Rectus abdominis muscle injuries are secondary effects of direct blows to the abdomen or indirect trauma by sudden or repetitive trunk movement, either rotation or flexion/extension. One retrospective study reported a prevalence rate of 29.5 % in the career of a professional tennis player. Injury happens during the cocking phase of the service motion when eccentric overload followed by forced contraction applies on the non-dominant rectus abdominis. Those muscle strains have also been described in many sports such as soccer, handball, baseball and their prevalence seems to increase especially in early season. Usually, the lesion is located in the rectus abdominis on the side of the nondominant arm or foot. Pain is usually peri- or infra-umbilical and is exacerbated by eccentric or concentric contraction. There is usually no ecchymosis because strains are mainly low-grade injuries [7–11].

A spontaneous rectus sheath hematoma can occur in elderly adults without history of trauma and with or without anticoagulation treatment [12]. Care should be taken in case of an abdominal mass with cyclical pain symptoms in middle-age women. Extra pelvic endometriosis which involves the abdominal wall muscles is an uncommon finding but it should be considered in the differential diagnosis of any abdominal swelling with or without a previous history of surgery [13]. A rare case of myositis ossificans involving the rectus abdominis muscle has been described in which the complete lack of history of trauma led to the suspicion of a malignant tumor, which was proved surgically [14].

No specific complications have been described besides those common to muscle strains. A fibrous scar can be responsible for chronic tenderness and pain and is usually treated by conventional physiotherapy and does not require further exploration. In our experience, some fibrous scars are responsible for small nerve entrapment and/or neuroma; they can be treated by surgical excision.

Side strains are mainly encountered in cricketers, javelin throwers, rowers, and ice hockey players. Patients present with sudden pain and point tenderness in the region of the lower costal margin. It is due to a tear of the internal oblique muscle from its rib or costal cartilage origin. Internal oblique muscle strains may also occur at the proximal insertion onto the iliac crest resulting from eccentric, unbalanced trunk rotation in tennis players. Such injuries may extend to the external oblique muscle [2, 8, 15]. Isolated external oblique muscle strains have been described in elite ice hockey players and usually cause groin pain.

The main differential diagnosis is rib stress fracture that occur in rowers, swimmers, golfers, and canoeists and which results from the repetitive forces exerted by the external oblique and serratus anterior muscles [16].

Ultrasound is an easy and commonly available technique that enables contralateral examination and appears to be sensitive to tears when used with a high-resolution transducer. Acute injuries are easier to diagnose when bleeding is present within the fibril disruption and the muscle edema. Hematoma appears anechoic at the acute phase and its echogenicity slightly decreases with healing. In our experience and in one study, MRI does not provide any additional information but the images are easier to understand [1]. Hematoma appears as a well-delineated high signal intensity area on both T1- and T2-weighted images within the muscle. Fluid-fluid levels and a concentric ring sign are commonly observed [17]. Hypertrophy of the rectus musculature on the non-dominant side is a normal finding which should not be taken for pathology. Coronal and sagittal images are the most useful for assessing the degree of fibril disruption and retraction.

Some studies have established that acute injury almost always occurs in the deep epimysium at the infra-umbilical level, approximately 5 cm from the umbilicus. Medial and anterior injuries are extremely rare. Tears may involve the whole thickness of the muscle or they can be partial [2, 9].