All athletes, depend on the ability of the heart, blood vessels, and lungs to supply oxygen and energy to working muscles. During training and competition, these demands grow, causing the heart to beat more rapidly and with greater force, the rate of blood flowing through the heart and to the muscles to increase up to 20-fold, and the lungs to fill up with more oxygen. Cardiovascular performance is limited by the ability to keep up with these increased demands and, more specifically, the body’s ability to consume oxygen. The need for oxygen is dependent on two key physiological parameters: (1) maximal aerobic capacity and (2) anaerobic threshold.

Maximal Aerobic Capacity

Maximal aerobic capacity, also known as VO2 max, is defined as the highest rate of oxygen consumption attainable during exhaustive exercise. This rate is supported by the maximum pumping capacity of the heart as well as the ability of the muscles to resist fatigue. Because the body requires oxygen to convert food into energy, the ability to consume large amounts of oxygen facilitates increased energy production for enhanced speed and endurance. Therefore, having a high VO2 max is like being equipped with the powerful engine of a race car. VO2 max provides a foundation for overall aerobic potential, making it an important performance variable for endurance sports such as the triathlon, cycling, rowing, distance running, cross-country skiing, and swimming as well as team sports requiring endurance such as soccer, basketball, and hockey.

VO2 max is often expressed as an absolute rate in liters of oxygen per minute but is more accurately represented as milliliters of oxygen per kilogram of body weight per minute, especially when evaluating differences between men and women. Values of aerobic capacity vary greatly between individuals and even between athletes competing in the same sport due to such factors as genetics, age, training status, sex, and body composition. Male athletes tend to have VO2 max values 15%-30% higher than female athletes, likely due to differences in body composition since research shows the aerobic capacity to decrease with increased body fat. Depending on baseline fitness, physical training can boost aerobic capacity by as much as 20%. Genetics, alone, has been shown to account for 25%-50% of the variance in VO2 max seen between individuals. VO2 max numbers typically range between 2.5 and 6.0 L/min, or 25-94 ml/kg/minute with untrained females and trained elite male endurance athletes falling on the low and high end of the spectrum, respectively.

Table 1: VO2max (ml/kg/min) in Various Sports  

Basketball 18-30 40-60 43-60
Cycling 18-26 62-74 47-57
Gymnastics 18-22 52-58 36-50
Squash 20-35 55-62 50-60
Soccer  22-28 54-64 50-60
Speed Skating 18-24 56-73 44-55
Swimming  10-25 50-70 40-60
Track and Field Running  18-39 60-85 50-75


VO2max is affected by altitude. Even after allowing for full acclimatization, athletes living, training, or competing at an altitude of sea level to 5,000 feet (1,524 m) should expect about a 5%-7% loss in aerobic capacity; for every 1,000 feet (305 m) of elevation gain above 5,000 feet (1,524 m), expect another 2% drop.

Improving Aerobic Capacity

Athletes wanting to improve maximal oxygen aerobic capacity should focus on increasing the rate of oxygen delivery to and consequent uptake by the muscles. It is well known that training has a profound impact on both. Recently, several nutritional ingredients and supplements, often marketed as oxygen enhancers, have shown promise in providing additional benefit. Research evaluating the impact various nutritional ingredients have on oxygen consumption, both via delivery and uptake, generally looks at one or several of the following five variables:   

  1. Cardiac output:

    During exertion, an athlete’s heart becomes a powerful pump, allowing for more blood, oxygen, and nutrients to be delivered to the working muscles and aiding the elimination of carbon dioxide, lactate, and other metabolic waste products that can exacerbate muscle fatigue and negatively affect performance. The actual amount of blood pumped by the heart each minute is known as cardiac output and is influenced by both stroke volume (the amount of blood that the heart pumps each time it beats) and heart rate (the number of times the heart beats in a minute). At rest, cardiac output for adults averages about 5-8 L/min, but for elite-level endurance athletes competing in cross-country skiing, this can increase to an amazing 40 L/min.
  1. Blood volume:

    Defined as the total volume of fluid, including red blood cells and plasma that circulates through the heart, arteries, and capillaries, blood volume is closely regulated by the kidneys. A boost in blood volume, naturally seen with physical training and further enhanced with heat and altitude acclimatization, occurs as a result of increases in antidiuretic hormones, aldosterone, and the plasma protein albumin. The primary benefits are enhanced oxygen transport abilities and consequent improvements in endurance performance and muscle recovery. A typical adult maintains a blood volume of 4.7-5 L, with blood levels for females slightly lower. An elite-level endurance athlete may have blood levels as much as 30% higher than those of the average adult.       
  1. Hemoglobin:

