Lecture notes of 13th and 14th class
Subject: Introduction in biomechanics
Lever Systems or Classes of Levers
Definition of Lever: A rigid bar that turns about an axis of rotation or a fulcrum.
Three points determine type of lever & for which kind of motion it is best suited
Axis (A)- fulcrum - the point of rotation
Point (F) of force application (usually muscle insertion)
Point (R) of resistance application (center of gravity of lever) or (location of an
external resistance)
First-class lever is a lever system in which the axis lies between the point of application of
the effort force and the point of application of the resistance force, without regard to the size
of EA or RA. As long as the axis lies between the points of application of the EF and RF, EA
can be bigger than RA smaller than RA or the same size as RA.
In a first-class lever system: EA may be greater than RA (A), smaller than RA (B), or equal
to RA (C).
Second-class lever is a lever system in which the resistance force has a point of application
between the axis and the point of application of the effort force, which always results in EA
being larger than RA.
A second-class lever system: The effort arm (EA) is always larger than the resistance arm
(RA).
Third-class lever is a lever system in which the effort force has a point of application
between the axis and the point of application of the resistance force, which always results in
RA being larger than EA.
A third-class lever system: The effort arm (EA) is always smaller than the resistance arm
(RA).
Muscles in First-Class Lever System
First-class lever is the pull of the supraspinatus on the humerus
AXIS
SMALL EFFORT ARM
LONG RESISTANCE ARM
COM
The supraspinatus is attached to the humerus on one side of the joint axis, and the point of
application of gravity (CoM) lies on the other side of the axis (inset). Regardless of whether
the muscle is working concentrically (EF) or eccentrically (RF), the muscle is part of a firstclass lever.
Muscles in Third-Class Lever System
Most of the muscle in body act in third order lever
The action of the quadriceps muscle on the leg foot segment against the resistance of gravity
and the weight boot serves as a typical example.
Quadriceps Pull
Small effort arm
Long resistance arm
Weight of the leg and boot
Muscles in Second-Class Lever System
Calf muscles (triceps surae muscle group, or Fms) acting on the foot segment (the foot being
considered as a rigid lever), lifting the body around the axis of the toes (metatarsophalangeal
[MTP] joints).
Mechanical Advantage
Mechanical advantage (M Ad) is a measure of the mechanical efficiency of the lever (the
relative effectiveness of the effort force in comparison with the resistance force). Mechanical
advantage of a lever is the ratio of the effort arm (MA of the effort force) to the resistance
arm (MA of the resistance force)
When EA is larger than RA, the M Ad will be greater than 1. The “advantage” of a
lever with a mechanical advantage greater than 1 is simply that the effort force can be (but is
not necessarily) smaller than the resistance force and yet will nonetheless create more torque
to “win.” The torque of the effort force is always greater than the torque of the resistance
force. If EA is greater than RA, then the effort torque can still be greater than the resistance
torque if EF is smaller in magnitude than RF.
In third-class levers, the M Ad will always be less than 1 because EA is always
smaller than RA (the effort force lies closer to the axis than the resistance force). A thirdclass lever is “mechanically inefficient” or is working at a “disadvantage” because the
magnitude of the effort force must always be greater than the magnitude of the resistance
force in order for the torque of the EF to exceed the torque of the RF (as it must for the force
to “win”).
In a first-class lever system, the EA can be larger than, smaller than, or equal to the
RA. However, because the distal attachment of a muscle tends to be closer to the joint axis
than is the point of application of an external force even when muscles are working on firstclass levers, muscles working in first-class lever systems (like those in third-class systems)
tend to be at a mechanical disadvantage.
1.
CONCEPT BUILDER NOTES
In all second-class levers, the mechanical advantage (M Ad) of the lever will always be
greater than 1. The magnitude of the effort force can be (but is not necessarily) less than the
magnitude of the resistance.
2.
In all third-class levers, the M Ad of the lever will always be less than 1. The magnitude of
the effort force must be greater than the magnitude of the resistance for the effort to produce
greater torque.
3.
The M Ad of a first-class lever can be greater than, less than, or equal to 1. However, it is
often true of first-class levers in the body that the MA of the muscle will be shorter than the
MA of the external force.
4.
When the muscle is the effort force in a lever with a M Ad of less than 1, the necessary
expenditure of energy to produce sufficient muscle force to “win” is offset by the need for
minimal shortening of the muscle to produce proportionally greater angular displacement and
angular velocity of the distal portions of the segment.
5.
When an external force is the effort force in a lever with a M Ad greater than 1 (e.g., a
second-class lever), the magnitude of the effort can be small in comparison with the
resistance, but less is gained in angular displacement and velocity.