5.2 Some Elementary Skeleto-Muscular Mechanics

Muscles and bones together work like machines within the laws of physics and chemistry. Lever and pulley systems are examples of simple machines found commonly in the human body.

LEVER SYSTEMS

See figure 5-3 for an illustration of the three classes of levers.

a. First Class. In a first class lever, the weight to be moved is at one end of the lever, the applied force is at the other end, and the fulcrum (the pivot or turning point) is between the two.

b. Second Class. In a second class lever, the weight to be moved is between the applied force and the fulcrum. This type of lever enables a weight to be moved with less force than would be required without a lever. (Many feel that there are no second class levers in the human body.)

c. Third Class. In a third class lever, the weight to be moved is at one end of the lever, the fulcrum is at the other end, and the applied force is between the weight and the fulcrum. This type of lever provides speed, but a greater amount of force is required for a given weight. This is the most common type of lever in the human body.

Figure 5-3. Types of lever systems.

SIMPLE PULLEY SYSTEM

a. In the human body when the tendon of a skeletal muscle slides over a round bony surface, the "system" acts like a simple pulley (figure 5-4). A simple pulley provides a change in the direction of the force or muscle pull. There is no change in the amount of force produced by the muscle. For example, the knee acts as a simple pulley by which the quadriceps femoris M. extends the leg.

Figure 5-4. A simple pulley (the human knee mechanism).

b. Sesamoid bones, such as the patella (kneecap), develop in tendons where pressure is applied to the tendon.

THE SKELETO-MUSCULAR UNIT

The skeleto-muscular unit (figure 5-5) is a working concept of muscle and skeleton producing motion. The components of an S-M unit are bones, a joint, and skeletal muscle(s).

Figure 5-5. The skeleto-muscular unit (arm-forearm flexion (3rd class lever system)).

a. Bones. Bones act as levers and as attachment sites for skeletal muscles.

b. Joint (Articulation). The joint is the center, fulcrum, point, or axis of motion.

c. Skeletal Muscle(s). Skeletal muscles apply the forces for motion. Any given motion utilizes a group of muscles working together. A skeletal muscle may serve only one of the three following major roles during a particular motion:

(1) Prime mover. The muscle which makes the main effort for a given motion is called the prime mover, or agonist.

(2) Synergist. A synergist is a muscle which assists the prime mover.

(3) Antagonist. An antagonist applies a force opposite to that of the prime mover.

(a) By opposing the prime mover, the antagonist helps control the motion.

(b) The antagonist also brings the limb or other part back to its original position.