    This is a protein in red blood cells that carries oxygen. Healthy levels fall between 13.8 and 7.2 gm/dL for men and between 12.1 and 15.1 gm/dL for women. Slightly lower levels in athletes, especially those engaged in endurance training and competition, are common and generally are caused by a phenomenon known as sports anemia. The expansion of plasma volume resulting from aerobic training reduces the concentration of red blood cells that carry hemoglobin. Despite a reduced concentration of red blood cells, the rise in plasma volume actually aids oxygen delivery to the muscles. Levels significantly lower than the norm, however, may indicate excessive fluid intake or a nutritional deficiency in iron, folate, vitamin B12, or vitamin B6, all of which can negatively affect cardiovascular performance.      
  1. Mitochondrial density and enzyme levels:

    Deep inside muscle fibers are microscopic structures called mitochondria, also known as powerhouses due to the role they play in energy production. The number of mitochondria present within the muscle, or mitochondrial density increases in response to calcium ion levels rising during muscle contraction and ATP levels failing to keep up with demands in skeletal muscle cells during exercise. Training, especially endurance-focused activity, not only helps to improve mitochondrial density but also facilitates an increased number of oxidative enzymes available to break down glucose, fat molecules, and certain amino acids before combining with oxygen to produce ATP energy for muscle contraction and other cellular functions. This adaptation in trained muscle allows for more fats to be used to generate ATP, thereby helping to spare muscle glycogen, critical for reaching peak endurance potential.  
  1. Capillary density:

    Improving capillary density, the number of tiny blood vessels within each muscle cell aids the distribution of oxygen- and nutrient-rich blood to the muscles; doing so also aids the clearance of lactate from the fast-twitch (Type IIa and Type IIb) muscle fibers and into the slow-twitch (Type I) muscle fibers for processing fuel, thereby reducing muscle fatigue, aiding muscle recovery, and maximizing endurance potential.

Anaerobic Endurance

Thought to be a better predictor of performance than aerobic capacity, anaerobic endurance refers to the ability of an athlete to sustain a maximum level of work in the absence of or with limited amounts of oxygen. Anaerobic endurance is a relevant performance variable for stop-and-go sports requiring intense bursts of energy: soccer, basketball, football, tennis, boxing, and hockey, among others; it also applies to certain styles of weightlifting when the goal is to keep the time between sets as short as possible. It becomes a relevant factor for endurance athletes competing in events such as a cycle or running race where a response to an opponent’s attack needs to be quickly countered (e.g., sprint to the finish line). During anaerobic activity, which involves rapid contraction of fast-twitch muscle fibers, the demand for oxygen and fuel exceeds the rate of supply, triggering a shift of focus from aerobic to anaerobic metabolism. For the first 10-20 seconds of anaerobic activity, energy is generated from stored ATP and the creatine phosphate (CP) energy system; beyond that, muscle glycogen breakdown and glycolysis provide additional energy. Unlike the aerobic fuel tank, however, whose energy supply can last hours, the anaerobic fuel tank can only supply enough energy for physical exertions lasting up to a few minutes before the fuel runs out or clearance of fatigue-inducing byproducts, specifically lactate, fails to keep up with their production, a state known as lactate or anaerobic threshold (LT).

Recreational athletes typically hit their LT near 65%-80% of their VO2 max, whereas elite and world-class endurance athletes tend to peak at 85%-95% of their VO2max.

Improving Anaerobic Endurance

To improve anaerobic endurance, an athlete should focus on increasing (1) the amounts of ATP and CP on hand, (2) the amount of glycogen available for breakdown, and (3) lactate tolerance and clearance ability of the muscle. These can be achieved through the incorporation of interval training (e.g., 100 m sprint repeats) into any exercise or training program. Such high-intensity efforts help improve the activity of glycolytic enzymes such as phosphofructokinase (PFK) and hexokinase (HK), important for better muscle-force generation and sustained contractions during exertion.

Understanding Nutrition

Athletes following healthy diets that are low in saturated fats and rich in whole grains, fruits, and vegetables can help protect against plaque buildup in the arteries that leads to blockages and restricted blood flow that negatively affect cardiac output. In addition, several ingredients, such as nitric oxide found in beetroot, can help dilate the blood vessels and allow for greater blood flow to the working muscles during exercise. There are several dietary supplements that have demonstrated promise in aiding anaerobic endurance, including creatine, which helps build total-body creatine stores, and buffering agents such as bicarbonate, phosphate, and more recently beta-alanine. These ingredients can help increase cellular and blood pH by countering the effects of hydrogen ion accumulation and consequent muscle fatigue during high-intensity training and competition.