What is Endurance?

What is endurance


Endurance can be defined as the ability to continue to endure a stress, hardship or level of suffering.

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

What is an endurance sport?

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

Endurance Sport Requirements

The main requirement for any endurance sport is the ability to sustain a high work rate for a prolonged period of time.

Whilst this is strongly influenced by physiological factors (efficiency of energy systems, aerobic capacity/VO2 max, lactate threshold, muscle strength, power and muscular endurance), psychology also plays a key part in success in endurance sport. As such, the ability to compete successfully in endurance events is affected by both our physiological fitness and our psychology.

Endurance training involves developing both general and event specific endurance. It also helps us to develop the mental toughness or “grit” needed to be able to achieve our peak performance.

What limits endurance?

The primary factor that limits endurance performance is fatigue. As fatigue builds up to a certain point an athlete’s work rate will begin to decrease. The actual point where fatigue limits performance (tolerance level) varies from one athlete to another. Some athletes are known to tolerate much higher levels of fatigue than others.

Whilst genetics and mental toughness contribute to our ability to tolerate fatigue. This is also highly trainable.

A key effect of endurance training is being able to resist fatigue. As such an athlete with a strong base of endurance will fatigue less easily, and will be able to outwork other less well-trained athletes.

What limits endurance exercise performance

Factors affecting endurance fitness

Cardiovascular endurance

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

The overall efficiency of the cardiovascular system is affected by a number of factors:

  1. The ability of the lungs to be able to inhale a large volume of air (tidal volume) and efficiently absorb oxygen from the air.
  2. The efficiency of the heart to pump large amounts of oxygenated blood (cardiac output) to the working muscles.
  3. The efficiency of the circulatory system to deliver the oxygenated blood to the working muscles. The efficiency of all parts of the cardiovascular system improves significantly with appropriate endurance training.

Muscular endurance

Muscular endurance is the ability of a muscle or a group of muscles to repeatedly exert a force for an extended period of time. As such, an athlete with good muscular endurance will be able to repeat a series of muscular contractions without fatiguing.

The greater the level of muscular endurance the more force the muscle can exert over time. Muscular endurance is affected by the percentage of different muscle fiber types (fast and slow twitch).

Muscular endurance can be improved through specific training that leads to improvements in the fatigue resistance of both slow and fast twitch muscle fibers. Part of the way training improves muscular endurance is through adaptations to the aerobic and anaerobic energy systems. Another factor is improved delivery of oxygen to the working muscles. And we also see increased muscle fiber recruitment, which effectively reduces the workload on individual muscle fibers.

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

Aerobic and anaerobic energy systems

Your muscle cells produce energy through both aerobic (requiring oxygen) and anaerobic (not requiring oxygen) metabolism. During endurance sports the majority of your energy needs are met through aerobic metabolism. However, as exercise intensity increases, aerobic metabolism is unable to meet all the cells energy requirements. This leads to a shortfall. To make up for this shortfall anaerobic metabolism has to increase proportionally, to the cells energy needs. The major factor here is increased recruitment of type II (fast twitch) muscle fibers at higher intensities.

You can see an example of how intensity affects the contribution of aerobic and anaerobic metabolism, in the table below. The table below highlights the contribution of different energy systems to endurance running performance over distances from 800m to 10,000m.

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

The contribution of aerobic and anaerobic energy system during 800m to 10,000m running.

Aerobic and anaerobic energy systems in endurance running events

As you can see, the aerobic energy systems provide the majority of energy during events lasting more than a couple of minutes. As such, endurance athletes should focus primarily on development of aerobic endurance.

However, it’s vital for effective long-term progression that endurance athletes also devote a percentage of training time to also developing anaerobic endurance. It’s common for endurance athletes to devote around 80-90% of training to aerobic endurance. A further 10-20% is devoted to a combination of aerobic capacity training, anaerobic endurance/capacity and sprint training. The proportion of each training type will vary depending on the training focus. It should also take account of different physiological factors between athletes – all athletes are different and don’t always respond the same way to different types of training.

Aerobic Endurance

Aerobic endurance refers to our ability to produce energy using aerobic metabolism. Not surprisingly, the limiting factor here is our ability to absorb, transport and use oxygen for energy production.

This can be broken 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, and;
  4. The efficiency of the muscles, and in particular the mitochondria, to produce energy via aerobic metabolism.

Some of the improvements seen with aerobic endurance training include:

  • Improvements in the efficiency of the heart, respiratory and circulatory systems.
  • Increased ability to absorb and transport oxygen.
  • Improved efficiency of muscles to absorb and utilise oxygen.
  • Increased efficiency of and total energy produced through the aerobic energy systems.
  • Greater stores of key aerobic fuels (muscle glycogen, intramuscular triglycerides) and our ability to utilise them during exercise.
  • Improved fatigue resistance of our respiratory, cardiovascular, and muscular systems. And, lead to
  • Improvements in the VO2 max, lactate threshold, exercise efficiency.
  • Increased recovery during and after exercise is also increase.
  • Improved ablility to tolerate higher training loads and high intensity training.
  • Increased ability to utilize lactate as a fuel source.
  • Higher levels of key aerobic enzymes.
  • Increased ability to utilize 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 that maximally stress the aerobic energy systems.

