Muscle injuries are one of the most common injuries sustained while playing sport and constitute between 10 and 55 % of all sporting injuries (Beiner and Jokl 2001; Best and Hunter 2000; Garrett 1996; Huard et al. 2002; Järvinen et al. 2005). Muscle injury results from a variety of mechanisms including muscle strain/tear, epimyseal fascial injury, delayed onset muscle soreness (DOMS), muscle contusion, muscle herniation and lacerations (Kijowski 2011; Kneeland 1997). Avulsions of the origin or insertion of muscles and tendinopathy is beyond the scope of this chapter.

Mimics of muscle injury on magnetic resonance imaging include—subacute denervation, muscle infarction, inflammatory myositis, normal immediate post exercise muscle oedema and deep venous thrombosis (Farber and Buckwalter 2002).

Muscle strain is the most common muscle injury sustained by both the general and sporting populations (Blankenbaker and Tuite 2010; Linklater et al. 2010; Shelly et al. 2009; Slavotinek 2010). Muscle strains occur when the muscle is subjected to an excessive tensile force, which leads to overstretching and rupture of myofbrils (Kijowski 2011). Most commonly, muscle strains occur during eccentric contraction. During an eccentric muscle contraction, muscle fibres are forced to lengthen, creating greater tensile forces than can be created during concentric contraction, when muscle fibres shorten. Injuries predominantly occur at the musculotendinous junction and may involve the intramuscular tendon.

The majority of muscle injuries are diagnosed clinically (Kijowski 2011). There is a spectrum in the clinical presentation of muscle strain injury, which may range from a severe tearing sensation to a mild tightening sensation. Historically, the most commonly used grading system was devised by O’Donoghue (1962) on the basis of the clinical impression of the extent of injury. Grade 1 injury (strain) consists of a minor degree of microscopic tearing with no macroscopic muscle fibre discontinuity. Grade 2 injury (partial tear) is defined as an incomplete disruption of muscle fibres. Clinically, grade 1 and grade 2 injuries can be difficult to distinguish—both grades result in loss of function and in some studies have shown no significant difference in time to return to sport (Blankenbaker and Tuite 2010). Grade 3 injury (complete tear) is a complete rupture of the muscle. This grading system fails to take into account the presence or absence of concomitant tendon injury and if present, the extent of tendon injury. There is scope for development of a new classification system which more precisely characterises the extent of muscle and tendon injury, hopefully providing a higher level of prognostic significance with respect to time to return to sport and risk of recurrent tear (Linklater et al. 2010).

There are specific characteristics of a muscle that makes it more susceptible to injury. Long, fusiform muscles that extend across two joints and have a propensity for eccentric contraction are more susceptible to injury. Examples of these muscles include biceps brachii, rectus femoris, long head of biceps femoris, semitendinosus and semimembranosus (hamstrings) and the medial head of gastrocnemius. A predominance of type II fibres may also predispose to injury.

Other factors which may predispose to muscle tears include: past muscle strain injury, low muscle strength, muscle fatigue, age, lack of warm-up, muscle temperature and poor flexibility (Garrett 1996; Mair et al. 1996; Orchard 2001).

The long head of biceps femoris, semimembranosus, and semitendinosus muscles consist of a proximal and distal “free” tendon and relatively extensive tendon–muscle interface which may be intramuscular (central tendon) or at the muscle surface, such that the proximal and distal tendons overlap or almost overlap. The central tendon is similar in configuration to the central rachis of a feather, with radiating myofibrils arising from it, exemplified by the proximal tendon of long head biceps femoris (Comin et al. 2013). As stated previously, the most common site of muscle strain/tear occurs adjacent to the muscle–tendon junction, which may be located at the central aspect of the muscle or eccentrically at the muscle surface (Blankenbaker and Tuite 2010; Noonan and Garrett 1992) (Fig. 1). A study by Weishaupt (2001) demonstrated that 33 % of hamstring tears occur at the proximal muscle–tendon junction, 53 % at the intramuscular muscle–tendon junction and 13 % at the distal muscle–tendon junction (Weishaupt et al. 2001). These findings therefore indicate that the most common site of muscle–tendon junction injury is within its intramuscular portion.

In a study by (Comin et al. 2013), central tendon disruption occurred exclusively in biceps femoris injury (12 of 45 cases). There were no central tendon disruptions of the semimembranosus or semitendinosus. In this same study, recovery times for injuries to the biceps femoris were significantly greater if there was central tendon disruption (Comin et al. 2013).

The investing fascia of a muscle can be described as a fascial ectoskeleton, providing an additional anchor point for muscle fascicles in addition to the proximal and distal tendons. Peripheral/epimyseal muscle injuries adjacent to the investing fascia/epimysium are less common than muscle–tendon junction injuries (Kijowski 2011). When they occur, they are thought to relate to differences in elasticity between epimysium and the adjacent muscle fibres (Blankenbaker and Tuite 2010). This is similar in pathogenesis to injuries at the muscle–tendon junction (Koulouris and Connell 2005). Peripheral/epimyseal injury has been described in the quadriceps and hamstring muscles, caused by an eccentric contraction (Connell et al. 2004; Cross et al. 2004). Differential contraction in two adjacent muscles may lead to injury involving both muscles.

