Coaching Athletes: Endurance Training Part 2

Delving into VO2 max (maximal oxygen consumption) and its significance in athletic performance and explore the trainability of different components, such as lung capacity, cardiac output, and capillary density, while suggesting exercise approaches to improve these factors. Additionally, this Part talks about lactate threshold training and the limits of trainability, advocating for a balanced approach that prioritizes an athlete's specific needs and the principle of "use it or lose it."

Coaching Athletes: Endurance Training Part 2

VO2 Max; Lactate Threshold; Limits on Trainability

By: Nick Soleyn, PBC, BLOC Editor in Chief

Read Part One | Read Part Three

Part One of this article discussed VO2 max and lactate threshold, describing their part in limiting athletes’ endurance. VO2 max is the maximum amount of oxygen that an athlete can use during exercise, while lactate threshold is the point at which lactate production exceeds lactate clearance. Both are important for endurance performance, and they can be improved through training. This second Part discusses the factors involved in improving these measures, which will serve as the basis for the specific training approaches of Part Three.

One note: Competitive athletes require a high level of fitness just to handle training, practice, and a competitive schedule in addition to the athlete’s other responsibilities. The importance of sleep, nutrition, recovery, mental health, and injury prevention cannot be overstated. There is no excuse for a coach to make demands of an athlete, especially a young athlete, that will sacrifice health or fitness in the name of winning. The athlete’s well-being is the implicit context of everything that follows.

Training VO2 Max

VO2 max or maximal oxygen consumption is the ability to deliver oxygen to muscles and utilize that oxygen during exercise. There are several factors that affect VO2 max, most of which have little to do with training:

  • Genetics
  • Age
  • Training
  • Nutrition
  • Sleep

Our focus has been on those things the strength and conditioning coach can help athletes develop in the gym.

As a measurement, VO2 max distills a multi-step process that involves the respiratory system, the cardiovascular system, and the blood supply to the exercising muscles (Joyner and Coyle 2008) into a single value. So, even though we can quantify VO2 max with a singular value, stated in milliliters per body mass per minute (mL/kg/min), athletes do not train VO2 max directly—different types of exercise improve different steps in this process, ultimately affecting how much oxygen is available or how well the body uses it. It helps to think of VO2 max as assessing the efficacy of a transport system. There are multiple parts to the system, each potentially holding up the overall process.

There are different types of evidence that help us identify which pieces of this transport system can be trained to improve VO2 max. Anecdotally, when athletes have cheated by blood doping—injecting oxygenated blood, red blood cell producing hormones, or synthetic oxygen carriers—they have exhibited both measurable improvements in VO2 max and improvements in performance (Bassett 2000).[1] Also, research comparing changes in cardiac output and other factors that affect total oxygen consumption has tracked changes in VO2 max to cardiac output, highlighting the heart’s ability to pump blood (Bassett 2000).[2] Muscles themselves don’t slow things down, since muscles have a high capacity for consuming oxygen. (Id., “The prevailing view is that in the exercising human VO2max is limited primarily by the rate of oxygen delivery, not the ability of the muscles to take up oxygen from the blood.”) But more blood reaching the muscles at the same time is linked to improved VO2 max, leading to some investigations into capillary density.

Finally, diffusion principles, used to measure the uptake and delivery of oxygen to peripheral tissues, say that V02Max is a function of cardiac output and the concentration of oxygen taken from the blood by skeletal muscles (Ozaki et al. 2013). This gives us three basic trainable factors to move the needle on oxygen consumption:

(1) Lung Capacity—how much oxygen is available for diffusion into the blood

(2) Cardiac Output—how much blood gets pumped to the muscles

(3) Capillary Density—how much gets delivered to the muscles

These factors are not the only ones that affect VO2 max, but they appear to be the most trainable. Fortunately, there is plenty of overlap in the types of physical activities that will improve them.

