The Mechanics of Muscle Movement
Most muscles have two or more points of attachment to the skeleton. The muscle is attached to a bone at each end by means of a cordlike extension called a tendon (Fig. 4-7). All of the connective tissue within and around the muscle merges to form the tendon, which then attaches directly to the periosteum of the bone (see Fig. 4-1). In some instances, a broad sheet called an aponeurosis may attach muscles to bones or to other muscles. In moving the bones, one end of a muscle is attached to a more freely movable part of the skeleton, and the other end is attached to a relatively stable part. The less movable (more fixed) attachment is called the origin; the attachment to the part of the body that the muscle puts into action is called the insertion. When a muscle contracts, it pulls on both points of attachment, bringing the more movable insertion closer to the origin and thereby causing movement of the body part. Figure 4-7 shows the action of the biceps brachii (in the upper arm) in flexing the arm at the elbow. The insertion on the radius of the forearm is brought toward the origin at the scapula of the shoulder girdle.
Figure 4-1 Structure of a skeletal muscle. (A) Structure of a muscle showing the tendon that attaches it to a bone. (B) Muscle tissue seen under a microscope. Portions of several fascicles are shown with connective tissue coverings.
Figure 4-7 Muscle attachments to bones. Three attachments are shown-two origins and one insertion.
Muscles Work Together
Many of the skeletal muscles function in pairs. A movement is performed by a muscle called the prime mover; the muscle that produces an opposite movement to that of the prime mover is known as the antagonist. Clearly, for any given movement, the antagonist must relax when the prime mover contracts. For example, when the biceps brachii at the front of the arm contracts to flex the arm, the triceps brachii at the back must relax; when the triceps brachii contracts to extend the arm, the biceps brachii must relax. In addition to prime movers and antagonists, there are also muscles that serve to steady body parts or to assist prime movers. These “helping” muscles are called synergists, because they work with the prime movers to accomplish a movement. As the muscles work together, body movements are coordinated, and a large number of complicated movements can be carried out. At first, however, the nervous system must learn to coordinate any new, complicated movement. Think of a child learning to walk or to write, and consider the number of muscles she or he uses unnecessarily or forgets to use when the situation calls for them.
Levers and Body Mechanics
Proper body mechanics help conserve energy and ensure freedom from strain and fatigue; conversely, such ailments as lower back pain-a common complaint-can be traced to poor body mechanics. Body mechanics have special significance to healthcare workers, who are frequently called on to move patients and handle cumbersome equipment. Maintaining the body segments in correct relation to one another has a direct effect on the working capacity of the vital organs that are supported by the skeleton. If you have had a course in physics, recall your study of levers. A lever is simply a rigid bar that moves about a fixed pivot point, the fulcrum. There are three classes of levers, which differ only in the location of the fulcrum (F), the effort (E), or force, and the resistance (R), the weight or load. In a first-class lever, the fulcrum is located between the resistance and the effort; a see-saw or a scissors is an example of this class (Fig. 4-8). The second-class lever has the resistance located between the fulcrum and the effort; a wheelbarrow or a mattress lifted at one end is an illustration of this class (Fig. 4-8 B). In the third-class lever, the effort is between the resistance and the fulcrum. A forceps or a tweezers is an example of this type of lever. The effort is applied in the center of the tool, between the fulcrum, where the pieces join, and the resistance at the tip.
The musculoskeletal system can be considered a system of levers, in which the bone is the lever, the joint is the fulcrum, and the force is applied by a muscle. An example of a first-class lever in the body is using the muscles at the back of the neck to lift the head at the joint between the occipital bone of the skull and the first cervical vertebra (atlas) (see Fig. 4-8). A second-class lever is exemplified by raising your weight to the ball of your foot (the fulcrum) using muscles of the calf. However, there are very few examples of first- and second-class levers in the body. Most lever systems in the body are of the third-class type. A muscle usually inserts over a joint and exerts force between the fulcrum and the resistance. That is, the fulcrum is behind both the point of effort and the weight. As shown in Figure 4-8 C, when the biceps brachii flexes the forearm at the elbow, the muscle exerts its force at its insertion on the radius. The weight of the hand and forearm creates the resistance, and the fulcrum is the elbow joint, which is behind the point of effort. By understanding and applying knowledge of levers to body mechanics, the healthcare worker can improve his or her skill in carrying out numerous clinical maneuvers and procedures.
Figure 4-8 Levers. Three classes of levers are shown along with tools and anatomic examples that illustrate each type. R = resistance (weight); E = effort (force); F = fulcrum (pivot point).