You’ve Got to Move It Move It
TYPES OF MUSCLES
Skeletal muscles are one of three types of muscles in the human body (the other two are cardiac and smooth). They are the engines that enable the human body to move and perform physical tasks as simple as holding a coffee cup or as complex as ballet dancing. Depending on how you divide up the skeletal muscles in the human body, there are well over 600 named muscles, with some estimates reaching more than 800.
Muscles of the Head and Neck
The lips of the human body, which can be so very expressive, are in fact a single oval-shaped muscle called the orbicularis oris. Likewise, surrounding each eye is a circular muscle called the orbicularis oculi, which is responsible for the movement of the eyelids. There are many other smaller muscles for facial expressions, including the frontalis, the forehead muscle that raises the eyebrows, and the buccinators, which form the cheeks and pull them inward when contracted. For chewing, no other muscle is as important as the masseter. Its insertion into the mandible leads to the powerful closing of the jaw during eating.
Several muscles of the neck help to move the head from side to side and up and down. The most prominent of these muscles is the sternocleidomastoid. It runs from the temporal side of the skull around to the ventral (front) side of the lower neck and connects with the sternum, which leads to the action of pulling the head to the side of the contracted muscle. Since this is a paired muscle, with one on the right and one on the left, they give the appearance of a V-neckline on the ventral lower neck.
On the dorsal (back-side) surface, a portion of a larger muscle, the trapezius, connects to the back of the skull as well as the cervical and thoracic vertebrae in a fanlike pattern and inserts into the scapula, allowing various movements of the scapula.
Muscles of the Chest and Shoulders
The muscle that forms the bulk of the human chest is the pectoralis major, often simply referred to as the pecs. Much like the trapezius, the pectoralis attaches to the sternum in a fan-shaped pattern, with the fibers coming to a point and inserting into the upper portion of the humerus of the forearm. Contraction of this muscle causes the humerus to be pulled toward the ventral midline of the body, therefore adducting the arm.
Anatomy of a Word
adduction and abduction
Adduction is a muscular action in which a body part is moved toward the midline of the body. You can remember this by saying the movement adds to the midline. If the body part is removed from the midline, the action is referred to as abduction (which means “taking away”).
Abduction of the arm is largely accomplished by the action of the deltoid muscle. This large muscle composes the bulk of the shoulder. It has dorsal, ventral, and medial portions attached to the scapula, clavicle, and humerus. Contraction of this muscle raises the arm. While the pectoralis and the deltoids are the most prominent of the muscles that move the arm, many other smaller and deeper muscles mediate all other actions of the arm and allow it a wide range of motion and action.
Muscles of the Arms
In the upper arm, two muscles work antagonistically, or against each other, in flexing (bending) or extending (straightening) the forearm. The large muscle on the ventral surface is called the biceps brachii, which is incompletely divided into a long and short head (hence the prefix bi-, meaning “two”). The biceps pulls the forearm closer to the upper arm when contracted and thus flexes the arm. Straightening the forearm is the task of the triceps brachii on the ventral side of the upper arm. The name of this muscle stems from its division into three heads, two of which are visible superficially with the third being deep.
The forearm has a highly intricate collection of small muscles that assist in moving it as well as moving the wrists and fingers. The muscles compose the most proximal portion of the forearm and connect to bones of the wrists and fingers via long tendons, which are used to perform specific actions. Flexors contract and extensors straighten.
Anatomy of a Word
proximal
Proximal generally means “closest to the body.” In some instances it is used to refer to the area closest to the point of attachment.
Additionally, other muscles mediate pronation (twisting of the hands from a palms-up position to a palms-down position). Conversely, other muscles called supinators rotate the hands back to a palms-up position.
Muscles of the Back and Hips
Starting at the highest aspect of the back (dorsal surface) is the aforementioned trapezius muscle. The remainder of the superficial back muscle is the large fan-shaped latissimus dorsi. Attached along the vertebral column from the thoracic region downward to the sacrum, it extends to a point under the arm and inserts into the humerus. In bodybuilders, contraction of this muscle causes the edge of the muscle to extend out laterally on both sides of the body, much like the hood of a cobra fanning out around the head.
Other deeper muscles of the back include the rhomboids (major and minor), which also attach to the vertebral column and cause various movements of the scapula when contracted.
Making up the dorsal hips (also called rear, posterior, and buttocks), are the three gluteus muscles. The gluteus maximus is the largest of the three and produces the bulk of the posterior hip tissue. When contracted, this muscle extends the hip and brings the thigh into a straight line with the hip. Along the side of the hip near the waist is the gluteus medius, which along with the deep gluteus minimus abducts and rotates the thigh to the side.
Muscles of the Abdomen
While not everyone has a clearly visible 6-pack, everyone does possess the paired rows of five muscle bundles that lie along the ventral midline of the body, called the abdominal muscles, or rectus abdominis. It is the middle three muscle bundles that are often called the 6-pack. Along with the rectus abdominis, the following muscles are responsible for tensing the abdominal wall and compression of the abdominal contents:
· From the rectus abdominis running along the side of the body is the external oblique muscle.
· Just beneath the external oblique is the internal oblique.
· The transversus abdominis, the deepest of the abdominal muscles, lies underneath the internal oblique muscle.
Muscles of the Legs
The ventral surface of the thigh is made up of a major group of muscles collectively referred to as the quadriceps (quadriceps femoris). This group comprises the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The prominent rectus femoris is the middle and superficial muscle of the group, with the vastus lateralis and medialis on the outside and inside of the thigh, respectively. The intermedius is a deep muscle lying underneath the rectus femoris. Together, these muscles function to extend the knee (straighten the leg).
On the back of the thigh (dorsal side), the group of muscles that function to flex (bend) the knee and pull the heel upward toward the hip is called the hamstring group. The two superficial muscles of this group are the biceps femoris and the semitendinosus. These paired muscles lie along the midline of the dorsal thigh, with a deeper, more medial (inside) muscle called the semimembranosus completing the group.
The calf muscle is formed from the two-headed gastrocnemius muscle. This muscle is connected to the heel (calcaneus bone) via the long, tough calcaneal (Achilles) tendon. Contraction of this muscle pulls the heel upward and straightens the foot. Imagine someone standing flat-footed and then rising up on the tips of her toes, like a ballet dancer. As in the hand, many smaller muscles function to flex, extend, and rotate the foot in several different directions.