There are layers of muscles in the muscular system. Muscles visible at the body surface are often called externus and superficialis, and they typically serve important functions to stabilize a joint or cause movement. With the naked eye it is often possible to identify the muscle group responsible for a certain action.
Major muscle groups of the body are shown in Fig. 1.11. The axial musculature begins and ends on the axial skeleton. Belonging to the group of axial musculature are the muscles of the head and neck that move the face, tongue, and larynx. The muscles of the spine include flexor and extensor muscles of the head, neck, and spinal column. The oblique and rec-tus muscles form the muscular walls of the trunk. In the chest area these muscles are partitioned by the ribs, but over the abdominal surface, they form broad muscular sheets. Trunk muscles keep the internal organs of the body intact, and in that function, they are similar to the corset that nineteenth-century women were obliged to wear in the Western world.
The muscles that stabilize the shoulder, hip, and the limbs are called the appendicular musculature. These muscles account for approximately 40% of the human musculature. The appendicular musculature is divided into two groups: (1) the muscles of the shoulders and upper extremities (arm, forearm, hand) and (2) muscles of the pelvic girdle (hip joint) and lower extremities (thigh, leg, foot).
Some of the muscles of the appendicular musculature act on a single joint. These are called monoarticular muscles. Gluteus maximus, the major muscle group of the buttocks, is a monoarticular muscle; it only acts on the hip joint. Other muscles may act at two or more joints. For example, the hamstring muscle, the semitendinosus and biceps femoris, traverses two joints and acts both on the hip and the knee. These muscles have the capacity to extend at the hip and flex at the knee. The quad muscle, rectus femoris, and the calf muscle, gastrocnemius, also act on two joints and as such are called biarticular muscles. What is the advantage of having polyarticular muscles in the human body? A plausible answer to this question may be that biarticular muscle, by affecting two joints at
front of arm (biceps)
front of arm (biceps)
inner thigh calf
(adductors) (gastrocnemius )
Figure 1.11a,b. Major muscle groups of the human body: front view (a) and back view (b). (Copied with permission from Bruce Algra Fitness Chart series).
the same time, helps prevent ligament injury in sudden acceleration and deceleration. The rate of rotation of a joint has to be zero at the instant the joint is fully extended. Otherwise, structures traversing the joint run the risk of damage. it has been suggested that biarticular muscles prevent the rotational energy of segments from reaching levels that could lead to injury to the ligaments and tendons.
Of the nine muscles that cross the shoulder joint to insert on the humerus, only the superficial pectoralis major, latissimus dorsi, and deltoid muscles are prime movers of the arms. The remaining six are syner-gists and fixators.
The pectoralis major muscle group of the chest originates in the cartilages of ribs two through six in the front, on the body of the sternum, and the medial portion of the clavicle, and inserts on the humerus (Fig.1.12). This fanlike muscle is of the convergent type and belongs to the appen-dicular musculature. it is responsible for pulling the upper arm across the body. it is a major climbing muscle in the sense that if the arms are fixed above the head, the massive power of the muscle can be used to pull the trunk upward. Because it is a triangular muscle, the line of action of the
force it generates may vary depending on which fibers are activated. As such the pectoralis flexes, adducts, and medially rotates the humerus. The muscle group under pectoralis major is the pectoralis minor. Its function is to move the shoulder girdle. it originates in ribs one through three and inserts at the scapula. Pectoralis minor depresses and protracts the shoulder, rotates the scapula, and elevates ribs if the scapula is made stationary with the use of other muscles that move the shoulder girdle.
Trapezoid muscles are located on the upper region of the back just below the neck. They originate along the middle of the neck and back and insert upon the clavicle and the scapula. The left and the right trapezoid muscles form a broad diamond. They are responsible for elevating the shoulders (shrugging) and for extending the head backward. Similar to the pectoralis major, this muscle group is innervated by more than one nerve, and specific regions can be made to contract independently. As a result, both the direction and the magnitude of the trapezoid muscle force vary greatly.
Latissimus dorsi is the largest muscle group of the upper body (Fig. 1.12). It is located on the back side of the body below the shoulder blades,
stretching between the thoracic vertebrae and the humerus. It is a climbing muscle responsible for pulling the arm downward and backward against resistance. This is a very powerful and important muscle. In people using crutches, the latissimus dorsi pulls the trunk forward relative to the arms. Because this muscle attaches to the pelvis, in patients with paralysis of the lower half of the body it can be used to produce movement of the pelvis and the trunk. Patients wearing calipers and using crutches can produce a modified gait by fixing the arms and hitching the hips by the alternate contraction of right and left latissimus dorsi.
Deltoids of the shoulder, a multipennate muscle group, consists of 11 muscles located on the upper side of the arms. Deltoids originate on both the clavicle and scapula and end on the humerus. This muscle group is responsible for raising the upper arm forward, lifting it sideways away from the body, and for rotating the arms front and back.
