Running involves leg extension and carefully choreographed motion, but the movement of muscles occurring inside of the leg is actually galvanized by the contraction of small fibers. The contraction of calf muscles is one of the largest contributors to locomotion and grants ankles the freedom of movement necessary for running.
Outer Muscle
According to the training company BetterU, Inc., the gastrocnemius is the large muscle that can be seen on the outer part of the calf. It originates on the femur behind the knee and crosses two joints down to where it is attached to the Achilles tendon. The two major sections of this muscle are the medial and lateral.
Inner Muscle
The soleus is the muscle that rests beneath the gastrocnemius on the rear of the leg. It also runs from just below the knee to the Achilles tendon. Compared to the structure on other animals, the human soleus and gastrocnemius are relatively differentiated, but the muscle pair is bound together tight enough to form a composite muscle called the triceps surae.
Function
Both the gastrocnemius and soleus are large and powerful muscles that allow the ankle a wide range of motions. The one difference between the gastrocnemius and soleus is that each muscle maintains a different and varying degree of contraction for every possible position of the ankle and leg. These muscles are involved in such activities as walking, dancing and running.
Running Motion
According to sports coach Brian Mackenzie, running requires two major phases: the driving phase and recovery phase. Each phase has a primary agonist muscle---the muscle that causes a particular motion to occur when contracted. During the driving phase, the gastrocnemius is the agonist muscle that causes the plantar flexion motion in the ankle, which elevates the heel and dips the foot forward. The agonist muscle during the recovery phase, however, is the tibialis anterior muscle near the front part of the lower leg. This causes the foot to rock back, which is also known as a dorsi flexion motion.
Types
Running motions with longer drives such as sprinting require both the quadriceps in the thigh and the calf muscles in the lower leg to generate power. Long distance running, on the other hand, requires a motion in which the feet are kept close to the ground. This is also known as "shuffling" and it is maintained with a stiff ankle and straight knee. The power generated from contraction is shifted from the calf muscles to the quadriceps.
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