REVIEW OF LEVERS AND THEIR RAMIFICATIONS IN THE HUMAN BODY
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The arrangement of the axis (fulcrum), the resistance, and the effort force applied to the lever determine the type of lever.
First-class lever – the axis (A) is in the middle between the effort force (EF) and resistance (R). The effort force and
resistance are on opposite ends. If the axis moves closer to the resistance arm, mechanical advantage increase (the effort
arm is longer than the resistance arm). The longer the effort arm, the easier the work is (more torque is applied with minimal
muscle exertion).
Second-class lever – R and EF are on the same side of the axis. The EF arm is longer than the R arm. Second class
levers favor mechanical advantage (power).
Third-class lever - R and EF are on the same side of the axis. The R arm is longer than the Effort arm. Third class levers
favor speed and range of angular motion, b/c they lack mechanical advantage. Third class levers depict force, but favor
speed.
Different class levers provide increases or decreases in three variables:
1. Mechanical Advantage
2. Speed of movement
3. Range of motion
A decrease in one of these variables is accompanied by an increase in one or both of the other variables. For example, if
the effort arm is longer than the resistance arm in a first-class lever, a in mechanical advantage results. However, the R
moves a shorter distance, and thus slower, than the EF. Hence one variable increases, two decrease. If the R arm is
greater than the effort arm, a gain in speed and range of motion results, but mechanical advantage is reduced.
The resistance arm is longer than the effort arm in third-class levers. Consequently, there is a reduction in mechanical
advantage, but a gain in speed and range of motion. If a effort torque greater than the resistance torque is applied, the R will
move rapidly and through a range greater than the effort.
Since most of the bony levers have a F arm that is shorter than the R arm (i.e., origin with respect to insertion), the human
body is generally considered to be much better equipped to make fast movements than forceful ones. The most common
human body lever is third-class. A couple of exceptions should be noted.
The triceps brachii is a first-class lever. The effort force point is the insertion of the tendon on the olecranon process of the
ulna. The resistance point is the COG of the forearm. The axis is the elbow. Since the R arm is greater than the effort arm,
the triceps brachii yields a gain in speed and range of the hand with a reduction in mechanical advantage.
Triceps brachii
- elbow extension
- first-class lever
EF A
R
Brachioradialis
- elbow flexion
- second-class lever
A
R
EF
Biceps brachii or brachialis
- elbow flexion
- third-class lever
A EF
R
The brachioradialis is a second-class lever. The effort force point is the insertion on the radial styloid process. Since the
effort arm is longer than the R arm, this muscle has a gain in mechanical advantage with a reduction in speed and range.
The biceps brachii and brachialis are third-class levers. The effort force point for the biceps brachii is the insertion on radial
tuberosity. Such a lever suffers a reduction in mechanical advantage with a gain in range and speed. Since these muscles
are strong enough to overcome most external torques, they result in increased speed and range of the hand.
Adding external weights to a body segment can cause a shift in the location of its COG (i.e., location of R), which can result
in a change in the class of lever. For example, without an external weight, the brachioradialis is a second-class lever. With
an external weight, it becomes a third class lever.
The addition of a weight (as when lifting weights) changed the brachioradialis from
a muscle that with high mechanical advantage to one that results in a gain in
speed and range. As such, the brachioradialis works with or compliments the
actions of the biceps brachii and brachialis muscles.
A
EF
R
Some individuals may be endowed with muscle insertions that are farther from their joints than in the average person. Only
a small difference is necessary to give considerable mechanical advantage. Consider the third-class lever, for example.
Muscle insertions further down the bone increase the effort arm length and allow better control/power of the limb, thus,
strength/torque; However, don’t lose sight of the fact that a short effort arm is necessary if great speed is desired of the limb.