Rectus abdominis is an axial muscle group that is arranged in parallel between the chest and the pelvis. It originates at the hip bone and inserts at ribs five through seven and at the lower tip of the sternum. It is responsible for spinal forward flexion and is used to contract the upper body toward the lower body. Obliques, located on both sides of the abdomen, are also part of the axial muscle group. They originate at the vertebrae and insert at the rib cage. They are responsible for moving the upper body from side to side.
Erector spinae is the main muscle group in the lower back. The erector spinae muscles include superficial and deep layers that align approximately along the long axis of the body. When contracting together, they extend the spinal column. When only the muscles on one side contract, the spine is bent laterally. Overall this muscle is responsible for extending or straightening the upper body from a bent-over position.
Biceps of the upper arms originates on the scapula and ends on the radius of the forearm. It is a parallel muscle (Fig. 1.12). Its major responsibilities are to supinate the forearm (as in inserting a corkscrew) and to flex the forearm upward toward the shoulder (as in pulling out the cork). Because the biceps is biarticular, it assists pectoralis major in the flexion of the upper arm. However, in the midst of other strong muscle groups that move the shoulder, its effect on the shoulder is of secondary importance. The brachialis and brachioradialis act as synergizers of biceps in flexing the lower arm.
Triceps, a group of three muscles, is located on the back of the upper arm (Fig. 1.12). The long head of triceps brachii originates on the scapula and the lateral and the medial heads on the humerus. All three insert on the ulna. Triceps function as agonists to biceps; they are responsible for straightening the arm at the elbow. The triceps extend the forearm during push-ups, and in various forms of pushing and punching. They are essential to execute a karate chop. A person in a wheelchair uses triceps to push the wheel around and propel the chair forward. In extending the forearm, triceps are aided by synergizers such as anconeus. In addition to these major muscle groups that move the shoulder, upper arm, and forearm, there are a large number of muscles that are specialized to flex and extend the wrist and to move the hand and the fingers. In fact, the arm contains 72 muscles, most of which are found in the lower arm. Their functions are to move the hand and the wrist. Small muscles in the hand control the finger and thumb movements.
Gluteus maximus, gluteus medius, and gluteus minimus are the main muscle groups that make up the buttocks. They originate at different locations on the hip bone and insert on the femur. Gluteus maximus extends the thigh in such activities as stepping up onto a stool, climbing stairs, and running. With the hamstrings it raises the trunk from a flexed posi tion. Gluteus medius and gluteus minimus abduct and medially rotate the thigh and support the pelvis in walking and running. Adductors are five muscles that are located on the inside area of the upper leg; their function is to bring the legs together. They are monoarticular muscles.
Quadriceps (the vastus lateralis, the vastus intermedius, the vastus medialis, and the rectus femoris) are located at the front of the leg above the knee (see Fig. 1.12). They are responsible for straightening the lower leg at the knee joint. We use quads in stair climbing, squats, and walking and running. In the standing position this muscle group performs very little action because the knees are in the locked position. That is why a person collapses when their knees are knocked from behind.
Hamstrings are the primary muscles located at the back of the thighs (Fig. 1.12). They are the semimembranosus, the semitendinosus, and the biceps femoris, which is of the bipennate type. These are responsible for contracting and extending the lower leg and for raising the heel toward the buttocks. Another important function is raising the trunk from a flexed position. This action requires a great deal of power because the weight of the trunk acting on the other side of the hip joint produces a large moment and these muscles work with a very short lever arm in the hip joint. This difficult task explains why hamstrings typically are quite bulky.
Gastrocnemius, the muscles of the calves, are located on the back side of the lower leg below the knee (Fig. 1.12). They are responsible for raising up onto the toes. These muscles are essential for such activities as running, walking, and jumping.
According to the rate at which a muscle generates force or shortens following activation, it falls into one of these three categories: fast, slow, and intermediate. Fast fibers are large in diameter. The fast muscles have the capacity to contract within 0.01 seconds (s) or less following stimulation. However, because readily available chemical energy within a muscle cell can be rapidly exhausted in sustaining contraction, fast fibers, fatigue easily. Slow fibers are about half the diameter of fast fibers, and they contract three times slower than a fast muscle. Intermediate fibers contract at intermediate rates. The percentage of fast versus slow fibers in each muscle is genetically determined, and there are significant individual differences. Marathon runners with a high proportion of slow muscle fibers in their leg muscles outperform those with faster muscle fibers.
The proportion of intermediate fibers within a muscle tissue changes with physical conditioning. Repeated and exhaustive contraction of a skeletal muscle leads to the hypertrophy or enlargement of the muscle. Following repeated stimulations that cause near maximum tension, the cross-sectional area of the fiber as well as the number of contractile fibrils increase. If used repeatedly for endurance events, fast fibers begin to function as intermediate fibers. On the other hand, when a skeletal muscle is not stimulated by a motor neuron on a regular basis, it loses muscle tone and mass. Muscle atrophy is initially reversible, but dying mus cle fibers are not replaced. When a muscle tears or strained, it was believed until recently that the damaged muscle had to depend on local cells to repair injury. However, recent research indicates that bone marrow contains cells that repair damaged muscles. This finding raises the exciting possibility that it may one day be possible to replenish degenerating muscles with fresh cells from the patient's bone marrow.
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