Every individual on our planet Earth has some mass; of course, gravity plays a key role here. A human's mass varies depending upon his life stage. For instance, as soon as a child puts his feet on the ground, he starts carrying weight in the form of his or her own body weight and as age progresses, he starts carrying more weight. In fact, not only the body weight, we often carry additional load as well. This could be in the form of a school bag or some household stuff. Thus, by default, we start getting the benefit of weight training by putting some load on the muscles.
One can compare this with Astronauts who spend a good amount of time exploring space where gravity is minimal, and they return to earth with a reduced skeletal muscle mass and bone density.
PRIMARY ROLE OF THE MUSCLE IN THE HUMAN BODY
WHAT MAKES A HUMAN BODY MOVE?
Bones, ligaments and muscles are the structures that form levers in the human body to create human movement. In simple terms, a joint (where two or more bones join together) form the axis, and the muscles crossing the joint apply the force to move a weight. So basically, muscles help us in basic locomotion!
Therefore, if muscles are that important in order to create any comfortable body movement, then it's necessary for us to make them stronger. How? Well, we all know the answer, Weight Training!