The Muscular System 43attachment and distal attachment. While this termi-
nology works well for the extremities, it is sometimes
necessary to use additional terminology for the
head, neck, and trunk, such as superior attach-
ment/inferior attachment or medial attachment/
lateral attachment. The use of these alternative
terms to origin and insertion better reflect the con-
cept that when a muscle contracts, it exerts equal
force on each attachment and tends to pull both
attachments toward each other. Which end actually
moves depends on the other forces that come into
play, such as the relative mass of the adjacent seg-
ments or the stabilizing action of adjacent muscles,
and whether the distal segment of the limb is fixed or
moving. For example, when flexing the elbow as you
lift a weight (biceps curl), the proximal attachment
stays stationary, and it is the forearm and distal attach-
ment of the biceps brachii
that move. In many everyday
movements of the limbs (espe-
cially of the upper extremity),
this pattern occurs where the
distal segment moves while
proximal segments are readily
stabilized due to greater mass.
It is also generally the easiest
way to think about learning the
actions of a given muscle and
can be termed the customary
action of a muscle. However,
there are cases in functional
movement where the distal
segment is stationary and the
proximal segment moves, or
where both segments simul-
taneously move. When the
proximal segment moves, it
can be termed a reversal of
customary action of a muscle.
A pull-up offers an example of
a reversal of customary action
for the biceps brachii, where
the proximal segments and
attachments (humerus and
trunk) move, while the distal
segment (forearm) stays sta-
tionary with the hands fixed
on the pull-up bar. Similarly,
lifting the thigh to the front
while the trunk stays station-
ary (figure 2.8A) would reflect
customary action, whereas lift-
ing the trunk while the thigh
stays stationary would reflect
reversal of customary action of the iliopsoas muscle
(figure 2.8B).Line of Pull of a Muscle
The location of the proximal and distal attachments
of a muscle relative to the axis of the joint is funda-
mental in determining the type of joint movement
that will occur. Although technically the line of action
or line of pull of a muscle is the net effect (resultant)
of applied forces on a common point of attach-
ment of every fiber contained within a given muscle
(Levangie and Norkin, 2001), one can roughly
approximate this resultant force, or “pull,” by draw-
ing an imaginary double-headed arrow with its base
at each attachment and pointing toward the center
of the muscle as seen in figure 2.9A. In instances inFIGURE 2.8 Customary and reverse muscle actions of the iliopsoas. (A) Raising
the thigh and (B) raising the trunk.
Sacramento Ballet dancer Merett Miller.