The calf muscles, or triceps surae, consist of three separate muscles in the back of each lower leg: the gastrocnemius muscle, the soleus muscle and plantaris muscle. The gastrocnemius muscle raises the heel when the knee is in an extended, straight position, and it is mainly used for walking and running. The soleus muscle's primary function is flexion of the ankle joint to extend the foot down when the knee is in a bent position and to balance the leg on top of the foot when the body is in an upright position. The plantaris muscle works with the gastrocnemius muscle to extend the ankle when the foot is in a raised position or to bend the knee when the foot is in a fixed position.
The connective tissue of the three calf muscles join to form the Achilles tendon, which connects the calf muscles to each foot. The calf muscles are very powerful and are used to flex the ankle and knee, muscle movements that are required to walk, run, stand and jump. When walking, the calf muscles lift the heel from the ground, supporting the body on the raised foot, so that the opposite leg can swing forward.
The gastrocnemius muscle is the largest and most superficial of the three calf muscles. It originates from behind the knee on the femur and extends down to the heel. The gastrocnemius muscle is prone to involuntary nighttime leg cramps or spasms that are usually referred to as charley horses. These involuntary, painful contractions of the gastrocnemius muscle usually occur either on the verge of sleep or wakefulness, and they can last anywhere from a few seconds to several minutes.
The soleus muscle is a broad, flat calf muscle located immediately in front of the gastrocnemius muscle. It helps the body balance on the feet and keeps it from tipping forward. The soleus muscle is also referred to as the skeletal-muscle pump, because it is responsible for pumping venous blood from the extremities back up to the heart.
The plantaris muscle is a superficial calf muscle that is not considered vital to motor function, and it is not found in 7-10 percent of the population. When present, the plantaris muscle is located between the gastrocnemius muscle and the soleus muscle. It is considered to be of minimal importance in calf muscle function, but the plantaris muscle sustains damage when the Achilles tendon is ruptured.