Lung Capacity

The first step in this oxygen transport system is the most basic: breathing. Oxygen in the air, the efficiency of the athlete’s respiratory system, and the demand for oxygen during exercise each affect how easily an athlete’s lungs can oxygenate his blood. Of these, a person can most directly affect lung capacity through exercise. Lung capacity, however, is often not a factor in limiting maximal oxygen uptake. This limitation is seen in two cases. First, athletes who train or play at altitude (3,000–5,000 meters above sea level) exhibit a limitation in the ability to bring in enough air to sustain intense exercise. Second, researchers have observed lung capacity limitations in some highly trained athletes whose maximal cardiac output uses up the available oxygen so quickly that they cannot supply enough, even with their highly trained respiratory muscles (Bassett 2000).[3] Whether someone can train their lung capacity to bypass these limitations is unclear. Despite that, research continually cites the cardiorespiratory system trifecta—heart, lungs, and blood—as the primary limiters of VO2 max, suggesting that ignoring lung capacity would be unwise for the athlete (See, e.g., (Bassett 2000)).[4]

Lung capacity probably is not the best name for the adaptation that relates to VO2 max. While lung capacity includes the ability to supply the blood with more available oxygen, it is also limited by the ability of the respiratory muscles to continue the bellow-like operations without getting fatigued. Studies have shown that the fatigability of the respiratory muscles is trainable (See, e.g., Wylegala et al. 2007). While there are specific breathing-based exercises for these improvements, these have been targeted at people recovering lost lung function due to stroke or disease or to specialized uses like divers. There is little evidence to show that athletes need to do targeted lung-training exercises.

In general, training one’s lung capacity is easy. Exercise that elevates the athlete’s breathing for extended periods of time will improve or maintain the lungs’ capacity and efficiency (Bassett 2000). The approach outlined in Part Three advocates regular aerobic conditioning that, in addition to regular practice and play, should allay any concerns about an athlete’s lung capacity.

Cardiac Output

In the next step, oxygen diffuses from the lungs into the blood, where it gets pumped via red blood cells to the muscles. During exercise, muscles need more oxygen, so the athlete’s breathing and heart rate go up. The heart beats harder as well as faster, increasing the stroke volume—how much blood it moves with each pump. Heart rate and stroke volume make up the athlete’s cardiac output, measured by blood volume moved per minute. If the body cannot keep up with energy needs, the athlete gets fatigued.

While the complete relationship between VO2 max and cardiac output is not nearly this simple, it has been observed that an increase in cardiac output through training will generally lead to an increase in maximum oxygen uptake (up to a point and with diminishing returns among trained athletes). Elite athletes may have VO2 max values 50-100% above normally active people (Joyner and Coyle 2008). It is unclear, however, how much of that increase is due to the plasticity of a human’s oxidative capacity and how much is due to non-trainable factors like genetics.

For trained athletes, much of the research for improving VO2 max focuses on high-intensity training (sometimes called sprint training) that takes place at or even exceeds the athlete’s VO2 max intensity, generally at 85% to 95% of the athlete’s maximum heart rate. A meta-analysis of 19 studies that measured changes to VO2 max from interval training suggests that intensity, rather than specific protocol, matters most for improving VO2 max. That analysis collected studies using long- and short-duration, high-intensity protocols (defining short as <60 seconds and long durations as >60 seconds) performed at intensities >80% of maximum intensity (de Oliveira-Nunes et al. 2021). This is close to our typical recommendations for High-Intensity Interval training for athletes and non-athletes but gives us some flexibility to make our intervals and work-to-rest ratios more specific to some kinds of sports situations.

Capillary Density

The next step in maximal oxygen consumption is the rate of transfer of oxygen and nutrients from the bloodstream to the tissues and the transfer of waste products from the tissues to the bloodstream. The body adapts at this transfer site by increasing the density of transport sites by increasing the number of capillaries.

