from high-intensity intervals reach a limit after about two or three all-out efforts. This finding led him to design a 10-minute workout combining easy pedaling with just two 20-second maximum sprints. In a 2017 meta-analysis, Vollaard confirmed that routines using up to ten 30-second all-out bursts don’t enhance aerobic fitness any further than his shorter "2×20-s" approach. He called this method SIT, suggesting it avoids many of the challenges that make other high-intensity protocols difficult for average people. In a BBC Horizon program in February 2012, Professor Jamie Timmons of the University of Loughborough walked Michael Mosley through a version using three sprints instead of two, performed three times weekly for 30 minutes total, including warm-up and recovery time.
Regimen comparison
A study looked at two types of high-intensity training, comparing HIIT with eight one-minute efforts at 85% of maximum Watts, with one minute of easy recovery at 25% Wmax between them, versus SIT using eight 30-second bursts at 130% Wmax and 90 seconds of recovery at 25% Wmax. Both routines lasted the same total time—24 minutes including warm-up and cooldown. The researchers concluded that HIIT was the better choice, but noted that the differences between the two approaches were small. They added that people should pick based on personal preference since the results were very similar.
-
Read this chapter
Overview
Altitude training involves endurance athletes spending weeks at high elevations—ideally above 2,400 meters, though often at intermediate altitudes due to limited access. At these heights, air contains about 20.9% oxygen but lower barometric pressure reduces partial pressure. The body adapts by increasing red blood cell mass and hemoglobin levels or changing muscle metabolism. Athletes may retain elevated red blood cell counts for up to two weeks after returning to sea level, which proponents believe offers performance edge. Some athletes live permanently at high altitudes, only coming down to compete, though their training can be compromised by reduced oxygen availability during workouts. This training can also be simulated using altitude tents, rooms, or mask-based systems that lower oxygen content while keeping barometric pressure constant. Another method, called hypoventilation training, reduces breathing frequency during exercise to decrease oxygen levels in blood and muscles.
Background history
The 1968 Olympics in Mexico City, at an elevation of 2,240 metres, sparked intense interest in how altitude affects athletic performance. During those games, endurance events saw significant below-record finishes, while sprint races broke all kinds of records. Before the games, experts had speculated that the thin air would hurt long-distance running but not significantly impact short, powerful sprints. They believed this was due to less air resistance and the fact that sprinting relies more on anaerobic energy. These observations led to further research into altitude training, which eventually gave rise to specific methods aimed at preventing underperformance at high elevations.
Live-high, train-low
Living high and training low is a method where athletes sleep at high altitudes to boost red blood cell production and VO2 max while training at sea level to maintain intensity. This approach allows adaptations like increased erythropoietin levels without full oxygen reduction during workouts. Studies show mixed results due to individual differences, time spent at altitude, and training type. Athletes doing mainly anaerobic activities may not benefit much since their energy doesn't rely heavily on oxygen. An optimal setup involves living at 2,100–2,500 meters and training below 1,250 meters. Good locations include Mammoth Lakes, Flagstaff, and the Sierra Nevada near Granada. A study found performance improvements persisted even after 18 days of simulated altitude with sea-level training. Critics argue that red blood cell counts return to normal quickly after returning to sea level, and that altitude sickness can reduce training effectiveness. Extreme altitudes above 5,000 meters may also cause muscle loss over time.
Live-high, train-high
When athletes follow a live-high, train-high approach, they spend both their days and training sessions at elevated altitudes. Their bodies are constantly exposed to lower oxygen levels, which causes VO2 max to drop by roughly 7% for every 1000 meters above sea level. This means that during exercise, the body can't take in as much oxygen as it would at lower elevations. As a result, performing any set pace or effort requires working at a higher relative intensity than usual. The ongoing stress of this hypoxic environment pushes the body to adjust and become more efficient over time.
Repeated sprints in hypoxia
In repeated sprints in hypoxia, athletes perform short, all-out sprints under 30 seconds, followed by minimal rest while training in low-oxygen conditions. The rest period is less than four times the sprint duration, meaning for every 30-second effort, there’s less than 120 seconds of recovery. After a four-week training period, those using this method were able to complete more sprints before exhaustion compared to a control group doing the same exercise in normal oxygen levels. The body responds to low oxygen by increasing blood flow and helping muscles recover more quickly. These changes may improve power output and delay fatigue during high-intensity activity. This approach is still being studied, but early results show promise for enhancing performance.
Artificial altitude
Artificial altitude training uses systems that simulate high-altitude conditions without athletes having to travel. In Finland, Heikki Rusko created a “high-altitude house” where people live and sleep in low-oxygen air—about 15.3% oxygen, similar to altitudes used for training—but train at normal oxygen levels outside. This method improved EPO and red-cell counts. Some athletes use simulated altitude for hypoxic exercise, allowing intense workouts at lower intensity, reducing stress on injured muscles. But just exercising in low oxygen doesn't change blood parameters like hemoglobin. Companies like Hypoxico, Inc., which pioneered these systems in the mid-1990s, offer such setups. Alternatively, Neil Stacey from South Africa suggested the opposite: using oxygen enrichment to create a training environment with higher-than-normal oxygen pressure, aiming to boost training intensity.
Principles and mechanisms
Altitude training works because of how air pressure changes as you go higher, making the air thinner and less dense. At sea level, there are more gas molecules in each breath, but at high altitude, those numbers drop, lowering the partial pressures of oxygen and nitrogen in your body. This triggers a range of physiological responses. Researchers debate what the main benefit is. American scientists Ben Levine and Jim Stray-Gundersen say it's mainly due to an increase in red blood cell volume. Others, like Australian Chris Gore and New Zealand’s Will Hopkins, argue that improvements come more from changes in how efficiently your body uses oxygen.
Increased red blood cell volume
At high altitudes, reduced oxygen stabilizes HIF1 protein, prompting kidneys to release more erythropoietin (EPO), which signals bone marrow to produce more red blood cells for better oxygen transport. Some athletes respond strongly while others see little improvement. Scientists don't know how long adaptation takes, as studies vary based on altitude duration. EPO is naturally produced but also made synthetically for treating kidney failure and chemotherapy patients. Over thirty years, some athletes have abused it through blood doping and injections to boost performance. This artificial increase in red blood cells can lead to thickened blood, raising risks of clots, heart attacks, or strokes. Still, altitude training can safely raise EPO levels naturally without crossing into dangerous doping practices.
Read
Free to download, keep and share. For general information only — not professional medical, legal or financial advice. Please consult a qualified professional.