What is Endurance?

What is endurance?


We can define endurance as the ability to continue to endure a stress, hardship or level of suffering.

In sport, endurance is the ability to sustain a specific activity (endurance running, cycling, swimming, rowing, cross-country skiing etc) for a prolonged period.

What is an endurance sport?… An endurance sport is any sport in which there is a requirement to sustain an activity level while enduring a level of physical stress.

Endurance Sport Requirements

The fundamental requirement for any endurance sport is the ability to sustain a submaximal work rate for a prolonged period.

While this is massively influenced by physiological factors (efficiency of energy systems, aerobic capacity/VO2 maxlactate threshold, muscle strength, power and muscular endurance); psychology also plays a key part in success in endurance sport. As such, the ability to compete strongly in endurance events is influenced by our physical fitness and psychological strengths.

Endurance training involves developing both general and event specific endurance. And the mental toughness or “grit” needed to achieve our peak performance.

What limits endurance?

The primary factor that limits endurance exercise is fatigue. When fatigue reaches a certain point, an athlete’s work rate decreases. The exact point where fatigue limits performance (tolerance level) varies and some athletes can endure much greater levels of fatigue than others.

While genetics and mental toughness contribute to fatigue tolerance, it’s highly trainable.

From this, it’s clear that a key effect of training is being able to resist fatigue. Therefore, an athlete with a solid base of endurance will fatigue less quickly, and will outwork other less trained athletes.

What limits endurance exercise

Factors affecting endurance

Cardiovascular fitness

Cardiovascular endurance refers to the capacity of our lungs, heart, and circulatory system to transport oxygen during prolonged exercise.

Several factors affect the overall efficiency of the cardiovascular system:

  1. The capacity of the lungs to inhale large volumes of air (tidal volume) and efficiently absorb oxygen.
  2. The ability of the heart to pump large amounts of blood (cardiac output) to the working muscles.
  3. The efficiency of the circulatory system to transport oxygenated blood to the working muscles.

Through training, we improve the efficiency of all parts of the cardiovascular system.

Muscular endurance

Muscular endurance is the ability of a muscle, or a group of muscles, to exert a force for a prolonged period. An athlete with good muscular endurance can repeat a series of muscular contractions without fatiguing.

The greater the level of muscular endurance, the more force the muscle can exert over time. One factor here is the ratio of different muscle fibre types (fast and slow twitch).

We can improve this through specific training that improves the fatigue resistance of slow and fast twitch muscle fibres.

Training improves this in several ways…

  • Adaptations to the aerobic and anaerobic energy systems.
  • improved delivery of oxygen to the working muscles (greater cardiovascular fitness)
  • Increased muscle fibre recruitment – reduces the workload placed on individual muscle fibres which improves fatigue resistance.

Both cardiovascular endurance and muscular endurance are key components of fitness.

Aerobic and anaerobic energy systems

Muscles produce energy through aerobic (requiring oxygen) and anaerobic (not requiring oxygen) metabolism. During endurance sports, we meet most of our energy needs through aerobic metabolism. However, as exercise intensity rises (such as during high-intensity interval training) anaerobic metabolism becomes proportionally more important.

A major factor here is the increased recruitment of type II (fast twitch) muscle fibres. At low intensities we recruit primarily slow twitch muscle fibres. However, as exercise intensity increases, there’s a shift towards increased recruitment of fast twitch muscle fibres. One point to note is slow twitch muscles fibre recruitment doesn’t decrease at higher intensities, rather we get increased recruitment of fast twitch (type IIa and IIb fibres).

In the table below, you can see the proportional contribution of aerobic and anaerobic metabolism during different running events (800m to 10,000m).

It’s important to note that the contribution of aerobic and anaerobic systems will vary between individuals, and is affected by gender, age, genetics, and the performance/conditioning level of the individual.

Aerobic and anaerobic energy systems in running events

As you can see, the aerobic energy systems supply most of our energy requirements during events lasting longer than a couple of minutes. As such, endurance athletes should concentrate primarily on developing aerobic fitness.

However, it’s vital for effective long-term progression that endurance athletes also devote a proportion of training time to developing anaerobic fitness and conditioning. It’s common for endurance athletes to devote as much as 80-90% of training to aerobic endurance.