Anaerobic Endurance

When we talk about anaerobic endurance we are talking about exercise where there is 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 high exercise intensities, aerobic metabolism cannot produce enough energy to meet all of the demands of the working muscles. The main reason for the limitation of the aerobic system at high intensities is the ability to deliver enough oxygen to the working muscles and the fatigue resistance of your muscle fibres.

When it comes to muscle fibers, slow twitch muscle fibers exert a much lower amount of force per contraction compared with fast twitch fibers. As exercise intensity increases the additional energy requirements are increasingly met by your fast twitch muscle fibers (Type IIa and IIb). This leads to an increase in anaerobic metabolism.

It’s important to remember, that at these higher intensities, aerobic metabolism still provides a large amount of the total energy. It’s just that fast twitch muscle fibers, and anaerobic metabolism becomes increasingly important.

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

Firstly, it leads to a build-up of hydrogen ions, which increase muscle acidity and can interfere with exercise performance.

Secondly, it rapidly depletes the key stores of muscle glycogen.

Despite this the development of anaerobic endurance is important in many endurance events. In fact, anaerobic endurance training has been found to be important for maximising performance potential in endurance events even where aerobic metabolism is the primary energy system. While this might sound counter-intuitive, but anaerobic endurance training can really help to maximize aerobic endurance.

Some of the 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 fibers.
  • Improved efficiency of exercise (lower oxygen cost)
  • Significant improvements in endurance exercise performance – importantly, the improvements are generally seen more quickly with anerobic endurance training.
  • Improvements in VO2max, heart stroke volume.
  • Increased glycogen stores.
  • Increased levels of key enzymes.
  • Improves 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 just above the VO2max using either short, or longer recoveries.
  • Short intervals at just above VO2max intensity*, with short active recoveries. Example session: 20minutes of 30secs at just above VO2max intensity (either speed or power), 30secs at just below 50% of VO2max intensity (either 50% of VO2max speed or power). Great for developing both aerobic and anaerobic conditioning as well as lactate clearance.
  • Longer VO2max intervals*, with longer recoveries. Example session: 5 x (3minutes at VO2max intensity, 3minutes at 50%VO2max intensity). Great for developing both aerobic and anaerobic conditioning, with increased emphasis on aerobic conditioning.
  • Anaerobic conditioning intervals. Example sessions: 5 x 300m at 800m pace, 4minute jog recovery. Another good option is short hill repeats of around 30seconds duration.

*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 in that it is not time dependent – power relates to force exerted over a given time, whereas muscular strength relates to the maximum force you can exert. 

Whilst having good muscular strength may not seem important for endurance athletes it plays a key role. The main reason for this is that an athlete with greater strength will find it easier to work at the lower intensities required during endurance sport. The key is to develop strength without significantly affecting bodyweight.

Mental toughness

Mental toughness – often referred to as ‘grit’ – is another key area that influences endurance performance. Endurance athletes need the ability to be able resist the sensation of fatigue that call out to us slow down during endurance events. 

Mental toughness is developed over time, through exposure to fatigue during training sessions. You’ve probably heard the phrase “get comfortable with being uncomfortable”. Well, this really is a key factor with endurance sport. But more importantly, we need to get comfortable with the intensity, or fatigue level, that we will encounter during competition. 

What does this mean?…Put simply, fatigue is specific to the event you’re competing in. 

So, if you’re a 1500m runner then you need to be comfortable with the fatigue associated with 1500m pace. If you’re a marathon runner get used to the prolonged levels of fatigue experienced at marathon pace. And if you’re a time trial cyclist you need to be comfortable with the 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

Endurance athletes need a good 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 endurance as well as developing specific endurance for the marathon. General endurance is important for long term development and involves training all the components that affect endurance performance.

Event specific endurance relates to the development of the specific endurance requirements for the athlete’s chosen event. As an example, both an 800m runner and 10k runner will have similar levels of general endurance. However, the 800m race requires a different training emphasis compared with a 10k runner. As such the 800m runners event specific training will be different compared with 10k running training.

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

An example of sports requiring very specific endurance include: rowing, swimming, cycling, running and cross country skiing. 

An example of a sport where a more general level of endurance is required is crossfit. Why is that?…Since, CrossFit athletes compete in many varied disciplines they require a more general level of endurance – it’s very difficult to develop high levels of specific endurance when training for more than one discipline.

Endurance Training Summary:

  • An endurance sport is one in which the main requirement is the ability to sustain a submaximal activity level for a prolonged period of time.
  • Therefore, the main requirement of an endurance sport is to sustain a high workrate for a prolonged period of time.
  • The main limiting factor in endurance sport is fatigue.
  • Factors affecting endurance performance include cardiovascular, muscular and aerobic and anaerobic endurance.
  • Muscular and mental strength (grit) also play a key role in endurance performance.
  • Endurance should be developed both as general endurance and event specific endurance.
  • The majority of endurance training should be focussed on developing and maximising aerobic capacity and efficiency.
  • Anaerobic energy systems contribute a relatively small amount of energy during endurance events. Despite this development of specific anaerobic endurance is often vital for success in many endurance sports.
  • Some of the measurable physiological factors that contribute to endurance performance include: Maximal oxygen uptake (V02max), The Economy of Motion Oxygen Economy, The Lactate Threshold Anaerobic Threshold, The velocity at V02max (vV02max), The Sustainable % of 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.