"It is more than the simple athleticism of the mens sana in corpore sano. The conception of the body as perfect instrument of the increasing intelligence pervades." (Ezra Pound)
What is a healthy body? We often associate health with a certain kind of look, promoted by health magazines, television and movies. A healthy body "looks good". It has a certain leanness; it is slim and muscular. It is also clean and (often) relatively hairless (at least neatly trimmed). It has a fresh smile. Or a serious, determined look on its face. Its eyes are clear, bright.
In a more substantial sense, being in good health simply means not suffering from any diseases and, more generally, a healthy body is a resilient body—one that doesn't get sick. We can add to this that a healthy body has strength and endurance. It is capable of effort. It also has a certain way of carrying itself. It is comfortable within its skin.
Health is, fortunately, still associated with moderation, balance, the middle way. Body builders are not "pictures of health". They are artists, working at the extreme limits of what their bodies can accomplish. Even professional athletes, like cyclists and football players, are not what we immediately think of when we think of healthy people. They are successful, to be sure, but we understand that they are sacrificing their bodies to the cause.
Like wealth, health is about knowing when you've got enough. The healthy body has the strength it needs to do the work that needs to get done. If you live on the fifth floor and can take the stairs easily, two steps at a time, you're demonstrating health. If you can get out of bed in the morning without grumbling (too much), you're demonstrating health. If you can stay awake, concentrate, and make intricate movements as needed you're in good, practical health. You've got good posture. You move gracefully. Etc.
The capacity for work has become an almost legal definition of health. The body is healthy so long as it can contribute to the gross national product. An unhealthy body, accordingly, is more difficult to insure. It is more likely to get hurt and sick.
I want to add a more subjective but very important aspect of health. It is a capacity for experience of a particular kind, namely, a capacity for pleasure. A healthy body is able to enjoy life.