A further 10-20% is then focussed on a combination of aerobic capacity training, anaerobic endurance/capacity and sprint training. The proportion of each will vary depending on the athlete’s training focus. We also need to account for physiological differences between athletes – all athletes are unique and don’t always respond the same way to different types of training.

Aerobic Fitness

Aerobic endurance refers to the capacity to produce energy using aerobic metabolism. The limiting factor here is the ability to absorb, transport and utilize oxygen for energy production.

We can break this down into four parts:

  1. Efficiency of the lungs to inhale and absorb oxygen
  2. The efficient transportation of oxygenated blood around our body via our heart and circulatory systems
  3. The ability of the muscles to absorb oxygen from the blood
  4. The efficiency of our muscles, and specifically the mitochondria, to produce energy via aerobic metabolism.

Some improvements from aerobic endurance training include:

  • Improvements in the efficiency of the heart, respiratory and circulatory systems.
  • Increased ability to absorb and transport oxygen.
  • Improved capacity of muscles to absorb and use oxygen.
  • Increased efficiency of aerobic energy systems.
  • Greater stores of key aerobic fuels (muscle glycogen, intramuscular triglycerides) and the ability to use them during exercise.
  • Improved fatigue resistance of respiratory, cardiovascular, and muscular systems.
  • Improvements in the VO2 max, lactate threshold, exercise efficiency.
  • Increased recovery during and after exercise.
  • Improved tolerance of higher training loads and high-intensity training.
  • Increased efficiency and capacity to use lactate as a fuel source.
  • Higher concentrations of key aerobic enzymes.
  • Increased capacity to use fat stores for energy, helping to spare muscle glycogen.

Examples of aerobic endurance training include moderate and longer duration exercise at low intensities, tempo and lactate threshold training, high-intensity interval training.

Anaerobic Endurance

When we talk about anaerobic endurance, we are referring to exercise where there’s increased reliance on anaerobic energy systems. The term anaerobic means ‘without oxygen’. So, anaerobic endurance refers to our ability to perform work in situations where anaerobic energy systems are playing an increased role in energy metabolism.

At higher intensities, aerobic metabolism cannot generate enough energy to meet all the requirements of the working muscles. The primary reason for the limitation of the aerobic system is the ability to deliver enough oxygen to the working muscles and the fatigue resistance of your muscle fibers.

With muscle fibers, slow twitch muscle fibres exert less force per contraction compared with fast-twitch fibres. When exercise intensity increases, the additional energy requirements (force production) are met by fast twitch muscle fibres (Type IIa and IIb). This leads to an increase in anaerobic metabolism.

As mentioned earlier, at higher intensities, aerobic metabolism still contributes a sizeable amount of the total energy. It’s just that fast twitch muscle fibres, and anaerobic metabolism becomes increasingly important.

Although anaerobic metabolism can supply energy at much faster rates, there is a trade-off.

First, it leads to a buildup of hydrogen ions. This increases muscle acidity and can interfere with exercise performance.

Second, it rapidly diminishes the key stores of muscle glycogen.

Despite this, the development of anaerobic endurance is important in many endurance events. In fact, it’s necessary for maximising performance potential in endurance events, even where aerobic metabolism is the principal energy system. While this might sound counter-intuitive, anaerobic training really can help to maximize performance in aerobic events.

Some improvements seen through anaerobic endurance training include:

  • Improvements in neuromuscular coordination, muscle strength and power.
  • Increased fatigue resistance of both type 1 and type 2 muscle fibres.
  • Improved efficiency of exercise (lower oxygen cost)
  • Significant improvements in endurance exercise performance – importantly, the improvements occur more rapidly with anaerobic training.
  • Improvements in VO2 max, heart stroke volume.
  • Increased glycogen stores.
  • Increased levels of key enzymes.
  • Improved strength of muscles and joints
  • Increased buffering capacity of muscle cells – ability to tolerate higher acidity levels.

