What primarily determines a person's endurance in exercise?

The primary factor that determines a person's endurance in exercise is maximum oxygen uptake, commonly referred to as VO2 max. This measurement reflects the maximum amount of oxygen that an individual can utilize during intense exercise. A higher VO2 max indicates a greater capacity to deliver and utilize oxygen in the muscles, which is crucial for sustaining prolonged physical activity.

Endurance activities, such as running or cycling, rely heavily on aerobic metabolism, which requires a steady supply of oxygen. Thus, an individual's ability to effectively use oxygen can greatly influence performance in endurance sports. Training can improve VO2 max, which is why athletes often engage in aerobic workouts to increase their endurance levels.

While muscle size and strength, dietary habits, and genetic predisposition can play a role in overall fitness and performance, they are secondary factors relative to the fundamental importance of effective oxygen utilization in sustaining endurance activities. Muscle size alone does not determine how efficiently an athlete can perform at aerobic levels, and while a well-balanced diet can support an athlete's training, it is VO2 max that serves as a more critical determinant of endurance capacity.