Endurance is the ability of an athlete to withstand external physical pressures over time, or to maintain competitive and training focus under pressure. It is a component of many aspects of sport; endurance sports have come to have a well-understood and distinct meaning.
Endurance sports are those that rely primarily on the aerobic system to provide energy for the performance of the activity. The aerobic energy system, in contrast to the anaerobic systems, both lactic and alactic varieties, will produce energy in a steady mode, where energy can be produced in relatively constant amounts for long periods. The anaerobic system draws upon stores of the sugar glucose, which is stored by the body in both the liver and in the muscles in the form of glycogen. The glucose will be employed by the body in a chemical process whereby the actual aerobic fuel, adenosine triphosphate (ATP), is created and burned through the introduction of oxygen transported by the erythrocytes, the red blood cells of the bloodstream. The red blood cells also remove the carbon dioxide and other wastes in the production and consumption of ATP.
The greater the efficiency of the delivery and removal mechanisms as supported by the blood and the underlying cardiovascular system, the greater the capability of the anaerobic system to produce energy, which is another definition of human endurance. Any sport or training program that serves to increase the strength of the heart, by requiring the heart to pump more frequently over time, will tend to increase the volume of blood that the heart is able to pump through the cardiovascular system. An increase in blood volume will result in greater amounts of glucose and oxygen being transported for energy production, as well as more speedily removing waste.
Sports scientists generally agree that athletes will enjoy measurably increased endurance if, with no prior aerobic training, they seek to attain a workout level of four times per week for a minimum of 30 minutes per training session. Aerobic fitness increases with both the duration and the intensity of the training. Although useful as a guideline only, the aerobic range, which is the heart rate at which benefits to the aerobic system and corresponding endurance occur, is generally believed to be 60-85% of the athlete's maximum heart rate. Unlike other muscles of the musculoskeletal system, the heart muscle does not sustain cellular damage that requires repair to be carried out by the body.
Many aerobic sports are well known for their cardiovascular strengthening powers. Distance running, cross-country skiing, distance swimming, cycling, and distance forms of canoeing and kayaking are such disciplines. The combination of calisthenics, dance routines, and jumping exercises made popular in the 1970s took the name "aerobics" from the purpose of the activity. A sport does not have to be a particular designated aerobic activity to provide aerobic benefits to the participant. Any athletic activity modified to generate endurance that elevates the heart rate and requires the body to produce energy aerobically will achieve similar results. Examples are interval running, carried out to shorten the rest period between each work portion.