Endurance is a term widely used in sport and can mean many different things to many different people. In sports it refers to an athlete's ability to sustain prolonged exercise for minutes, hours, or even days. Endurance requires the circulatory and respiratory systems to supply energy to the working muscles in order to support sustained physical activity.
When most people talk about endurance they are referring to aerobic endurance, which is often equated with cardiovascular fitness. Aerobic means "with oxygen" and during aerobic exercise the body uses oxygen to help supply the energy needed for exercise.
The objective of endurance training is to develop the energy production systems to meet the demands of activity for as long as they are required.
Energy Pathways - How Foods Fuel Exercise
The body converts food to fuel via several different energy pathways. In the simplest terms, the body can convert nutrients to energy with or without the presence of oxygen.
These two energy systems are called:
Aerobic metabolism (with oxygen)
Anaerobic metabolism (without oxygen)
These pathways can be further divided. The three energy systems most commonly mentioned in exercises include:
ATP-CP (Anaerobic) Energy Pathway - which supplies short bursts of energy lasting up to 10 seconds.
Anaerobic Metabolism (Glycolysis) - which supplies energy for short, high-intensity bursts of activity lasting several minutes.
Aerobic Metabolism - which supplies most of the energy needed for long duration, less intense exercise and requires plenty of oxygen. The waste products, carbon dioxide and water, are removed in sweat and exhalation.
Aerobic Metabolism and Endurance
Most often it's a combination of energy systems that supply the fuel needed for exercise, with the intensity and duration of the exercise determining which method gets used when. However, aerobic metabolism fuels most of the energy needed for long duration or endurance exercises.
Athletes continually strive to push their capacity to exercise harder and longer and increase their endurance. The factors that limit sustained high intensity efforts include fatigue and exhaustion. Sport training has been shown to modify and postpone the point at which this fatigue occurs.
VO2 Max and Aerobic Endurance
VO2 max or maximal oxygen uptake is one factor that can determine an athlete's capacity to perform sustained exercise and is linked to aerobic endurance. VO2 max refers to the maximum amount of oxygen that an individual can utilize during maximal or exhaustive exercise. It is measured as milliliters of oxygen used in one minute per kilogram of body weight. It is generally considered the best indicator of cardiorespiratory endurance and aerobic fitness. Elite endurance athletes typically have a high VO2 max. And some studies indicate that it is largely due to genetics, although training has been shown to increase VO2 max up to 20 percent. A major goal of most endurance training programs is to increase this number.
Muscle Fiber Type and Endurance
High level endurance athletes often have a higher proportion of slow twitch (Type I) muscle fibers. These slow twitch fibers are more efficient at using oxygen (and aerobic metabolism) to generate more fuel (ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.