According to the sports medicine literature, MRI is a sensitive modality for confirming strain in the muscles of the calf muscle complex and for accurately identifying the location of injury within the muscle-tendon-bone unit (Koulouris et al. 2007; Menz and Lucas 1991). However, MRI would only be recommended particularly in high-performance sports in cases where a rapid and accurate diagnosis is required and in which a thorough US study does not identify any injury (Balius et al. 2014).

Previously not appreciated is the high number of dual injuries (although the most commonly injured muscle of the calf muscle complex is the medial head of the gastrocnemius in the proximal musculotendinous junction), as well as strains involving the soleus muscle, which may potentially be of prognostic significance. Soleus muscle injuries have been rarely described in the echotomography literature; it is likely that these injuries have been underdiagnosed, and thus MRI should be considered the modality of choice in elite athletes and preferred over US (Kerkhoffs and Servien 2014; Koulouris et al. 2007; Hayashi et al. 2012).

MRI is commonly used to locate the lesion and assess its severity. The extent of musculotendinous injuries and the associated architectural distortion is assessed using axial, sagittal, and coronal images arranged along the long and short axes of the included musculotendinous unit. The axial plane is helpful to evaluate muscle contours and to delineate the musculotendinous junction and its exact anatomical relation to focal lesions (Hayashi et al. 2012; Palmer et al. 1999), while the coronal and sagittal planes are utilized to survey the longitudinal extent of the injury (Valle 2011).

Musculotendinous units are commonly altered in all forms of acute traumatic injuries. Normal skeletal muscles, compared with other soft tissues, show intermediate to low signal intensity on both T1-weighted (T1-w) (short TR/TE) and T2-weighted or short tau inversion recovery (STIR) (long TR/long TE) images (Deutsch and Mink 1989).

Fluid-sensitive sequences, i.e., fat-suppressed T2-weighted (FS T2-w) or fat-suppressed proton density-weighted (FS PD-w) images, and turbo spin echo (TSE) and STIR sequences, are suitable for detecting edematous changes (hyperintense feather-shaped) in the musculotendinous unit, and for delineating and locating intramuscular or perifascial fluid collections or hematomas as hyperintensity (Hayashi et al. 2012). T1-w TSE sequences are used to visualize atrophy and fatty infiltration and to differentiate between edema (hypointensity) and hematoma (hyperintensity) (Deutsch and Mink 1989). But in chronic muscle injuries, T1-w images may not show any signal abnormalities in small tears (Hayashi et al. 2012).

Findings in musculotendinous strains and tears include rupture or discontinuity of gastrocnemius muscle fibers, fluid signals consistent with hemorrhage and hematoma at the musculotendinous junction, and retraction of the torn muscle fibers. MRI also allows differentiation between gastrocnemius and Achilles tendon injury, which can help to improve the treatment (Campbell 2009).

In musculotendinous strains without a tear, some fiber disruptions are seen, but muscle functions are maintained. On MRI, interstitial edema and hemorrhage are present at the musculotendinous junction and extend into the adjacent muscle fascicles, producing a feathery appearance (i.e., hyperintensity) on fluid-sensitive sequences. In the presence of partial tears (without retraction), in addition to interstitial edema and hemorrhage, hematoma at the musculotendinous junction and perifascial fluid collection appear as hyperintensity on fluid-sensitive sequences. Hematoma in this region is associated with a complete musculotendinous rupture. Clinically, muscle function is completely lost, with a palpable gap and retraction of muscle fibers. MRI and US may be useful for the preoperative assessment of the extent of retraction (Hayashi et al. 2012).

In an MRI series of 23 injuries to the distal gastrocnemius that occurred in 20 patients, myotendinous strains were the most common injuries (43%); partial tears (30%) and complete tears (22%) of the myotendinous junction or proximal Achilles tendon were less frequent. Injuries to the medial head of the gastrocnemius in the myotendinous junction were more frequent than those to the lateral head (86% vs. 14%) (Weishaupt et al. 2001).

MRI is also helpful in diagnosing tears of the plantaris or soleus muscles. Soleus injuries can occur throughout the extent of the muscle; those occurring at the perimeter of the muscle and not at the tendons are classified as myofascial (epimysial) strains, those at the myofascial junction of the soleus with the gastrocnemius are classified as posterior myofascial strains, and those at the junction between the soleus and the deep posterior compartment of the leg are classified as anterior myofascial strains (Campbell 2009; Balius et al. 2013)., Soleus injuries can also occur in the musculotendinous junction, involving the distal intramuscular tendon, or the proximal medial and lateral aponeuroses. As shown in a retrospective study of 55 cases of soleus muscle strains, 31 strains were considered musculotendinous, of which 14 were located at the medial aponeurosis and 10 were located at the central tendon; 24 of the soleus muscle strains were myofascial, of which half were anterior myofascial (Balius et al. 2013).

