When you think of body organs, your muscles might not be the first thing that comes to mind. However, the muscular system is one of the 11 organ systems in the human body. In addition to your muscles, this system contains tendons — connective tissue that attaches muscle to bone to allow your skeleton to move.
In addition to muscle tissue, the muscular system organ also includes tendons — the structures that connect your muscles to bone.
Read more: Big Versus Small Muscles
Types of Muscle Tissue
There are three main types of muscle tissue — visceral, cardiac and skeletal. Visceral muscle is also called "smooth muscle." When compared to its counterparts under a microscope, it has a smooth appearance. Cardiac and skeletal muscles both have striations, or dark and light stripes running across them.
Visceral muscle can be found inside other organs such as your stomach, blood vessels, liver and intestines. It's this type of muscle that moves food through your digestive system and moves your lungs when you breathe. Your brain automatically, or involuntarily, controls visceral muscle, without your having to think about it.
Cardiac muscle is specific to the heart. Like visceral muscle, it is also involuntary. However, your brain does not directly control your heart. While the brain influences how quickly your heart beats, electrical impulses within the heart cause it to contract. Even if the brain dies, the heart will continue to contract.
Skeletal Muscle System
Skeletal muscles are attached to the bones of your skeleton. There are more than 700 muscles in the human body. Even the movement of your eyeball requires the work of six different muscles. Contraction of skeletal muscles is voluntary — if you want to get something off the shelf, your brain sends electrical messages along nerves to tell your arm muscles to move.
Muscles are grouped according to which part of the skeleton they move. Axial muscles move your head, neck, spine, trunk and pelvic floor. Appendicular muscles move your arms and legs, or appendages.
Understand Your Muscles
Each muscle in your body has its own name. Muscles are named based on several factors, including their function, location, how many bony attachments they have and their shape. For example, the pronator quadratus muscle in the forearm performs pronation — rotating your hand to a palm-down position. The adductor magnus on your inner thigh adducts or moves your thigh in toward the middle of your body.
The supraspinatus muscle is named for its location — supra, or "above," and spinatus "spine of the scapula." This muscle sits on the back of your shoulder blade, above a bony ledge called the spine of the scapula. The sternocleidomastoid muscle, also named for its location, attaches to three bony landmarks — the sternum, clavicle and mastoid bone of your skull.
Where Muscles Attach
Muscles have an origin and an insertion. The origin is typically attached to a stable part of the skeleton, while the insertion is further away on a bone that does the moving. For example, the deltoid muscle on top of your shoulder originates on your shoulder blade and collarbone; inserts on your humerus, or upper arm bone; and is attached by a tendon. This muscle lifts your arm forward, out to the side and backward.
The quadriceps muscle group is named after its number of attachments. This large muscle mass on the front of your hip and thigh has four separate origins that come together to form one large tendon at your knee. Individually, these muscles are called the rectus femoris, vastus lateralis, vastus intermedius and vastus medialis. The biceps brachii and triceps brachii muscles are also named for their multiple attachments.
Some muscles are named after their shape. The serratus anterior muscle at the front of your chest looks like a serrated knife or saw. The large trapezius muscle on your back resembles a trapezoid.
Read more: Human Leg Muscles & Tendons
Muscular System Function
The main function of the skeletal muscle system is movement. However, muscular system organs provide protection to other organs. For example, your abdominal muscles protect internal organs such as your intestines and bladder. Skeletal muscles also help maintain your body temperature when you get cold by shivering.
Muscles work together to produce movement. The main muscle that is performing a movement is called the agonist, or prime mover. For example, when you bend your elbow, your biceps muscle is the agonist. However, the biceps does not work alone. Synergists, or helping muscles, such as the brachialis and brachioradialis also bend the elbow.
To allow your elbow to bend, muscles on the opposite side of your elbow have to relax. The opposing muscle group is called the antagonist. The triceps muscle on the back of your arm straightens your elbow, making it the antagonist during elbow flexion.
Read more: How Do the Biceps & Triceps Work Together?
Those Tough Tendons
Force generated by your muscles is transmitted to your tendons, which in turn, causes your skeleton to move. Tendons are stiffer than muscles, and very strong. For example, tendons in the bottom of your foot can withstand more than eight times your body weight.
Nevertheless, tendons are susceptible to injury, particularly with overuse of a particular muscle group. Because they are stiff, tears can occur when a tendon is overstretched.
Some tendons are enclosed in sheaths, or fluid-filled tunnels that help hold them in place and glide smoothly. For example, each individual tendon that moves your fingers and toes has its own sheath.
Muscles and Aponeuroses
Another less commonly known muscular system organ is the aponeurosis. This structure is made of tendon-like connective tissue and also connects muscle to bone, muscle to muscle, and muscle to tendons. Unlike a tendon, aponeuroses are thin, flat sheets of connective tissue.
The Musculoskeletal System
Because they are intricately linked, the muscular and skeletal systems are often talked about as the musculoskeletal system. This system also includes bone, cartilage — the padding between bones — and ligaments that attach bone to bone.