1103 Chapter 11. The Muscular System
11.1 Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems
Learning Objectives
By the end of this section, you will be able to:
- Describe how muscles are attached to bones to produce movement
- Explain how the location of the insertion and origin of a muscle determine the type of movement accomplished by contraction of that muscle
- Describe the principle of muscular antagonism in movement, using the forearm as an example
- Explain how synergists influence the type of movement accomplished by contraction of a muscle
- Define the following terms: level, fulcrum, resistance, effort
- Describe three kinds of levers and give an example of each type in the human body
- Calculate the force that has to be exerted by the biceps brachii to maintain the forearm in equilibrium when the hand is holding a mass of 10 kilograms
- Describe one mechanical disadvantage and two advantages of the insertion of the biceps brachii being near the elbow joint
To move the skeleton, the tension created by the contraction of the fibers in most skeletal muscles is transferred to the tendons. The tendons are strong bands of dense, regular connective tissue that connect muscles to bones. The bone connection is why this muscle tissue is called skeletal muscle.
Interactions of Skeletal Muscles in the Body
To pull on a bone, that is, to change the angle at its synovial joint, which essentially moves the skeleton, a skeletal muscle must also be attached to a fixed part of the skeleton. The moveable end of the muscle that attaches to the bone being pulled is called the muscle’s insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. During forearm flexion—bending the elbow—the brachioradialis assists the brachialis.
Although a number of muscles may be involved in an action, the principal muscle involved is called the prime mover, or agonist. To lift a cup, a muscle called the biceps brachii is actually the prime mover; however, because it can be assisted by the brachialis, the brachialis is called a synergist in this action (Figure 1). A synergist can also be a fixator that stabilizes the bone that is the attachment for the prime mover’s origin.
A muscle with the opposite action of the prime mover is called an antagonist. Antagonists play two important roles in muscle function: (1) they maintain body or limb position, such as holding the arm out or standing erect; and (2) they control rapid movement, as in shadow boxing without landing a punch or the ability to check the motion of a limb.
For example, to extend the knee, a group of four muscles called the quadriceps femoris in the anterior compartment of the thigh are activated (and would be called the agonists of knee extension). However, to flex the knee joint, an opposite or antagonistic set of muscles called the hamstrings is activated.
As you can see, these terms would also be reversed for the opposing action. If you