Capillaries are the smallest and most numerous types of blood vessels and play a crucial role in the exchange of oxygen, nutrients, and waste products between the bloodstream and muscle tissues. One of the key factors of VO2 max is the amount of oxygen taken up by the muscles from the blood. Improvements in this measure are generally attributed to an increase in capillary density and in “muscle mitochondria content and enzyme activity” (Ozaki et al.). The former of which is trainable—to a point—resulting in muscles’ increased ability to extract and use oxygen. Since capillary density is involved in delivering oxygen to muscles, it has a theoretical proportional relationship with V02 max.

Improving VO2 max

The general advice from the American College of Sports Medicine recommends performing aerobic exercise three to five days per week to improve VO2 max. Aerobic exercise increases the demand for oxygen, leading to an increased blood flow to the muscles and stimulating new capillary growth, in turn increasing the overall capillary density at the muscles. There also appears to be a dose-dependent relationship between VO2 max and intensity. It is generally thought that training at intensities of 85-100% of a person’s maximum heart rate is an effective way to improve VO2 max. For under-trained and understrength people, there have been some observed improvements in VO2 max just from resistance training (Ozaki et al.). Possible mechanisms for these improvements are increases in cardiac output, capillary density, and increases in muscle mass (and, thus, blood flow to the exercising muscles) (Ozaki et al.). The ceiling for resistance training improvements in VO2 max is probably not very high, but it is one more reason resistance training acts like a scattergun of salutary improvements for untrained people.

Lactate Threshold

As we discussed earlier, an athlete’s lactate threshold has a strong connection to her endurance. A high lactate threshold means that the athlete can work at high intensity for sustained periods. At intensities above a person’s lactate threshold, she will start to fatigue quickly, but the mechanism by which fatigue results from an increased concentration of lactate is unclear.

As we discussed earlier, athletes can improve their intensity at lactate threshold by improving VO2 max, becoming more efficient, and increasing the threshold itself. Lactate threshold is expressed as a percentage of VO2 max. By increasing maximal oxygen uptake, any percentage of that also increases. By improving efficiency or economy of movement, a person can work harder using less energy, meaning their speed or power at lactate threshold will improve. Finally, there are marked differences in the percentages of VO2 max that make up the lactate threshold in untrained people (around 50% of VO2 max) and highly trained athletes (around 75% of VO2 max). As discussed, improving VO2 max and improved efficiency for sports is beyond this article. Next, we need to identify ways that an athlete can improve their percentage of VO2 max that makes up their lactate threshold—direct threshold training.

Traditionally, lactate threshold training involves extended periods of sustained, high-moderate intensities. The rationale behind this is, basically, “do the thing you want to get better at.” Accordingly, lactate threshold training usually means sustaining a pace that is at one’s lactate threshold for 20–30 minutes. Over time, the athlete should find that the pace and duration of these workout increases. This may be due to improve efficiency, mental toughness, or direct improvements to the threshold. Most likely, it is a combination of all three.

Research has also shown that lactate threshold improves with HIIT training (Esfarjani and Laursen 2007) and strength training (Marcinik et al. 1991).[5] So, the first improvements an under-trained athlete might see in this limitation will likely come from general strength and conditioning training. The more trained the athletes are, the more targeted training must become. Strength training alone will not improve an athlete’s lactate threshold for very long, nor will the athlete’s generally improved fitness translate to better staying power. HIIT workouts and threshold training workouts target the athlete’s ability to operate at this relatively high intensity for extended periods. HIIT causes lactate production at high intensities and forces the body to clear out waste products in increasingly short rest intervals. This works particularly well for athletes whose sports require a lot of starts and stops, high intensity with moderate rests.

