Anatomy and Kinesiology
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Chapter 3
Popliteus. This small muscle, located just behind the
knee, is also a prime mover for knee flexion. It is said
to be an important stabilizer for the back and lateral side of the knee.
Ankle Joint
Fig. 3-20. Ankle Joint
Fig. 3-21. Posterior Leg Muscles
Fig. 3-22. Anterior Leg Muscles
The ankle joint actually refers to two different sets of
articulations, the talocrural and subtalar joints. The
talocrural joint is a hinge joint and is the junction of the
distal ends of the tibia and fibula with the talus. It is worth
noting that the medial side of this joint is buttressed by an
extremely strong ligament, making ankle sprains in which
the ankle rolls toward the midline (eversion sprains) fairly
uncommon. The lateral side of the ankle, however, has
three rather small ligaments that provide less structural
support for the ankle joint. As a result, ankle sprains in
which the ankle rolls away from the midline (inversion
sprains) are much more common. There are two joint
actions of the talocrural joint.
1. Ankle dorsiflexion—movement that brings the top of
the foot toward the shin
2. Ankle plantar flexion—movement that brings the sole
of the foot downward (pointing the toes)
The subtalar joint is the articulation of the talus with
the heel bone, or calcaneus. There are also two joint
actions of the subtalar joint.
1. Ankle eversion (pronation)—movement of the ankle
in which the sole turns or lifts outward and the medial border of the foot (the arch) tends to flatten
2. Ankle inversion (supination)—movement of the
ankle in which the sole turns or lifts inward and the
medial border of the foot (the arch) tends to lift
Major Muscles of the Ankle Joint
• Anterior tibialis. Just as its name implies, this long
shin muscle lies in front of the tibia bone, inserting
onto the medial side of the foot. It is a prime mover
in ankle dorsiflexion and inversion. The anterior tibialis is opposed by the gastrocnemius and soleus.
• Gastrocnemius. This is the largest and most obvious
calf muscle; it is a two-joint muscle, crossing both the
knee joint (where it is an assistor for knee flexion)
and the ankle joint. The gastrocnemius has two heads
that fuse distally to form the Achilles tendon, which
attaches to the calcaneus or heel bone. It is a prime
mover for ankle plantar flexion.
• Soleus. The soleus lies under the gastrocnemius, however, unlike the gastrocnemius, it does not cross the
knee joint. The soleus connects to the calcaneus via
the Achilles tendon. It is a prime mover for ankle
plantar flexion.
Personal Fitness Training: Theory & Practice
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Chapter 3
Anatomy and Kinesiology
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Posterior tibialis. This muscle is the deepest posterior
muscle and is responsible for ankle inversion when the
ankle is plantar flexed. It inserts onto the medial underside of the foot, helping to maintain the longitudinal arch.
Extensor digitorum longus. Responsible for ankle dorsiflexion and ankle eversion, the extensor digitorum longus
also lifts the four smaller toes.
Peroneus tertius, peroneus longus, and peroneus brevis.
The three peroneal muscles are all prime movers for ankle
eversion; the peroneus tertius is also responsible for ankle
dorsiflexion.
Fig. 3-22. Lateral Leg Muscles
Roles Muscles
Play
Depending on the work being performed, muscles can assume different roles
or functions. For example, when performing a biceps curl, the biceps muscle
performs the role of the agonist, or prime mover. Yet during a triceps kickback exercise, the biceps assumes the role of the antagonist, or opposing muscle (the triceps is now the agonist). The biceps can act as an assistor during
shoulder front raises (the anterior deltoids are the agonist muscles) and as a
stabilizer when holding the elbow at 90° of flexion while performing wrist
exercises. Most muscles have the potential to perform these various roles,
depending on the exercise and the position of the body. Note the following
definitions.
• Agonist—prime mover, or the contracting muscle that is responsible for
the movement that you see
• Antagonist—the muscle that works in opposition to the prime mover
and reflexively elongates to allow the agonist to contract and move the
joint
• Assistor—the muscle that assists in performing a movement, but is not
a prime mover; sometimes called a secondary mover
• Stabilizer—the muscle that maintains a static or isometric contraction
to anchor or support the movement of other primary movers. Torso
muscles, for example, are important stabilizers of the spine during daily
activities; ideally, they help maintain proper posture and aid in the prevention of back problems.
• Synergist—textbooks disagree on the definition of a synergist. Some
describe it as a stabilizer; some describe it as an assistor. The muscle is
contracting synergistically along with the prime mover in some way.
Muscle Actions
When muscles work, or develop force or tension, they are said to contract.
However, this implies that the muscles shorten. Muscles sometimes shorten
when they work, but they can also lengthen or even stay the same length and still
produce tension. There are three main types of muscle actions.
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