Muscles are the contractile component of the musculo-skeletal system. Basically, muscles act like bungee cords that expand and contract in order to move the bones and joints of our body. Muscles have a few basic structures, and several terms associated with them.
MUSCLE BELLY:
The belly of a muscle is what we associate the most with the term “muscle”. It is the middle section of the muscle, and is composed of many striated or straight muscle fibers. It is in the muscle belly that the contractile tissue resides which allows muscles to lengthen and contract. In the analogy of the bungee cord, this would be the elastic part in the middle of the cord.
TENDON:
The tendon is the structure that attaches the muscles of our body to the bones of our body. Tendons are composed of extremely strong fibres, and are often surrounded by a sheath so that the entire complex acts like a sword in a scabbard (ie- sliding back and forth). In the bungee cord analogy, the tendons would be the “hook” part of the cord.
ORIGIN:
The origin of a muscle is the bone, or part of a bone, where the muscle is said to arise from. Rule number one of anatomy is that A MUSCLE WILL ALWAYS PULL TOWARD ITS ORIGIN. For example, the biceps has its origins at two points on the shoulder blade, and thus as it pulls it brings the upper arm upwards towards the shoulder.
INSERTION:
As opposed to the origin of a muscle, the insertion is the bone (or part of a bone) where the muscle attaches. To use the above example, the biceps originates on the shoulder blade and ten ATTACHES onto an area of the radius in the forearm.
RULE #1 :THE ORIGIN AND INSERTION OF A MUSCLE WILL ALWAYS BE ON SEPARATE SIDES OF A JOINT, IN ESSENCE CREATING A ROPE AND PULLEY EFFECT!
INNERVATION:
Nerves arise in the cortical area of the brain, descend the spinal cord, and exit at various levels in the spine to feed into the surrounding musculature and initiate movement. Basically, the muscular nerves act like a telephone system allowing our brain to “dial up” a muscle and tell it to move.
ACTION:
The action of a muscle is, quite simply, what the muscle does. For example the action of the quadriceps muscle is to extend the leg at the knee, the action of the triceps muscle is to extend the arm at the elbow. Knowledge of the origin and insertion of a muscle, and always heeding Rule #1 of muscles, will allow anyone to understand what the action of a given muscle will be. When we know what the action of a muscle is, we know how to train it, to rehabilitate it, to stretch it and to prevent further injury!