Athletic exercise usually involves both quantity and quality; therefore, it is difficult to differentiate between them in training. For instance, when a swimmer sprints, the distance and time of the event represent volume, and the velocity of performance indicates intensity. Placing different relative emphasis on these components in training yields different effects on the body's adaptation and training status. The higher the intensity and the longer it is maintained, the higher the energy requirements and the more stress on the CNS and athlete's psychological sphere.