In the leg, as throughout the limbs, functionally related groups of muscles are contained in fascial compartments . These compartments are bound by a thick canvas-like covering of deep fascia and share a neurovascular supply. The soft tissues of the leg are divided into four of these compartments: the anterior, the lateral, the superficial posterior, and the deep posterior. These four compartments are divided by crural (leg) fascia, also called fascia cruris, which is the deep fascia of the lower leg; more posterior, it is known as the popliteal fascia.

Although the main focus of this book is on the muscles of the superficial and the deep posterior compartments , the proximity of the lateral and posterior compartments as well as their functional overlap necessitates careful delineation of lateral and posterior compartments clinically. The lateral compartment houses several clinically important structures, including the common peroneal nerve, the peroneus longus, and the peroneus brevis. The peroneus longus is the more superficial of the two muscles; it everts and plantarflexes the foot (Fig. 1.2) [3].

The most distal part of this peroneus longus consists of a very long tendon. The peroneus brevis is located just anterior to the peroneus longus. The common peroneal nerve moves from its proximal posterior location behind the fibular head to an anterior lateral position as it transverses between the fibular head and shaft. Both muscles are supplied by the superficial peroneal nerve, which also provides sensory innervation to the dorsum of the foot.

The superficial posterior compartment consists of the gastrocnemius , soleus, plantaris, and tendo calcaneus (Achilles tendon) . As a functional unit, the superficial posterior compartment is critical in maintaining functions of standing posture and gait. As a group, they primarily assist in flexing the knee and plantarflexing the foot.

The gastrocnemius is the largest and most superficial muscle of the superficial posterior compartment. Its heart-shaped landmarks are easily visible through the skin in well-trained athletes. The more proximal part of the gastrocnemius (its “belly”) originates from two heads, which are connected to the femoral condyles by tendons. More specifically, there is a medial and a lateral head. The medial head tends to be the larger of the two. On its distal end, the muscle becomes narrower and blends to form a union with the Achilles tendon. Starting above the knee and extending beyond the ankle to the calcaneus, the gastrocnemius crosses two joints. This biarthodial architecture conveys a unique functional continuity with the hamstrings in initiating knee flexion and limiting knee extension. Rarely, individuals may not have a lateral head of the gastrocnemius , or a third head may be present [5]. The tibial nerve provides the innervation of the gastrocnemius.

The soleus , located just deep or anterior to the gastrocnemius, is a broad and flat muscle. The soleus has a central tendon and pennate structure. Unlike the gastrocnemius, which spans two joints (the knee and the ankle), the soleus originates on the posterior tibia and crosses only the ankle joint. It is attached proximally to the posterior border of the tibia and fibula by an arch-like aponeurosis with a tendinous gap centrally. The posterior tibial artery and tibial nerve pass through this gap to enter the deep posterior compartment . The more distal end of the muscle connects to the gastrocnemius, which then, as noted above, forms the Achilles tendon . As a result, there is a strong synergistic relationship between the gastrocnemius, the soleus, and the Achilles tendon, which is important in regard to the injuries discussed in the following chapters. Please see Figs. 1.3 and 1.4 for illustration of the mentioned muscles.

The soleus has a high percentage of fatigue-resistant muscle fibers and plays a critical role in maintaining standing posture and walking gait. The soleus muscle is supplied by two branches of the tibial nerve. Accessory soleus muscles are well-known but uncommon congenital anomaly [6]. An accessory soleus can be a source of exertional pain due to an inadequate blood supply from the posterior tibial artery.

The plantaris is a rather small muscle and actually has been found absent in a number of individuals. It originates from the lateral supracondylar line deep to the lateral head of the gastrocnemius . From this lateral attachment on the femur, it moves medial as it extends distal. The muscle ends shortly after crossing the knee joint, where at approximately the origin of the soleus, it transitions in a long, thin tendon that traverses between the medial head of the gastrocnemius and the soleus before inserting on the medial calcaneus either independently or by means of the Achilles tendon . In the setting of an Achilles tendon rupture and a preserved independent attachment of the plantaris tendon to the calcaneus, the ability to plantarflex the ankle can remain potentially confounding for the clinical diagnosis. The plantaris is supplied by the tibial nerve. It is referred to by some as the gastrocnemius’s “little helper,” as it acts in conjunction with the gastrocnemius in an accessory fashion. Like the gastrocnemius, it crosses both the knee and ankle but due to its small size and absence in some individuals, the functional role of the plantaris is traditionally thought to be relatively insignificant and it is considered by many to be a vestigial muscle. Indeed, the plantaris tendon is harvested for autograph tendon repair for this very reason [7]. However, the pattern of a small muscle running in parallel with a larger prime mover has been termed parallel muscle combination, and in these combinations the small muscle has been found to have a high density of muscle spindles. Some authors have proposed these muscles may serve an important proprioceptive function, providing neurologic feedback to regulate movement [8]. It should be noted that discussion regarding the function of the plantaris can be influenced by larger arguments regarding the implications of vestigial muscle as evidence for evolution [9].