Biomedical Engineering Reference
In-Depth Information
a
b
c
F
A
F
A
F
A
F
R
F
R
F
R
Fig. 5.6
Examples of levers in the human body. Levers of first, second and third class are
represented in (
a
), (
b
), and (
c
), respectively. In all types,
F
R
is the weight and
F
A
is the muscle
force
5.5.4.3 Third Class Levers
This class of lever is very common in the human body. In this case, the arm of
resistance is always longer than the action arm, causing a mechanical disadvantage
(mechanical advantage equal to 0.1 or smaller). On the other hand, a small
shortening of a muscle causes a great arc of motion, being able to transport a
relatively small load over a large distance. There are several examples: the deltoid
acting on the glenohumeral articulation, the superficial flexor of the toes in the
interphalangic articulations, the radial extensor of the wrist, the anterior tibia at the
ankle articulation, and the brachial biceps at the elbow.
The first step in the analysis of a lever is to identify its axis of rotation. Then we
should identify the lines of action of the muscle force (action) and of the resistance
force (load). Finally, the arms of these forces are determined. Figure
5.6
shows
three examples of levers in the human body, with one of each type. The forces of
resistance (load) and of action (muscle) and the fulcrum are represented for each
type of lever.
Exercise 5.8
Flex and stretch your arm, observing carefully the action of the
brachial biceps and the triceps muscles. Represent each situation by the
corresponding diagrams and classify the levers.
Exercise 5.9
Consider an arm under the action of the brachial biceps acting
as a flexor in several postures, as illustrated in the figure of Exercise 5.9.
Represent this muscle action in each case, identify its line of action, and deter-
mine the respective arm of the action force. Determine the angle in the case of
maximum torque.
