Lever Systems

Lever Systems

Most skeletal muscles use leverage in order to move. To understand further, a lever is any rigid structure that moves on a fixed point, which is known as a fulcrum, when force is applied. This applied force is known as effort, which is used to move a load, which is a resistance. The body’s joints act as fulcrums, while the bones act as levers. Muscle con-traction creates the effort applied at an insertion point on a bone. The bone itself as well as its overlying tissues and anything else being moved together act as the load.

Levers

The concept of levers involves the movement of loads that are heavier, or the movement of loads at a faster speed or over a further distance. When a load is close to a fulcrum and effort is applied further away from the fulcrum, just a small effort over a relatively large distance can move a large load over a small dis-tance (FIGURE 10-2). This operation or mechanical­ advantage is often referred to as a power lever. Think of a car jack, in which a heavy vehicle is moved upward for only a small distance, in order to change a tire. The muscle effort needed to make the jack work is very small.

However, when a load is far from a fulcrum and effort is applied near the fulcrum, the exerted force must be greater than the load that will be supported or moved. This operation or mechanical disadvantage is often referred to as a speed lever (FIGURE 10 -3). They allow a load to be moved quickly, over a large distance, with a wide range of motion such as using a shovel to dig a hole. Small differences in the muscle’s insertion site compared to the joint can create large differences in the amount of force needed to move a certain load or resistance.

Classes of Levers

There are three basic classes of levers, which can be understood by their comparison to certain muscle actions in the body:

■■ First-class levers: Effort is applied at one end while the load is at the other end and the fulcrum is somewhere in between such as in scissors or seesaws. In the body, first-class leverage occurs when the head is lifted off the chest. Some first-class levers operate for strength (mechanical advantage) and some operate for speed and distance (mechanical disadvantage).

■■ Second-class levers: Effort is applied at one end of a lever while the fulcrum is at the other end and the load is in between such as in a wheelbarrow. This type of lever is not common in the body, but exists such as when we contract the calf muscles to elevate the body and stand on our toes. Second-class levers have mechanical advantage since muscle insertion is always farther from the fulcrum than the load. These are levers of strength and do not have much speed or range of motion. An example is contracting the calf muscles to elevate the body on the toes.

■■ Third-class levers: Effort is applied between the load and fulcrum operating at a mechanical disadvantage such as in forceps or tweezers. The majority of skeletal muscles utilize third-class levers. Examples include how the biceps muscle lifts the distal forearm and anything being carried by the hand. These levers allow a muscle to be inserted very close to a joint where movement occurs. Rapid and extensive movements are allowed, such as when throwing a ball, and there is not much shortening of the muscle. Related muscles are usually powerful and thicker than other muscles. Third-class levers are the most common lever systems in the body.

Superficial Skeletal Muscles

There are more than 600 skeletal muscles in the human body. Superficial muscles are located near the surface of the body. FIGURES 10-4 and 10 -5 show anterior and posterior views of the superficial skeletal muscles.

Axial and Appendicular Muscles

The muscles are subdivided into the axial and appen-dicular sections. The axial muscles affect the head, spinal column, and rib cage. They allow for position-ing of the head and spinal column, move the rib cage, and assist in the breathing process. They do not move the body or support the pectoral ­girdle, pelvic­ girdle, or limbs. About 60% of the ­skeletal ­muscles are axial muscles. The appendicular­ muscles stabilize and move the appendicular skeleton, and make up the remaining 40% of skeletal muscles.

The major axial and appendicular muscles are superficial and larger than many other skeletal mus-cles. They cover deeper and smaller muscles that are not visible, unless the overlying muscles are removed or reflected, which means to cut them and pull them out of the way.

It is important to understand the term ­innervation, which is the distribution of nerves to a region or organ. For example, many head and neck muscles are innervated by the cranial nerves. These nerves begin in the brain, passing through the foramina of the skull. Also, spinal nerves connect to the spinal cord, passing through the intervertebral foramina. Spinal nerves may form a complex plexus or network, after they exit the spinal cord. A single branch of a plexus may contain axons from several of the spinal nerves.

Muscles of Facial Expression

The muscles of facial expression begin on the sur-face of the skull. At their insertions, the epimy-sium fibers weave with those of the superficial fascia and the skin’s dermis. The skin moves when these muscles contract. FIGURE 10-6 shows various facial ­expressions, listing the muscles that are used to make them. TABLE 10-1 describes the muscles of facial expression.

Muscles of the Tongue, Pharynx, Larynx, and Palate

The muscles of the tongue have names that end in glossus­. The muscles of the pharynx initiate the swal-lowing process. The laryngeal muscles control the posi-tion of the larynx. The muscles of the palate ­elevate the soft palate and nearby portions of the pharyngeal wall, and they also pull the entrance to the auditory tube open. TABLE 10-2 describes the muscles of the tongue, pharynx, and palate.