Limits on Trainability and the “Use It or Lose It” Principle

VO2 max is highly plastic, meaning we can observe vast differences between untrained and trained people. This does not necessarily mean it is highly trainable, however. We have identified the structures that limit endurance and how each responds to physical activity. Two questions remain: How much the strength and conditioning coach can affect the endurance of a competitive athlete? And, how do we know whether training was successful? Many athletes and coaches choose to bypass these questions and take a more is better approach, choosing to pursue conditioning and endurance above almost any other physical trait. It is unclear how much an already fit athlete can improve their VO2 max and lactate thresholds through training. Instead, we have to operate with the understanding that a person can affect these measurements. It is up to the coach to identify the priority that conditioning should take for the athlete and to adhere more to the use it or lose it principle: an athlete must maintain high levels of activity to maintain the salutary adaptations to that activity.

Regarding the second question, coaches and athletes should not expect in-the-gym endurance training to immediately or automatically improve an athlete’s performance. Raising the athlete’s ceiling helps remove limitations, but whether the athlete can rise to the level of those limitations is another matter.

[1] While blood doping can certainly increase the amount of oxygen that reaches the muscles, it is illegal and unethical in most sports. Moreover, it can have serious health risks, including blood clots, stroke, and heart attack.

[2] From Basset, 2020: “However, it must be remembered that maximal cardiac output is not the dominant factor limiting VO2max in exercise with an isolated muscle group (i.e., one-legged cycling). Whole-body VO2max is primarily limited by cardiac output, while for exercise with small muscle groups the role of cardiac output VO2max is considerably less important.”

[3] They observed this limitation by supplementing test subjects with enriched O2 and looking for increased exercise capacities.

[4] Basset, 2020: “In the field of exercise physiology, when limiting factors for VO2 max are discussed, it is usually with reference to human subjects, without metabolic disease, undergoing maximal whole-body exercise, at sea level. Under these conditions, the evidence clearly shows that it is mainly the ability of the cardiorespiratory system (i.e., heart, lungs, and blood) to transport O2 to the muscles, not the ability of muscle mitochondria to consume O2, that limits VO2 max. We conclude that there is widespread agreement with regard to the factors limitingVO2 max, and that this agreement is based on sound scientific evidence. In general, the 75 years of subsequent research have provided strong support for the brilliant insights of Hill et al.”

[5] But see, Jung 2003, discussing Marcinik et. al: “It is important to note that these subjects were untrained, allowing greater room for improvement compared with a trained population. Additionally, the mode of training (circuit-training) may have generated a sufficient stimulus for the untrained subjects to observe an improvement in LT. A second explanation may also be worth considering. Following a resistance-training program the muscle fibres are capable of producing more absolute force; therefore, the fibres would work at a lower percentage of maximum strength during endurance exercise, compared with pre-training.”


Bangsbo, J. 2015. “Performance in Sports – With Specific Emphasis on the Effect of Intensified Training.” Scandinavian Journal of Medicine & Science in Sports 25 (December): 88–99.

Bassett, David R. 2000. “Limiting Factors for Maximum Oxygen Uptake and Determinants of Endurance Performance:” Medicine & Science in Sports & Exercise, January, 70.

Esfarjani, Fahimeh, and Paul B. Laursen. 2007. “Manipulating High-Intensity Interval Training: Effects on , the Lactate Threshold and 3000m Running Performance in Moderately Trained Males.” Journal of Science and Medicine in Sport 10 (1): 27–35.

Joyner, Michael J., and Edward F. Coyle. 2008. “Endurance Exercise Performance: The Physiology of Champions: Factors That Make Champions.” The Journal of Physiology 586 (1): 35–44.

Oliveira-Nunes, Silas Gabriel de, Alex Castro, Amanda Veiga Sardeli, Claudia Regina Cavaglieri, and Mara Patricia Traina Chacon-Mikahil. 2021. “HIIT vs. SIT: What Is the Better to Improve VO2max? A Systematic Review and Meta-Analysis.” International Journal of Environmental Research and Public Health 18 (24): 13120.

Ozaki, Hayao, Jeremy P. Loenneke, Robert S. Thiebaud, and Takashi Abe. 2013. “Resistance Training Induced Increase in VO2max in Young and Older Subjects.” European Review of Aging and Physical Activity 10 (2): 107–16.




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