Considering the number of body systems that must interact (musculoskeletal, cardiovascular, respiratory, nervous, etc.), athletic performance is one of the most complex human traits. Perhaps the first noticeable difference between athletes of different specialties is in body morphology (i.e., height and body composition), with specific body types naturally suited to specific sports. Beyond body morphology, endurance, strength, and power are primary factors underlying athletic performance.
Aerobic endurance is the ability to sustain an aerobic effort over time, such as distance running or cycling. At the most basic level, aerobic endurance requires the ability of the cardiovascular system to deliver oxygen to the working muscles and the ability of the muscles to utilize that oxygen. The most common quantification of endurance is the maximal rate of oxygen uptake (VO2max). However, VO2max does not perfectly correlate with endurance performance (e.g. marathon running), as other factors such as economy and ventilatory threshold also influence performance.