Psychology = Sagittarius is the virtue & principle of spiritual Sovereignty and trust in one faith. This energy is the masculine side of Jupiter, so it makes you want to express your wisdom and knowledge before receiving experiences, which can make you seem chaotic or separate or detached since it is expressed through the element of fire, which deals with burning through physical reality. This is how you know that your “Spirit” is using the masculine side of Jupiter, which is the (Third eye Chakra) when you want to act on what makes you Expand, aka long-distance travel, whether it’s physical or mental. Furthermore, Sagittarius rules the muscular system, which is made of different muscle fibers that allow moment (Mutable fire) by muscle contraction and lengthening; if you do any form of exercise or yoga, you gain or extract some form of wisdom & knowledge from experience.
Physiology = The muscular system is composed of specialized cells called muscle fibers. Their predominant function is contractibility. Muscles, attached to bones or internal organs and blood vessels, are responsible for movement. Nearly all movement in the body is the result of muscle contraction. Exceptions to this are the action of cilia, the flagellum on sperm cells, and the amoeboid movement of some white blood cells. The integrated action of joints, bones and skeletal muscles produces obvious movements such as walking and running. Skeletal muscles also produce more subtle movements that result in various facial expressions, eye movements, and respiration.