Skeletal muscles make up the largest tissue of a body, which composes almost the half of the total body weight (up to 40% according to McCall [1], Freckleton and Pizzari [2]). The specificity of active sports, in particular football, is an increased traumatic risk. From 25–50% of all sustained injuries comes for muscle injuries [3, 4, 5]. Most muscle injuries (90%) are bruises or stains and the remaining 10% are tears [6–8]. The most serious types of injuries can cause chronic post-traumatic pain, dysfunction of the ligaments, individual muscles or their groups. Almost all muscle injuries (95% according to Hallen and Ekstrand [4]) can be divided in four groups: hamstring injury, adductor injury, quadriceps injury, and the calf muscle injury. Among the above groups, hamstring injuries are the most common, presenting a third of all cases. Overall, the number of hamstring injuries varies depending on the competition. In football, for example, their frequency is 52 per 1000 players per year [9].
The hamstrings consist of three muscles that can be found at the back of the thigh covering the hip and knee joints: Musculus semitendinosus, Musculus semimembranosus, and Musculus biceps femoris [10]. The semitendinosus muscle is located closer to the medial edge of the posterior femur. Musculus biceps femoris is situated along the lateral edge of the posterior thigh and consists of two parts, long head (caput longum) and short head (caput breve). The long head of biceps femoris has a common origin with semitendinosus as a proximal tendon from the ischial tuberosity. After an approximately 9-cm distance, this common tendon divides [11] and the Musculus semitendinosus becomes separated from the ischial tuberosity at a greater angle of inclination as compared to the biceps femoris. A recent study has shown that at this angle, the proximal part of the semitendinosus muscle is susceptible to injury [12]. Further, the semitendinosus muscle passes downwards, to a long tendon which follows a path around the medial epicondyle of the femur to the anteromedial surface of the tibia and attaches to a tibial tuberosity. At the attachment point, this tendon forms a pes anserinus superficialis with tendons of two other muscles, gracilis and sartorius. The semitendinosus muscle is frequently has a tendinous intersection in the middle and is approximately 44.3 cm in length [11]. Separated from the common tendon, the long head of the biceps femoris merges with the short head to form a strong belly, which runs down to a long narrow tendon. The latter goes posteriorly around the lateral epicondyle to the head of the fibula. The semimembranosus muscle is located at the medial edge of the posterior femur. It originates with a flat tendon on the ischial tuberosity that narrows, goes around the medial epicondyle, and inserts at the posteromedial aspect of the medial condyle of the tibia. Semimembranosus extends approximately 38.7 cm [11].
A hamstring muscle injury typically occurs during eccentric loading (stretching) when the contracting muscle fibers generate even more tension in the muscles. Considering the absorption of kinetic energy during the eccentric phase of muscle contraction as the main function of hamstrings, the posterior compartment of the thigh is susceptible to trauma from stress, especially in the region adjacent to MTJ (musculotendinous junction) [10, 13]. Recent studies have shown that hamstring injuries vary depending on the type of the training load. There are 2 types of injuries known, the high-speed running type [14] and the stretching type [15]. Hamstring injuries due to high-speed running is most frequent among football players, basketball players, tennis players or athletes engaged in similar sports (these are sports that require sprinting with sudden stops) and is closely associated with damage to the long head of the biceps femoris [14]. Hamstring injuries due to stretching occur at the extreme joint load (i.e., at hip flexion and at knee extension). Frequently, they involve more than one muscle but the most common combinations comprise the semimembranosus muscle and its free proximal tendon [15]. These types of injuries vary in prognosis regarding the time to full recovery, which is normally longer for injuries due to stretching [16]. Alongside the mechanism of injury, its exact location also affects the prognosis. Hamstring strains usually occur in the proximal parts of the muscle and are more problematic because they are located closer to their origin in the ischial tuberosity [16]. The involvement of the free proximal tendon is also associated with a longer recovery period [14].
As with other muscle injuries, hamstring strains are classified depending on a condition, which they cause [17] and which is associated directly with the degree of damage to the muscular-tendon node. In the clinical classification, three levels of severity are distinguished: mild (first degree), moderate (second degree) and severe (third degree) hamstring strains [17]. This classification system, however, is rather fuzzy and does not take into account the effect of an injury site on symptom duration and full recovery time. For instance, a small rapture in the proximal part of the biceps femoris can have more serious consequences than a severer injury located more distally [17]. Therefore, it is preferable to have a clinical classification system that is built around these factors too.
Imaging can provide detailed information about the nature and extent of damage to the hamstring muscles. Magnetic resonance imaging (MRI) and ultrasound are the common instrumental methods of choice [18, 19]. To evaluate the hamstring muscle damage based on imaging findings, the Peetrons classification [20] for ultrasound and its modification for MRI are used [21]. This classification defines four grades of muscle injury: grade 0 with no injury; grade I with a focal fiber rupture or edema, without architectural distortion (less than 5% of muscles injured); grade II with a partial focal fiber rupture, with/without fascial damage (5%, to 30% of muscles injured); and grade III with a focal fiber rupture to complete muscle rupture with retraction and fascial damage (more than 30% of muscles injured). Grades 0 to I correspond to a mild hamstring strains, grade II falls within the moderate level of severity, and grade III is associated with severe clinical cases.
Despite possible inaccuracies due to pain, the range of motion (ROM) variables can reveal a decrease in flexibility for the injured leg. An impairment in the back of the thigh is best defined by evaluating the passive and active ROM through a series of straight leg rise and knee extension tests. When the patient is lying supine, the strength of his hamstring muscles can be measured by means of a knee flexion test and a hip extension test. Testing both legs allows detecting a decrease in the strength of the injured leg but keep in mind that a ROM deficit may be implicated by the presence of pain, rather than damage to muscle fibers [16].
For this reason, a deep understanding of hamstring injury processes is required. A right treatment, a more detailed research and evidence on the most effective treatments may be useful in recovery prognosis and enable a full, effective and quick return to sport activities. This article examines cases of hamstring injury in futsal players and finds association between the active ROM deficit and the full recovery time.