If there is one aspect of specialist, sports-oriented fitness which embodies the greatest part of the lay ideal, it is probably endurance fitness: the ability to continue a demanding physical activity many times longer than the untrained person can. Whether the challenge is a London- Brighton cycle race, an ascent of the Matterhorn, or a Channel swim, the fundamentals of this category of fitness are the same. Each of these activities is trained for in essentially the same way namely, by covering large mileages several days a week for many months, with few if any periods of exertion that are flat out, either in strength or speed. Each activity is, in turn, necessarily aerobic: an activity performed in balance with oxygen intake and consequently requires that the heart can pump blood to the working muscles at several times its resting rate throughout the long duration of the exercise; also that the lungs can adequately oxygenate this enhanced blood flow as long as the exercise continues. Cardio-respiratory fitness is thus a common feature of all endurance events, though they differ in the skeletal muscles used, and the movement patterns these muscles perform.
When muscles have been endurance-trained they are typically only a little larger than before the training began, months or years before. They become furnished, however, with a much more copious system of blood capillaries. Within the muscle fibres, mitochondria, the organelles involved in oxidative energy provision, may be 2-3 times more numerous than in untrained or differently trained fibres. Connective tissues within the muscle as well as the associated tendons and ligaments are stronger too. The nervous system must also participate in the training, for patterns of movement in the exercise concerned are usually measurably more economical than before the regime began.