The Muscular System 45It is easier to understand rotary motion if we look
at how a lever works. Put simply, a lever is a rigid
bar revolving about a fixed point called the axis,
pivot, or fulcrum. In the human body, bones serve
as levers, and the interposed joint serves as the axis
(A). Muscles attach to the levers or bones, and when
they contract they generally produce rotary motion
about the fixed or relatively fixed axis of the joint (in
reality, the axis of joints tends to shift slightly with dif-
ferent ranges of motion). This tendency of muscles
to produce rotation in one direction, termed effort
(E), is countered by the tendency of the resistance
to produce rotation in the opposite direction (R).
Which of these three components—effort, axis, or
resistance—is between the other two determines
which of the three classes a lever belongs to as shown
in figure 2.10. It is easier to remember the difference
between levers if you remember that the middle com-
ponents spell “ARE” when you proceed from first- to
third-class levers. So, with a first-class lever the axis
is in the middle, and effort and resistance are on
opposite sides of this axis. This arrangement can be
used for balance or to gain either effective force or
range of motion, depending on the relative distances
of effort and resistance from the axis. Seesaws, scis-
sors, and crowbars can function as first-class levers.
In the body, the atlanto-occipital joint (A) functions
in a first-class lever system, where the weight of the
head (R) is balanced by the extensor muscles of the
head/neck (E).
With a second-class lever, the resistance is between
the axis and effort, making the distance of the effort
from the axis always greater than the distance of the
resistance. This arrangement magnifies the effective-
ness of the effort, allowing for less force to be used
to overcome a given resistance. The wheelbarrow,
nutcracker, and lug nut wrench can all function
as second-class levers. This type of lever probably
does not exist in its pure form in the human body.
However, there are cases in closed kinematic chain
movements in which the muscle acts on the proxi-
mal segment while the distal segment (e.g., foot or
hand) is fixed, so that the distal portion of the body
acts like a third-class lever. For example, in a relevé,
the dancer rises on the toes (metatarsophalangeal
joints—A) when the weight of the body (R) is over-
come by contraction of the calf muscles (E).
In the third-class lever, the effort lies between
the resistance and the axis, making the distance of
the resistance from the axis always greater than the
distance of the effort. This arrangement favors range
of motion and speed, at the expense of the effective-
ness of the force. Tweezers and many sport imple-
ments (bats, rackets, paddles) are used as third-class
levers. Most muscles of the human body when they
act in open kinematic chain movements function as
third-class levers. An example is the deltoid muscle
(E) acting to produce abduction of the arm (R) at
the glenohumeral joint (A).Torque (Moment of Force)
Adding a little more detail to this concept, the capac-
ity or effectiveness of a force to produce rotation
is termed torque (L. torqueo, to twist) or moment
of force. The amount of torque acting to rotate a
given system can be calculated by multiplying the
amount of force times the perpendicular distanceDifferent Actions of Pectoralis Major
The differing functions of the clavicular and sternal portions of the pectoralis major can be demon-
strated with the following exercise.- Shoulder flexion and extension. Sit with your hands clasped at shoulder height with both elbows
extended. Pull down with the right arm (shoulder extension) and up with the left arm (shoulder flexion)
simultaneously such that no net movement of the shoulders occurs. Note the lower sternal portion
of the pectoralis major contracting on the right side of your chest and the upper clavicular portion of
the pectoralis major contracting on the left side of your chest. - Shoulder horizontal adduction. Press both your hands and arms toward each other so that no
net movement of the shoulder occurs (isometric horizontal adduction). Note the clavicular and sternal
portions of the pectoralis major contracting on both the right and left sides of your chest.