First, in fitness training, we usually refer to skeletal muscle. Skeletal muscle covering our whole body is striated muscle, which is controlled by somatic motor nerves, and we can actively control it (we can't actively control smooth muscle controlled only by autonomic nerves).

Second, muscle is made up of a large number of muscle fibers thinner than hair. One of the characteristics of muscle fibers is that every time skeletal muscle exercises, a very small amount of muscle fibers will be pulled and broken (note that this is only a very small injury, unlike the large-scale tearing of muscle fibers during muscle strain), and then the newly grown muscle fibers will be stronger than before. This is an ability that organisms have acquired in long-term evolution, which can be seen in creatures with muscle tissue. With this ability, we can ensure that biological individuals can better change their body shape and physical fitness with the environment, making them better at running, swimming or climbing trees.