Hamstring strain injury (HSI) is the most common injury in many sports, including high-speed running (Orchard et al. 1997). The incidence and recurrence rates of HSI are high in many sports (Brooks et al., 2006; Engebretsen et al., 2010). The proportion of HSI occurring during international athletic competitions (11.4%) is the highest among all injuries (Edouard et al., 2014). The prevalence of HSI in rugby players ranges from 6% to 12% (Brooks et al., 2006). Development of effective prevention programs and identification of the risk factors for HSI are under development. Van Mechelen et al. (1992) advocated the "sequence of prevention" of sports injuries. Many studies of sports injuries have been conducted based on this concept, which consists of the following four steps:
Establishing the extent of the sports injury problem (incidence and severity).
Establishing the etiology and mechanism of injury.
Introducing preventive measures.
Assessing their effectiveness by repeating Step 1.
Although many researchers have investigated the risk factors for HSI (Step 2), several unidentified factors (age and history of HSI) might be associated with the future occurrence of HSI (Freckleton and Pizzari., 2013). We consider that there are two reasons for the lack of clarity regarding the risk factors for HSI.
The first reason is that HSI can occur in various situations. HSI mainly occurs in two situations (Askling et al., 2006): high-speed running (high-speed running type) or extreme stretching of the hamstring by physical contact, falling, or dancing (stretching type). The risk factors for HSI will differ according to these situations. Most previously published studies of the risk factors for HSI have involved rugby, soccer, or Australian rules football players (Brooks et al., 2006; Orchard et al., 1997). However, these sports include both of the above-mentioned situations. More studies of restricted injury situations are needed to fully examine the risk factors for HSI. In the present study, we investigated the risk factors for only high-speed running type HSI. Additionally, when the injury situation is limited to sprinting, the risk factors for HSI must be reconsidered. We hypothesized that the occurrence of HSI is related to the function and morphology of the hip muscles. Therefore, we added hip muscle function and morphology to the previously investigated risk factors.
The second reason for the lack of clarity regarding the risk factors for HSI is that many previous studies did not consider confounding factors. The most recent meta-analysis of risk factors for HSI (Freckleton and Pizzari, 2013) used 15 questions that modified Downs and Black's checklist (Downs and Black, 1998) to evaluate the quality of previous studies. The question that revealed inadequacy in most studies was "Was there adequate adjustment for confounding in the analyses from which the main findings were drawn?" A history of HSI, age, and sex are probably confounding factors between the occurrence of HSI and expected risk factors. A nested case-control study design is suitable for excluding the effects of some confounding factors.
The purpose of this study was to examine the risk factors for high-speed running type HSI by focusing on hip and knee muscle strength, morphology, and range of motion (ROM) by fixing effects of confounding factors. We hypothesized that injured athletes have limited ROM or muscle strength of the hip joint compared with uninjured athletes.
The theoretical significance of this study is the ability to accurately assess the physical risk factors for HSI induced by high-speed running. The practical significance of this study is to facilitate understanding results of physical examination and corresponding to athletes who have risk of HSI.
Study sample and participant selection
This was a prospective nested case-control study involving 61 male track and field athletes (sprinters, long jumpers, and decathletes) (age, 19.6 [+ or -] 1.1 years; height, 1.74 [+ or -] 0. 05.m; body mass, 67.2 [+ or -] 5.0 kg). All athletes performed sprint running in their events. Athletes with any pain or discomfort at recruitment were excluded from this study. All participants provided written informed consent, and approval for the study was obtained by the Chukyo University Human Research Ethics Committee (approval No. 2016-4).
The preseason measurements were as follows:
Isokinetic hip flexion and extension muscle strength
Isokinetic knee flexion and extension muscle strength
ROM of hip flexion and extension
Passive knee extension test for assessing hamstring tightness
Muscle thickness of biceps femoris long head (BF1h) and gluteus maximus (GMax)
Questionnaire on whether the participants had a history of HSI
Assessments and measures
Isokinetic strength tests were performed using an isokinetic dynamometer (Biodex system 3; Biodex, Shirley, NY, USA). The participants performed all tests three times at the same angular velocity (60 deg/s). They conducted hip and knee tests on different days to exclude the effect of fatigue. We analyzed the peak torque per body weight (%) and agonist-antagonist peak torque ratio (hamstring-quadriceps ratio [HQ ratio] and iliopsoas--GMax ratio [IG ratio]).
In the hip test, the participant lay on the dynamometer, and his body was stabilized by straps on the chest, pelvis, and thigh of the untested leg. To prevent pelvic movement, the participant maximally flexed his neck and contracted his rectus abdominis. The knee angle was kept at 90 degrees throughout the measurement. The hip ROM was fixed at 115 degrees of flexion from full extension.
In the knee test, the participant was seated on the dynamometer, and his body was stabilized by straps on the chest, pelvis, and thigh of the untested leg. The knee ROM was fixed at 90 degrees of flexion from full extension.