174 Introduction to Anatomy and Physiology
Multiplanar Movements
A few movements of body segments do not
fall within a single plane. If you have ever
purchased running shoes, you may have heard
the terms pronation and supination used to
describe motions of the foot occurring
specifi at the subtalar joint (where the heel fically
and ankle bones meet). Pronation at the subtalar
joint is a combination of eversion, abduction, and
dorsifl Supination at this joint includes flexion.
inversion, adduction, and plantar flexion. fl
Moving a fi nger, arm, or leg in a rotational fi
manner such that the end of the segment traces a
circle is called circumduction (SER-kum-DUK-
shun), which you can see in Figure 5.14. And,
fi nally, touching any of your four fingers fi to the fi
thumb is known as opposition. Having an
opposable thumb gives us the all-important
ability to grasp objects.
Head and Neck Muscles
The muscles of the head and neck can
be divided into three groups: facial muscles,
chewing muscles, and neck muscles. The
difference between facial muscles and most
other muscles is that facial muscle insertions
connect them to other muscles or skin. When
these muscles contract, pulling on the skin, they
produce an array of facial expressions.
With the exception of the orbicularis oris,
which encircles the mouth, and the sheetlike
platysma on the front and sides of the neck, all of
the other head and neck muscles are paired—one
on the right and one on the left. The head and
neck muscles are displayed in Figure 5.15, and
1. In what direction are movements guided
on the sagittal plane?
2. What special movements occur on the
frontal plane at the hand? the foot?
3. What is the difference between pronation
and supination of the hand and the foot?
4. Describe circumduction.
5. Defi medial and lateral rotation. fine
Check Your Understanding
their locations and functions are summarized in
Figure 5.16.
Trunk Muscles
The trunk muscles provide stability for the
vertebral column. They are also responsible for
maintaining upright posture. American football
players train to strengthen the neck and trunk
muscles in an effort to maximize spinal stability
and minimize risk of injury to the delicate
spinal cord and internal organs (Figure 5.17).
Conversely, female gymnasts train to enhance
the fl exibility of the spine and are capable of fl
extraordinary spinal hyperextension, especially
during balance-beam and floor-exercise routines. fl
Collectively, the trunk muscles enable
fl exion, extension, hyperextension, lateral fl
fl exion, and rotation of the head and trunk. From fl
a functional perspective, the anterior abdominal
muscles also assist with urination, defecation,
forced expiration during breathing, and
childbirth. The all-important diaphragm muscle
regulates breathing. The trunk muscles also
Arthur Eugene Preston/Shutterstock, Inc.
Figure 5.17 Football players conditioning. Why is it
critically important for football players to strengthen
their neck and trunk muscles?