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FUNCTIONAL ANATOMY education The gastrocnemius is the largest and most superficial muscle of the calf, extending from the posterior knee to the heel. It is one of the three triceps surae (triceps = "three heads" and surae = "calf") muscles. The plantaris and soleus are also part of this group, which converges into the calcaneal tendon and inserts on the posterior surface of the calcaneus. Compared to the other two muscles, the bulk of the gastrocnemius is more proximal and divided between two large, symmetrical segments or "heads." The two heads of gastrocnemius are located medially and laterally, respectively, forming the inferior borders of the popliteal fossa at the knee. Moving inferiorly, they merge near the middle of the calf before descending to the calcaneal tendon. Significant variation occurs in the size and position of the two heads and provides a visual clue as to the genetic predisposition of individuals. Individuals with large gastrocnemius heads and a very short calcaneal tendon tend to have a greater propensity for explosive power. Those with smaller heads and a long calcaneal tendon are more suited to sustained or endurance activities. The structural differences are primarily the result of muscle fiber-type distribution, which is genetically determined (see Massage & Bodywork, "Skeletal Muscle Fiber," January/February 2017, page 43). Gastrocnemius contains mainly fast-twitch fibers, which are recruited rapidly but fatigue quickly, and is, therefore, more developed in individuals with a higher percentage of this fiber type. The gastrocnemius crosses both the knee and the ankle, allowing it to perform both knee flexion and ankle plantar flexion. Compared to the hamstring muscles, the gastrocnemius has relatively little mechanical advantage for knee flexion and serves primarily as a Gastrocnemius By Christy Cael 40 m a s s a g e & b o d y w o r k j u l y / a u g u s t 2 0 1 9 GASTROCNEMIUS Attachments • Origin: Medial head—Posterior medial condyle of femur • Origin: Lateral head—Posterior lateral condyle of femur • Insertion: Posterior surface of calcaneus via the Achilles tendon Actions • Plantar flexes the ankle • Flexes the knee Innervation • Tibial nerve • S1–2 Gastrocnemius: lateral head Gastrocnemius: medial head Soleus Peroneus longus Peroneus brevis Achilles tendon