The Muscular System

The muscles in the body do not only support the movement of the body, but it also helps stabilize the joints, maintain posture, and generate heat during activity. Muscles are attached to the bones of the skeletal system, with more than 600 named muscles that make up half a human’s body weight.

The Muscular-Skeletal system is both an organ and a biomechanical system. The skeletal system serves to protect the body’s organs, support the weight of the body, while the muscular system attached to these bones pulls them to allow for movement of the body.

Each group of muscles in the body has its own function, let’s start classifying them accordingly.

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Prime Movers

Prime movers are primary muscles that are involved in the activation of many major joints within the body.

Latissimus Dorsi, or is commonly referred to as the back muscles, is made up of two large muscles on the left and right posterior of the upper body. It allows the extension of the shoulder joint. In other words, by allowing the extension of the shoulder joint, adduction of the arm is possible. They are generally responsible for most of the pulling actions of the upper body.

Pectorals are more commonly known as the chest muscles. It is located on the right and the left side of the anterior side of the upper body. Made up of an upper and lower muscle group, its primary function is the abduction of the arms and flexion of the shoulder joints. This muscle group is responsible for pulling actions of the upper body.

Quadriceps is made up of four separate muscles that work together to allow the extension of the knee joint. It is located on the anterior, or front, of the upper thigh. These muscles allow the extension or raising, of the lower leg.

The function of the hamstrings is to allow for flexion of the knee joint. It is made up of three separate muscles and it is located on the posterior side or back, of the upper thigh. Hamstrings allows the bending of the knee and the lowering of the lower leg

The deltoids, or shoulder muscles, work along with the muscles of the chest and the back, to allow for abduction and adduction of the arms and the extension and flexion of the shoulder joint. It is made up of three separate parts–the anterior, or front shoulder, the medial, or middle shoulder, and posterior, or back shoulder.

Biceps and Triceps are muscles that are located on the front and back of the upper arm. The responsibility of these muscles is the flexion and extension of the elbow joint, or movement of the lower arm. The biceps are made up of two separate muscles while the triceps are made up of three separate muscles.

Antagonist Muscles

Antagonists muscles are muscles that oppose the motion of your body. The function of the antagonist, the abdominals, is to slow down and stop the spine if it moves too fast or too far backward.

On the opposite side of the body are the abdominal muscles. The rectus abdominis is the most well-known ab muscle. It runs from the bottom of the ribs down into the front of the pelvis and it acts to flex the spine, which is the opposite movement of the back extensor muscles. The external abdominal obliques and transversus abdominis also helps flex the spine.

The external abdominal obliques are the ab muscles on the sides of your torso, which runs from the bottom and sides of your ribs down into your pelvis. The transversus abdominis is the deepest ab muscle. It covers the bottom of your sternum, down to the pelvis, and back to the sides of your hips.

The rectus abdominis, external oblique, and transversus abdominis all flex the back, which makes them antagonists to the back extensor muscles.

Synergist Muscles

When a group of muscles work together to perform a given task this is known as a muscle synergy. They are muscles that stabilise a joint around where movement is occurring. They also help to create the movement. Other muscles, such as stabilizers, neutralizers, and fixators, that help stabilize the joint are also synergists. Fixator muscle is a stabilizer that acts to eliminate the unwanted movement of an agonist, or prime movers, origin.

Skeletal muscles can also be named based on these seven specific criteria.

The Location of the Muscles

Muscles are located all around the human body. The major muscles of the back, shoulder and chest are the Erector spinae, Latissimus dorsi, Trapezius, Rhomboids, Deltoid and the Pectoralis major. On the upper arm, there is the Biceps brachii and Triceps brachii. On the abdomen, there is the Rectus abdominis, External obliques and Internal obliques. On the hip region, you can find the Gluteus maximus, Gluteus medius, Gluteus minimus, and Hip flexors. On the lower body, there’s the Quadriceps and Hamstrings. And on the calves, there is the Gastrocnemius and Soleus

The Shapes of the Muscles

Muscles can appear circular in shape, they are muscles that surround an opening such as the mouth, and the eyes.

There is also the convergent muscles, or also sometimes known as triangular muscles, an arrangement that allows maximum force production.

Another is the pennate muscles, which can be divided into; Unipennate, arranged to insert in a diagonal direction onto the tendon, which allows great strength. Bipennate, which has two rows of muscle fibres, facing in opposite diagonal directions, with a central tendon, that allows even greater power but less range of motion, and the Multipennate, which has multiple rows of diagonal fibres, with a central tendon which branches into two or more tendons.

There are parallel muscles, they run parallel to each other and are sometimes called strap muscles. They are normally long muscles which cause large movements.

Fusiform muscles, sometimes included in the parallel muscles, are more spindle-shaped, with the muscle belly being wider than the origin and insertion.

The Size of the Muscles

Muscles are also grouped by their size. For example, the Gluteus maximus and the Gluteus minimus are both located in the hip region but they have different sizes and functions. The Gluteus maximus is responsible for the extension and lateral rotation of the leg, while the Gluteus minimus is responsible for the abduction and medial rotation of the leg.

The Direction of the Muscles

When looking at muscles, we can often see the appearance of lines running within. The lines are made of muscle fibres, and the direction that these fibres run in relation to the midline of the body is often used to provide partial names to different muscles. If the fibres of the muscle are parallel to the body’s midline, the muscle is often described as the rectus. An example of a muscle that have the term rectus in its name is the rectus femoris.

On the other hand, if the fibres of the muscle run at an angle to the body’s midline, they are said to run obliquely.

The Number of Origins

Most of the muscle names indicate the number of muscles in a group. An example of this is the quadriceps, it is a group of our muscles located on the anterior. There are also muscle names that can provide information as to how many origins a particular muscle has, such as the biceps, and triceps. The prefix bi indicates that the muscle has two origins and the tri indicates that there are three origins.

The Location of Attachment

Each muscle has two main points of attachment: the insertion point, and the origin point. The insertion point is the moveable end of the muscle, it is attached to the structure that is being moved, and the origin of the muscle is the end that doesn’t move, it is the fixed end.

We can also tell the location of a muscle’s attachment based on the appearance in its name. For example, sternocleidomastoid muscle of the neck has two origins, on thesternum (sterno) and clavicle (cleido), and it inserts on the mastoid process of the temporal bone.

The Type of Motion

The other way of knowing the name of a muscle is its action. Here are instances where muscles are named for the movement they produce, flexor (decreases the angle at the joint), extensor (increases the angle at the joint), abductor (moves the bone away from the midline), or adductor (moves the bone toward the midline).


Muscles are made up of a bundle of structures. When prompted, they contract to produce motion. Muscles also contribute to the function of the circulation of the blood, the mobility of the body, respiration, digestion, urination, childbirth, vision, the stability of the body, and posture.  Without muscles, your body wouldn’t be able to function.