Chapter 12. Biomechanics
Priscilla Stewart
Unit Outline
Learning Objectives
At the end of this unit, you should be able to:
I. Describe how muscles attach to bones to produce movement.
II. Describe the principle of muscular antagonism in movement, using the forearm as an example.
III. Define the following terms: lever, fulcrum, resistance, effort.
IV. Describe three types of levers and give an example of each type in the human body.
V. Explain the biomechanical principles and functioning of a lever system.
Part 1: Skeletal Muscle Anatomy
Muscle Attachment to the Skeleton
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.
To pull on a bone, that is, to change the angle at its synovial joint, which essentially moves the skeleton, a skeletal muscle must 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, whereas the end of the muscle attached to a fixed (stabilized) bone is called the origin.
Muscular Antagonism
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 the prime mover; however, because this muscle can be assisted by the brachialis, the brachialis is called a synergist in this action (Figure 12.1). A synergist can also be a fixator, which 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 shadowboxing without landing a punch, and thereby 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 is activated. These muscles would be called the agonists of knee extension. However, to flex the knee, an opposite or antagonistic set of muscles called the hamstrings is activated. Flexing the knee involves the hamstrings muscles as the agonists and the quadriceps femoris muscles as the antagonists. As you can see, the terms agonist and antagonist are associated with a particular movement, and these terms can be reversed for opposing movements. See Table 12.1 for a list of some common agonists and antagonists.
| Agonist | Antagonist | Movement |
|---|---|---|
| Biceps brachii (in anterior compartment of arm) | Triceps brachii (in posterior compartment of arm) | The biceps brachii flexes the forearm, whereas the triceps brachii extends it |
| Hamstrings (group of three muscles in the posterior compartment