Muscle contusions constitute 2 % of muscle injuries, most commonly involving the quadriceps muscle group. They occur when a muscle sustains a sudden blunt force/direct blow, causing compression of the deep portion of the muscle against the underlying bone (Järvinen et al. 2005). As a result, there is disruption of capillaries and muscle fibres and an intramuscular haematoma which forms between muscle fibres, without muscle fibre disruption in most cases. Occasionally direct blunt trauma may result in muscle fibre discontinuity, for example in surfboard injuries to the biceps brachii muscle.

Contusional injuries are typically deep within the muscle belly, often involve more than one muscle and tend to be less symptomatic and have a shorter clinical course compared to muscle strain injuries. Clinically there are three severity grades described: mild, moderate and severe, depending on the remaining range of motion (Jackson and Feagin 1973). The loss of range of motion does not necessarily correlate with the extent of the haematoma on imaging and does not always correlate with loss of function (Megliola et al. 2006).

DOMS is an indirect muscle injury with reversible structural damage to the muscle at a cellular level resulting in an acute inflammatory response and an increase in intracellular fluid (Armstrong 1984; Cleak and Eston 1992). DOMS is associated with eccentric muscle contraction and is directly related to the intensity and duration of physical activity (Cleak and Eston 1992). Acute grade 1 muscle strain may be differentiated from DOMS clinically. Grade 1 strains have an acute onset of pain and dysfunction. Symptoms associated with DOMS typically commence at 1–2 days after the causative activity, is greatest at 24–48 h and gradually resolves over the next 48–72 h (Armstrong 1984; Cleak and Eston 1992).

A compartment syndrome is elevation of pressure within a confined anatomic space resulting in a reduction in blood flow that threatens the viability and function of the tissues within the compartment (Stedman 2000). Compartment syndromes may present as a complication of an acute muscle injury. The most common causes include: fracture, blunt or penetrating trauma, burns, intramuscular haemorrhage and less likely, secondary to muscle tear. Patients commonly present with pain, which is disproportionate to the injury, exacerbation of pain when the affected muscle group is passively stretched. Neurological symptoms may also be present (Arciero et al. 1984; Davies 1979; Pearl 1981). Acute compartment syndrome is a surgical emergency requiring exploration and fasciotomy. In contrast, chronic exertional compartment syndrome involves exercise related pain which resolves with rest.

Muscle hernias are an uncommon complication of muscle/compartmental injury and represents herniation of muscle through a small fascial defect which may relate to prior penetrating injury or develop at a site of penetration of the investing fascia by a nerve (e.g. superficial peroneal nerve as it penetrates the investing fascia of the lateral compartment in the leg).

Complications of acute muscle injury include—haematoma, acute compartment syndrome, myositis ossificans and calcific myonecrosis (Counsel and Breidahl 2010).

Myositis ossificans is extensively presented in “MRI of muscle tumours” and thus here only briefly discussed in the direct context of muscle injuries.

As a complication of muscle injuries, heterotopic ossification occurs as myositis ossificans circumscripta. Following a muscle tear or a direct contusion and sometimes without a remembered trauma, a haematoma is initially present. This haematoma typically undergoes a maturation process exhibiting characteristic clinical, radiological and histological features.

Calcific myonecrosis presents as an expanding calcific mass that develops within a muscle post trauma. Most reported cases have occurred in the anterior and lateral compartments of the lower leg, with only single cases reported in the foot and forearm (Holobinko et al. 2003; Larson et al. 2004).

This entity most commonly occurs due to compartment syndrome post fracture of the tibia or femur and more rarely after neurovascular injury (Early et al. 1994; Milisano and Hunter 1992; Renwick et al. 1994; Viau et al. 1993). The reported interval from injury to presentation ranges from 10 to 64 years (average, 36 years) (Wang and Chen 2001).

The pathogenesis of calcific myonecrosis is unknown. An entire single muscle is replaced with central liquefaction and peripheral calcification.

The radiographic findings demonstrate a fusiform mass, which involves the entire muscle, with linear, peripherally oriented, plaque-like amorphous calcification. Differential diagnoses on radiographs include tumoral calcinosis, calcific tendonitis, calcific bursitis, synovial osteochondromatosis, synovial sarcoma, chondrosarcoma, osteosarcoma, and myositis ossificans (Olsen and Chew 2006).

On MRI, the mass is diffusely T2 hyperintense/fluid signal, while other areas of the lesion demonstrate heterogeneous, intermediate signal (Muramatsu et al. 2009; Ryu et al. 1996).

The role of imaging in acute muscle injuries includes establishing the site and extent of injury and providing some prognostic guidance regarding return to sport. In general the threshold for imaging is substantially lower in the elite athlete, when compared to the recreational athlete. In the recreational athlete, depending on the clinical context, there may still be a role for imaging to exclude an injury requiring surgery such as a hamstring origin avulsion (Colosimo et al. 2005; Cross et al. 2004; Folsom and Larson 2008; Slavotinek et al. 2002). In the elite athlete there may be a role for imaging in monitoring healing of a muscle injury.

Both MRI and ultrasound (US) may be utilised for the diagnosis of muscle injury (please see also “Imaging the skeletal muscle – when to use MRI and when to use ultrasound”). In Europe, US is often used as the first-line imaging modality. Ekstrand et al. (2012) have investigated which imaging methods are used to investigate hamstring injuries in 23 European professional soccer teams and revealed the following distribution: 58 % were examined by MRI, 29 % by US only and 40 % by both MRI and US. Interestingly, 13 % were managed clinically without the use of imaging. In North America and Australia, MRI is more commonly utilised in the first line assessment of acute muscle injury. The accuracy of MRI is superior to US, particularly with deep muscle injuries (Guillodo et al. 2011).