Examples of ways to develop anaerobic endurance include:

  • Intervals completed above VO2max intensity, using either short or longer recoveries.
  • Short intervals at above VO2max intensity*, with short active recoveries. Example session: 2 x 8-10minutes of 30secs at just above VO2max intensity (either speed or power), 30secs recovery at just below 50% of VO2max intensity (either 50% of VO2max speed or power). These are great for improving both aerobic and anaerobic conditioning and lactate clearance.
  • Longer VO2max intervals*, with longer recoveries. Example session: 5 x (3minutes at VO2max intensity, 3minute recovery at 50%VO2max intensity). Great for developing both aerobic and anaerobic conditioning, with greater emphasis on aerobic conditioning, and fatigue resistance.
  • Speed endurance intervals. Example sessions: 5 x 200m at 90-95% effort, 4-5minute jog recovery. Another option is short hill repeats of around 30seconds duration. These are excellent for developing anaerobic capacity, neuromuscular coordination, and exercise efficiency. Read more about sprint training for distance runners.

*These sessions are effective for the development of both aerobic and anaerobic metabolism.

Muscle strength

Muscle strength is our ability to exert force during a single maximal effort. It differs from muscular power because it is not time dependent – power refers to force exerted over time, whereas muscular strength relates to the maximum force you can exert. 

While having good muscular strength may not sound important for endurance athletes, it plays a key role. The major reason for this is an athlete with greater strength will find it easier to work at the sub-maximal intensities required during endurance sport. The key is to improve strength without significantly affecting bodyweight.

Mental toughness

Mental toughness – often referred to as ‘grit’ – is another key area that influences performance. Endurance athletes need the ability to resist the sensation of fatigue that cries out to us “slow down” or “stop” during endurance events or challenging training sessions. 

We develop mental toughness over time, through exposure to fatigue during training sessions. You’ve likely heard the phrase “get comfortable being uncomfortable”. Well, this really is true with endurance sport. But the truth is we need to become comfortable at the intensity, or fatigue level, encountered during competitions. It’s no good being comfortable at 400m pace if you’re an ultra runner!

Why is that?… Put simply, fatigue is specific to the event you’re competing in. 

So, if you’re a 1500m runner, you need to be comfortable with the fatigue associated with 1500m pace. If you’re a marathon runner, get used to the prolonged, but lower levels of fatigue, experienced at marathon pace. And if you’re a time-trial cyclist, you need to be comfortable with the top levels of muscular fatigue associated with the power, cadence and cycling position (TT-position) experienced during a time trial.

This brings us on to the importance of specificity and the difference between general and event specific endurance.

General and event specific endurance

Clearly, all athletes require a level of general endurance. They also need to develop event specific endurance. As an example, a marathon runner will develop a good level of general as well as specific endurance for the marathon.

General endurance is essential for long-term development. It involves training all the components that affect performance.

Event specific endurance refers to the development of the specific endurance requirements for the athlete’s chosen event. As an example, both an 800m runner and 10k runner should develop similar levels of general endurance. However, the 800m race requires a distinct training focus compared with a 10k runner. And the 800m runners event specific training will be very different, compared with 10k running training.

Importantly, some sports require a much greater level of specific training, whereas others require a more general endurance. 

Examples of sports requiring highly specific endurance include rowing, swimming, cycling, running and cross-country skiing. 

An example of a sport requiring a more general level of endurance is CrossFit. Why is that?… CrossFit athletes compete in many varied disciplines. For this reason, they require a much greater level of general endurance – it’s very difficult to develop top levels of specific endurance when training for many events!


  • An endurance sport is one where the fundamental requirement is the ability to sustain a submaximal activity level for a prolonged period.
  • The primary limiting factor is fatigue. This has physiological and psychological components.
  • Factors affecting performance include cardiovascular, muscular, aerobic and anaerobic endurance.
  • Muscular and mental strength (grit) play vital roles.
  • We should develop both general and event specific endurance.
  • The primary focus of endurance training is on developing and maximising aerobic capacity and efficiency.
  • Anaerobic energy systems supply a comparatively small amount of energy during distance events. Despite this, the development of anaerobic conditioning is necessary for success in many endurance sports.
  • Some measurable physiological factors that contribute to endurance performance include: Maximal oxygen uptake (V02 max), The Economy of Motion Oxygen Economy, Lactate Threshold, The velocity at V02max, Sustainable %V02max, Peak power output – Power at (V02max), Maximal Lactate Steady State, Fractional utilisation – %V02max at lactate threshold.


Fallowfield, J.L. and Wilkinson, J.L. (1999). Improving sports performance in Middle and Long-Distance Running. Chichester: John Wiley and Sons, LTD.

Maughan R., Gleeson M., and Greenhaf P.L. (1997). Biochemistry of Exercise and Training. Oxford University Press, Oxford.

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