Calf Strain

Anatomy

The calf muscle is actually three separate muscles: the gastrocnemius, soleus, and plantaris. Each muscle has a different origin around the knee, but they all unite in the posterior (back) ankle to form the Achilles tendon. The Achilles tendon then inserts onto the heel just below the ankle joint. The calf muscles plantarflex the ankle (point the toes).

The gastrocnemius begins above the knee on the femur (upper leg bone) and is a powerful muscle used for sprinting and jumping. The soleus starts below the knee on the tibia (large lower leg bone) and is more of an endurance muscle used for prolonged repetitive motions such as walking or running. The plantaris is a small, almost vestigial muscle that begins above the knee. It contributes little to movement but can sometimes become injured and symptomatic.

Causes

Like the fraying of a rope under tension, calf strains are caused by a tearing of the muscle due to the application of a force across the muscle that is greater than the muscle can withstand. The degree [Page 219]of muscle tearing depends on the force applied and the strength of the muscle to withstand that force. Like a braided rope, the thicker the rope, the greater the strength, but at some point if enough force is placed across the rope, it will begin to fail. First, some strands will fray, weakening the rope, and eventually the entire rope will fail.

Calf strains most commonly occur in the medial (inside) aspect of the gastrocnemius muscle. This injury is often called “tennis leg” because it was first described in the late 1800s in conjunction with lawn tennis. The gastrocnemius is at high risk for strain because it crosses two joints (allowing for greater stretch of the muscle across two fulcrums), and it has a high density of Type 2 muscle fiber (fast twitch), which allow the muscle to generate a quick, powerful contraction. This combination of stretching the muscle and then contracting it quickly is similar to the action of a whip and leads to high degrees of force across the muscle and risks muscle strain. For this reason, calf strains have historically been given the moniker of coup de fouet, or snap of the whip.

Strains of the soleus are less common. The soleus begins below the knee and crosses only the ankle. The soleus is predominately made up of Type 1 muscle fiber (slow twitch). These factors make the soleus at low risk for strain.

Symptoms

Symptoms of calf strain should be unilateral. They should localize to the calf or posterior knee. The symptom onset should correlate to an injury or activity. They should be reproducible on muscle testing during the physical exam.

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