What does endurance refer to in the context of exercise?

Endurance in the context of exercise is best defined as the ability to sustain a relatively high level of physical activity over an extended period of time. This aspect of fitness specifically relates to how long a person can maintain an effort without succumbing to fatigue. When speaking about relative intensity, it refers to how hard a person is working compared to their maximum capacity. This can encompass various forms of endurance, including muscular endurance (the ability of a muscle or group of muscles to perform repeated contractions against a load) and cardiovascular endurance (the ability of the cardiovascular and respiratory systems to supply oxygen to the body during sustained physical activity).

The maximum weight lifted pertains more to strength rather than endurance, as it focuses on the maximal force a muscle can exert in a single effort. Heart rate variability relates to the variations in time intervals between heartbeats and is a measure of autonomic nervous system function, not directly indicative of a person's endurance capabilities in exercise. Speed of movement can be an outcome influenced by endurance, but it does not directly define what endurance entails; rather, endurance can influence how fast and efficiently one can sustain movement over time. Therefore, the concept of relative intensity maintained over time captures the essence of endurance in exercise.