The muscles of the body were designed to move in order to function properly. A muscle that is stagnant for a long time will eventually degenerate and lose its function. When muscles are exercised, they become stronger, revitalized and regenerated – not weak or worn out. Therefore, Maimonides considers physical exercise to be one of the most important conditions for maintaining a healthy body. Here’s what he says in Sefer Hamada:
They have given another principle with regard to physical well-being: As long as one exercises, exerts himself greatly, does not eat to the point of satiation and has loose bowels, he will not suffer sickness and he will grow in strength. [This applies] even if he eats harmful foods. [Conversely,] whoever is idle and does not exercise, or does not move his bowels when he has the need, or is constipated, even if he eats the proper foods and takes care to follow the rules of medicine, will be full of pain for all his days and his strength will fade away.
When Maimonides says “exercise” he is not necessarily referring to the kind of exercise we do today. He is talking about general physical exertion, so if a person’s job includes physical labor which activates his whole body, then he is not required to exert additional effort. However, a person whose work does not include this kind of physical labor must dedicate some time to physical exercise. He does not need to exercise in an extreme way, all he has to do is perform exercises that will rejuvenate and strengthen all of his muscles. This will ultimately reinvigorate the person to serve G-d.
Maimonides, who compiled the fourteen parts of Yad Hachazaka (and enumerates the laws associated with the spiritual service of G-d), felt the need to explain why he included the part that discusses physical health. He writes as follows:
Since maintaining a healthy and sound body is among the ways of G-d – for one cannot understand or have any knowledge of the Creator if he is ill, therefore, he must avoid that which harms the body and accustom himself to that which is healthful and helps the body become stronger.
There are many opportunities in life in which we can perform this mitzvah. For example, a person who shows up early to an appointment, will usually try to look for the closest parking spot – he may even argue with the other driver about the spot. But the truth is, that if we arrive early to a meeting, and we’re not in a rush, we should park the car a little farther and take advantage of the opportunity to walk by foot. The same is true for using an elevator. There are people who are willing to wait a long time for the elevator just to go up one floor. A wise person on the other hand, will seize the moment and take the stairs so he can activate the muscles of his body, and if time is not a factor for him, he should use the stairs even more.
Today however, we do not move enough. The advent of technology has replaced many of our physical tasks, making our daily activities insufficient in maintaining our physical health. Therefore, exercise regimens should be implemented. Devoting a little time for exercise everyday can provide the necessary results. However, the idea of working out in order to shape and design the body as a goal in itself must be obliterated! This mindset is completely false and creates an opening for all earthly desires. Instead, one should always remind himself that one day this body will return to the dust, as the Mishna states:
Know from where you have come, to where you are heading etc. From where have you come – from a putrid drop; to where are you heading – to a place of dust, worms and maggots…
The Greek culture, which rejected the concept of the soul, encouraged the idea that physical cultivation should be the greatest aspiration. It was the progenitor of all types of sports (the word “sport” comes from the name Sparta; the Greek city that it originated in), and pushed the spirit aside in order to focus solely on the body. However, in Judaism, physical exercise should be performed only for health reasons. The body’s function is to serve as a strong and healthy vessel for the containment of the soul. Its purpose is to help the soul withstand challenges and shape its spiritual world.
We will conclude this chapter with the wording of Maimonides who guarantees that all those who follow these guidelines, will have a long and healthy life (unless he was born with problematic health issues, has already damaged his health, or was previously harmed by a superior force). Here are his words:
Whoever conducts himself in the ways that we have drawn up, I will guarantee that he will not become ill throughout his life until he reaches advanced age and dies. He will not need a doctor. His body will remain intact and healthy throughout his life. One may rely on this guarantee unless [his body] was impaired from the birth, he was accustomed to one of the harmful habits from birth, or unless there’s a plague or a drought in the world.
Notes and Sources
 Rambam Hilchot De’ot 4:14-15
 Rambam Hilchot De’ot 4:1
 Avot 3:1
 Rambam Hilchot De’ot 4:20
Adapted from “The Keys to Life” by Rabbi Zamir Cohen