Imaging features of plantaris injury include hemorrhage and edema in the muscle, seen on T1 and T2 imaging (27); the fluid is located between the gastrocnemius and soleus muscles. Plantaris injury may occur at the myotendinous junction with or without a partial tear of the medial head of the gastrocnemius muscle; plantaris injury may also occur as an isolated injury or in conjunction with injury to the anterior cruciate ligament (ACL) (10 of 15 patients) (Helms et al. 1995).

Ultrasonography has proven to be an easy to perform and a fast and safe imaging modality for evaluating the size of the tears in patients with clinically suspected tennis leg (Bianchi et al. 1998). Ultrasonography allows dynamic imaging while the injured leg is maneuvered to elicit symptoms and aid in clarifying the diagnosis. Power Doppler is useful for identifying hyperemia associated with acute injuries (Blankenbaker and Tuite 2010).

Moreover, a large hematoma may be drained under US guidance after liquefaction of the hematoma has occurred. The sensitivity of US for acute fluid collection has been shown to be equal to that of MRI in some studies (Koulouris and Connell 2005), especially in larger tears where fluid collection occurs between the injured medial head of the gastrocnemius muscle and the soleus muscle (Bianchi et al. 1998).

The disadvantages of US are seen in follow-up imaging, because is very difficult to reproduce exactly the same imaging position and plane as those at baseline, and US cannot always differentiate between old and new lesions and those on follow-up visits (Hayashi et al. 2012).

The US technique discriminates partial tears from complete tears of the muscle, with case reports demonstrating the rate of occurrence of incomplete injuries as 33% to 75% (Campbell 2009). At the gastrocnemius muscle myotendinous junction, US usually shows discontinuity of fibers, and follow-up US shows union of this defect with hypoechoic tissue at 4 weeks (Bianchi et al. 1998). The lesions in a gastrocnemius muscle injury produce loss of the alternating linear hyperechoic and hypoechogenic structures of muscle fibers and normal muscle septum (Delgado et al. 2002).

Fluid collections are often a useful guide to the site of injury (Bianchi et al. 1998). Edema peripheral to a gastrocnemius muscle injury appears as a poorly defined region of increased echogenicity, and this can be an important echotomographic sign of a muscle injury, especially in small tears (Lee and Healy 2004). In the presence of partial tears, these pathological features are depicted as hypoechogenicity. Disruption of muscle fibers will be depicted as notable echo-inhomogeneity (Hayashi et al. 2012).

In patients with small tears, examined within a couple of hours of the injury, the absence of a definite hypoechoic or anechoic blood collection was reported to make detection of the tear difficult. Cautious assessment of the distal segment of the medial head of the gastrocnemius, however, revealed that muscle fibers and septa did not achieve an aponeurosis. Most of these injuries affected the most anteromedial portion of the gastrocnemius medial head and could be missed if this region is not evaluated carefully (Bianchi et al. 1998).

US also offers a quick and inexpensive imaging methodology to diagnose plantaris or soleus injuries.

Five sites of strain distribution have been identified within the soleus: three musculotendinous junction sites (proximal medial strains, accounting for 25.5% of all injuries, were the most common type of soleus muscle injuries, with proximal lateral strains accounting for 12.7%, and distal central tendon strains accounting for 18.2%) and two myofascial sites (anterior strains 21.8% and posterior strains 21.8%) (Balius et al. 2013).

US, compared with MRI, is not sensitive enough for detecting and assessing soleus injuries (Delgado et al. 2002; Balius et al. 2014), although the sensitivity is enhanced by thorough anatomically based US; also, the timing of the US examination (within 28.8 days of the injury in positive cases P = 0.003) may be important. Some authors observed that injuries at the myofascial junctions (10 out of 24; 41.7%) were more readily identified than those situated at the musculotendinous junctions (5 out of 26; 19.2%). This may be due to the presence of a fluid collection that appears to facilitate US visualization of myofascial injuries. Each type of soleus injury appears to have a characteristic US pattern based on a defect of connective expansions, the existence of small myofascial filiform collections, and the rarefaction of the fibrillar area (Balius et al. 2014).