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On the human body, the calf refers to the lower leg or the area between the knee and the ankle. Two bones of the skeleton — the heavy tibia and the more slender fibula — comprise the area known as the calf and support the area between the patella, or knee, and the ankle, or tarsus. In medical terminology, the word posterior means "behind." The muscles that lie behind the lower leg bones or in the location of the posterior calf include the gastrocnemius and the soleus. A minority of anatomists classify the posterior calf muscles as including a third muscle — the plantaris — in the overall structure known as the Triceps Surae.
The major posterior calf muscle is the gastrocnemius. This large and easily identifiable muscle, commonly known simply as the "calf," is the muscle most responsible for human ambulation. Its attachment to the posterior thighbone, or femur, and the heel, or calcanus, allows a person to walk by pushing the body weight forward from heel to toe on alternate legs in a process known as plantar flexion. The gastrocnemius attaches to the calcanus with the long Achilles tendon. Both of these posterior calf structures are known to be easily injured: the gastrocnemius through a strain and the Achilles tendon through inflammation secondary to overuse.
The soleus is the second muscle of posterior calf. Some anatomists consider the soleus as an inner portion of the gastrocnemius as opposed to a separate muscle. This thin, narrow posterior calf muscle lies underneath and below its curvy, thick partner in a marked contrast of opposites. The soleus is often difficult to differentiate from the gastrocnemius without exercises to define it. One such exercise that specifically works only the soleus muscle as opposed to the gastrocnemius is known as the seated calf raise.
The gastrocnemius muscle is prone to strains and cramping. Strains — muscle fiber tears graded along a continuum — are common when first beginning an exercise program or when a quick abrupt movement is initiated without previously completing the necessary warm-ups and stretching. Gastrocnemius cramps often occur secondary to low potassium levels or dehydration, both conditions that are common after a sporting or athletic event. Even elderly and sedentary individuals can suffer from painful calf cramps, particularly if they are dehydrated by diuretics taken to control their high blood pressure. Treatment for both populations includes rehydration and oral potassium in the form of orange juice or specialized sports drinks.