The soleus is broad and bulky, located superficially to muscles of the deep posterior compartment (flexor hallucis longus muscles, tibialis posterior, and flexor digitorum longus,) and deeply to the gastrocnemius and plantaris.

The soleus has two distinct proximal tendinous fibular and tibial origins. The fibular arises on the posterior aspect of the head. The tibial origin is at the inferior border of a tibial bony ridge called the tibial soleal line [2, 17].

Both proximal tendinous fibers merge to form the tendinous arch of the soleus, and extend proximally into the soleus belly to form a tendinous lamina called the intramuscular aponeurosis of the soleus. The soleus muscular fiber originates proximally on the latter and extends inferiorly, forming a bipennate pattern. The angle of pennation seems to be sex specific, with higher angle pennation in the male soleus muscle [18].

The majority of soleus fibers originate on the posterior aspect of the intramuscular aponeurosis and converge downward to finish on the anterior aspect and on the borders of a new tendinous lamina, the insertion lamina.

The anterior muscle fibers originate on the anterior aspect of the intramuscular aponeurosis and form distally a central intramuscular tendon.

In the distal segment of the soleus muscle two medial and a posterior sagittal intramuscular aponeurotic band converge with the insertion lamina [8, 19, 20]

This insertion lamina, central muscular tendon and sagittal and medial aponeurosis converge and fuse with the tendinous lamina issuing from the gastrocnemius muscle to form the Achilles tendon.

The role of the soleus muscle is to maintain posture and to assist low energy activities like walking. Moreover the soleus is the main plantar-flexor muscle of the foot and the most powerful muscle of those crossing the ankle.

The lesser susceptibility of soleus to musculotendinous lesions is due to its monoarticular pattern and its composition (96 % of soleus muscle fibers are type I) [21].

Soleus injuries occur when the ankle is passively dorsiflexed while the knee is flexed.

Balius in a recent work revealed the propensity of proximal strains to occur along the medial side, probably due to the increased length of the intramuscular tendon and a larger musculotendinous junction. It can also be explained by the fact that during a plantar-flexor movement the medial head provides 71 % of its force, while the lateral head contributes only a small part [21, 22].

Soleus injuries usually appear as a moderate diffuse pain with subacute presentation, resembling delayed onset muscle soreness, probably because the soleus is composed predominantly of type I fibers. The difficulty of distinguishing among gastrocnemius, soleus, plantaris, and proximal Achilles tendon injuries and soleus lesions is usually underestimated.

Furthermore, under-appreciation of soleus strains has likely played a role in the apparently decreased incidence of soleus injuries, which can be attributed in part to the traditional role that ultrasound has played as the imaging modality of choice. Owing to the gradual onset of the injury and the fact that it is usually well tolerated, it is felt that the frequency of soleus lesions is underestimated. Soleus injuries are easily detected on MRI [15, 21].

Soleus injury semiology and staging is overall the same as for gastrocnemius strains. On the other hand soleus injuries seem to be frequently underdiagnosed on ultrasound. Subtle grade 2 strains, where minimal macroscopic fiber disruption may be the only evident imaging feature (and where hemorrhage is absent), may be missed on ultrasound. Given the deeper location of the soleus muscle, resulting in relatively poorer image quality, and the anatomic complexity, it is likely that low grade and therefore subtle strains are missed, with only higher grade injuries visualized [18, 23].

As Balius noted in a recent study, the variability of injury localization makes ultrasound difficult. Nevertheless strains of the proximal medial musculotendinous junction are the most common type of soleus muscle injuries (56.4 % of all cases) [21, 22].

During ultrasound examination special attention should be given to detection of architectural anomalies of the central muscular and peripheral myo-aponeurosis junction. An isolated collection located in the inter-aponeurosis soleus-gastrocnemius space suggests careful examination of the soleus [1].

Thus, MRI is the best method to investigate injuries of the soleus (Fig. 21.4). Koulouris showed that the high prevalence of soleus injuries on MRI and the frequent association between different calf muscle injuries suggests that many cases are probably missed on ultrasound (Figs. 21.5 and 21.6) [15, 24].

Owing to the gradual onset of the injury and the fact that it is usually well tolerated, the frequency of soleus lesions is probably underestimated. Although soleus injuries are easily detected on MRI, owing to their good prognosis and the expense of MRI it is usually recommended only for high-level athletes

The plantaris muscle is the smallest muscle of the calf. It is missing in as much as 7–20 % of the normal population [25].

The plantaris arises from the supracondylar lateral area of the femur, with a short muscle body located between the aponeurosis of the soleus and lateral gastrocnemius, continues by a thin tendon distally located at the anteromedial border of the Achilles tendon and inserts into the flexor retinaculum or calcaneus. Its distal insertion is separated from the Achilles tendon.

In relation to its small size the mechanical role of the plantaris muscle (flexor of the knee, plantar flexor of ankle) is weak. Nevertheless itis an agonist of the anterior cruciate ligament and participates at the posterolateral complex of the knee.