A healthy body does not have to bulge with rippling muscles. In fact, this can be unhealthy. People are naturally different shapes and sizes, and this includes their muscles. So physical fitness also varies from one person to the next. Fitness is not just simple strength. It includes the body’s ability to move about and stay active, without becoming too tired or suffering sprains, strains or other injuries. Being fit means having flexible joints that do not ache, and muscles that have stamina so they do not run out of power or become weak and trembling.
Any form of sudden, extreme action can be bad for health and cause injury. This is why athletes and sports people do gentle warm-up exercises before they begin, and cooling down exercises afterwards. It is important to ask expert advice before beginning many types of activity.
Inside the body, the fitness of the heart and lungs is especially important. The heart should be able to speed up and pump more blood faster when muscles are active and need a greater blood supply. Similarly, the lungs should be able to breathe faster and deeper, to take extra oxygen from the air, which is then used by the active muscles.