Muscles of Mastication

The muscles of mastication move the mandible at the temporomandibular joint (TMJ). The strongest jaw muscle is the masseter. TABLE 10-3 describes the muscles of mastication and FIGURE 10-7 shows these muscles.

Muscles That Move the Head

The muscles that move the head originate from the axial skeleton. The major head flexors are the sternocleidomastoid­ muscles, assisted by the suprahyoid­ and infrahyoid muscles. Lat-eral head movements occur when muscles on just one side of the neck contract. Head extension is aided by the ­trapezius muscles in the back, but the ­primary ­extensors of the head are the splenius muscles­ lying deep below them (TABLE 10-4). The muscles of the face and anterior trunk are shown in ­FIGURE 10-8.

Anterior Neck Muscles

The anterior neck muscles, described in TABLE 10-5, affect the larynx, mandible, floor of the mouth, tongue, pharynx, clavicle, sternum, scapula, hyoid, and even the first rib.

Muscles That Move the Pectoral Girdle

The muscles that move the pectoral girdle help to elevate, depress, rotate, and move the scapula both ­laterally and medially. The clavicles rotate around their axes, providing stability and ­precision to the scapula. Most anterior muscles stabilize and depress the shoulder girdle. The scapular muscle attachments­ are arranged so that a single muscle cannot cause a simple linear movement by itself. Instead, several muscles must act together (TABLE 10-6). The muscles of the shoulders and back are shown in FIGURE 10-9.

Muscles of the Vertebral Column

The muscles of the vertebral column lie beneath the more superficial muscles of the back such as the ­trapezius and latissimus dorsi (TABLE 10-7). The erec-tor spinae extend the vertebral column when they contract together. When the muscles on one side con-tract, the vertebral column experiences lateral flexion.

Muscles That Move the Arm

The muscles that move the arm (TABLE 10-8) are easy to remember since they are grouped by their actions at the shoulder joints. Of all the arm mus-cles, the deltoid is the major abductor. The muscles of the anterior shoulders and arms are shown in FIGURE 10-10.

Muscles That Move the Forearm

The muscles that move the forearm are described in TABLE 10-9 and shown in Figure 10-10. These muscles­ as well as the muscles that move the hand mostly orig-inate on the humerus. However, the biceps brachii and the long head of the triceps brachii originate on the scapula.

Muscles That Move the Hands and Fingers

Several superficial as well as deep muscles of the ­forearm flex and extend the joints of the fingers. They are large muscles that actually end before ­reaching the wrists. Their tendons cross the wrist joints, increasing­ mobility of the wrists and hands. Strength and crude control of the hands and fingers are ­provided by the extrinsic muscles of the forearm, whereas fine hand control involves the smaller intrinsic muscles originating on the carpal and metacarpal bones. No muscles originate on the phalanges. The ­distal joints of the fingers function by only tendons. The muscles that move the hands and fingers are dis-cussed in TABLE 10-10.

Muscles of the Thorax and Abdominal Wall

The muscles of the thorax and abdominal wall are described in TABLE 10-11. The deep thorax muscles assist in breathing, with two primary layers of mus-cles partially forming the anterolateral wall of the thorax. Thoracic muscles are extremely short and mostly extend from one rib to the next. The most important muscle of inspiration is the diaphragm. The anterior and lateral abdominal wall has no ribs, but is composed of four pair muscles and their related structures. The abdominal wall consists of the external and internal oblique muscles. The rec-tus abdominis and transversus abdominis are other major muscles. Together, the abdominal muscles protect and support the viscera quite well, until they loose muscle tone.

Muscles of the Pelvis

The muscles of the pelvis (TABLE 10-12) are subdivided into the muscles of the pelvic diaphragm, urogenital diaphragm, and superficial perineal space.

Muscles That Move the Thigh

The muscles of the thigh are mostly anchored to the pelvic girdle ( TABLE 10-13). The ball-and-socket hip joints allow flexion, extension, abduction, adduction, circumduction, and rotation. The related muscles are among the strongest in the body. Most of the thigh flexors pass in front of the hip joints. The superficial muscles of the anterior hips and legs are shown in FIGURE 10-11 and the superficial muscles of the posterior hips and legs are shown in FIGURE 10-12.

Muscles That Move the Leg

The muscles that move the leg (TABLE 10-14) allow for flexion and extension at the knee joint. These muscles may originate on the pelvis or spine. There are medial, anterior, and posterior compartments of the thigh. There are also posterior muscles, which include the gluteals and lateral rotators.

Muscles of the Feet and Toes

The muscles of the feet and toes are extremely similar to those of the hands and fingers (TABLE 10-15). The leg muscles that enter the soles. There is a single mus-cle on the foot’s superior aspect, while there are several muscles on the plantar aspect, in four layers.