The hip joints are part of the pelvis from a functional point of view (Dalstra and Huiskes 1995; Oatis 1990). Numerous muscles and ligaments originating from and/or inserting into the pelvis all contribute to this function (Dalstra and Huiskes 1995; Oatis 1990). The trunk and the pelvis are joined at the sacroiliac joints, and the pelvis and the lower extremities are joined at the hip joints (Oatis 1990). The synergies between the muscles acting across the pelvis, sacroiliac joints and hip joints are important for achieving optimal function in movements that involve the extremities (Dalstra and Huiskes 1995; Snijders et al. 1993a, b). The pelvis is the centre of load transfer from the upper extremities and the trunk to the lower extremities and vice versa (Snijders et al. 1993a, b). The pelvis is dependent of both skeletal and articular stability as well as sufficient neuromuscular coordination controlling large synergistic actions and forces transmitted through the hip and pelvic ring (Hungerford et al. 2003; Snijders et al. 1993a, b). A number of muscle groups interact on the pelvis and hip, and the quadriceps, hamstrings, adductors, iliopsoas and abdominals are the primary muscle-tendinous structures at risk of being injured (Hölmich 2007). The adductor muscle group have been shown to be very important as stabiliser of the hip joint, and the interaction of the abdominal muscles (in particular the transverse abdominis muscle), the multifidus muscles deep in the low back and the pelvic floor muscles has been shown to be important for the integrity and stability of the trunk, lumbar spine and pelvis (Hodges et al. 1997; Hungerford et al. 2003; O’Sullivan et al. 2002). The precise functions of the iliopsoas are not fully understood, but this muscle group seems to work both as an important hip flexor and as a pelvic stabiliser, as well as a stabiliser for the lumbar spine (Andersson et al. 1995; Bogduk et al. 1992). The hamstring and quadriceps muscles (rectus femoris) are important prime movers developing large forces during lengthening contractions especially during high-speed running but also during accelerating and decelerating events, and therefore these muscle groups are highly stressed in most sporting movements.

These hip, pelvis, and trunk muscles are all constantly involved in most sports activities contributing to a considerable amount of eccentric and concentric work, including fast changes between these work forms (Barfield 1998; Brophy et al. 2010; Charnock et al. 2009; Dorge et al. 1999). This makes them highly important but also at risk of sustaining an injury (Hölmich 2007; Ekstrand and Hilding 1999; Nielsen and Yde 1989). Muscles are often defined and even named from the concentric action they have on the non-weight-bearing leg (Oatis 1990). However, these muscles also have a primarily eccentric function as stabilisers of the pelvis including the hip joints and the trunk (Morrenhof 1989). The abdominal muscles, including the external oblique, internal oblique, rectus abdominis and transversus abdominis, are also stabilisers of the pelvis, and in synergy with the muscles of the back, they control the movements of the trunk in relation to the pelvis and the legs (Snijders et al. 1993a, b). The tendons of the internal oblique and the transversus abdominis muscles insert into the pubic bone as they blend to create the conjoint tendon (the falx inguinalis) (Gibbon et al. 1999; Gibbon 1999). A close anatomical relationship exists between the conjoint tendon, the rectus abdominis sheath and the common adductor origin (Gibbon et al. 1999; Gibbon 1999). These structures undergo large eccentric forces and strains in sports where kicking, acceleration/deceleration, sudden change of direction, extreme positions and rotational movements of the hip and pelvis are included (Barfield 1998; Brophy et al. 2010; Charnock et al. 2009; Dorge et al. 1999).

The acute strain usually involves one or more muscle-tendinous structures. In most cases, the lesion is close to the muscle-tendinous junction, but in some cases, the tendon itself or the enthesis where the tendon inserts into the bone is the site of the injury. These injuries usually happen during forceful actions such as in kicking and skating and with other sporting movements where the muscle is being stretched during forceful contraction. The athletes are usually not in doubt that something happened: it hurts, the function of the limb is affected and sometimes a discoloration of the skin occurs in 24 h. In most cases, the athlete will have to stop the activity. In some cases, the athlete describes a snapping feeling in the groin, sometimes even accompanied by a sound. If not attended to appropriately, these injuries might develop into a more long-standing and sometimes chronic injury. In other cases, the hip and groin injuries have the characteristics of an overuse injury, and an exact moment of injury (the inciting event) can be hard to establish. In the beginning, there is only pain after activity with stiffness of the joint or the muscle group and decreased range of motion of the hip joint, developing to pain in the hip and/or groin at the commencement of sports activity. The pain will often disappear as the athlete warms up but will recur during the activity. If the athlete does not get appropriate treatment and instead continues to participate in the sport, the pain-free periods often become shorter, and finally all sports activities start to cause pain. At this point, even activities of daily living might be a problem. A similar behaviour and injury pattern can also be seen in athletes returning to sports too early after an initial acute hip and/or groin injury without receiving appropriate and/or sufficient treatment and rehabilitation. Athletes will often try to compensate for symptoms of hip and groin pain, and although they might succeed for a while, gradually, the pain and loss of function will take over, and the athlete will not be able to continue sports participation. Typically, athletes will seek treatment when their ability to produce fast movements such as in sudden changes of direction, sprinting and kicking is impaired and painful. At this point, specific structures are often already severely stressed. Therefore, early detection and load management are extremely important for these athletes to avoid some of the long-standing problems that may arise if acute or overuse hip and groin problems are initially ignored.