Muscle strain is a highly complex issue that has a direct impact on the performance of soccer players and the overall ranking of clubs in soccer leagues (Bisciotti et al., 2019; Biz et al., 2021; Danielsson et al., 2020; Ekstrand et al., 2011; Waldén et al., 2005a, 2005b; Woods et al., 2004). Hamstring muscle strain injuries (HSI) are the most common type of injury in soccer, with 12–17% of all injuries occurring during the soccer league season (Ekstrand et al., 2011; Hauge Engebretsen et al., 2010; Woods et al., 2004). Hamstring muscle strains are responsible for approximately 20–25% of all missed matches. Further, their recurrence rate is among the highest of all cases at around 13–22% (Bisciotti et al., 2019; Ekstrand et al., 2011; Waldén et al., 2005a, b). Therefore, the prevention and management of HSI are crucial to maintaining and enhancing the performance of soccer players for achieving positive club results.
To prevent HSI in soccer, it is important to consider that sprinting is the most common activity associated with these injuries in soccer players (Bisciotti et al., 2019; Danielsson et al., 2020; Freeman et al., 2021), despite being an essential skill for successful performance (Di Salvo et al., 2010; Faude et al., 2012; Hisdal et al., 2013; Andrzejewski et al., 2012; Unnithan et al., 2012). Around 80% of all recorded HSI in soccer players occurs during sprinting, while other activities, such as kicking, only accounted for approximately 20% (Bisciotti et al., 2019; Danielsson et al., 2020; Freeman et al., 2021; Kalema et al., 2021). Soccer sprints comprise three phases: acceleration, maximum speed, and deceleration (Mero et al., 1992). Several studies have found the kinematic and muscle activation demands to be significantly different for each phase (Buchheit et al., 2014; Debaere et al., 2015; Haugen et al., 2019; Higashihara et al., 2018; Macadam et al., 2019; Mero et al., 1992). These differences may lead to a nonuniformity in the mechanism of HSI among phases (Higashihara et al., 2018). Among the three, the acceleration phase, especially the initial time of this phase is particularly crucial to the short sprinting outcome and player performance because most sprinting in soccer matches is short, with a mean distance of less than 20 m (Buchheit et al., 2014; Di Salvo et al., 2010; Hisdal et al., 2013; Morin et al., 2015; Lockie et al., 2011; Wdowski and Gittoes, 2020). In addition, the hamstring muscle was demanded explosive work in the initial sprinting acceleration because it is a major contributor to generating horizontal impulse, which is necessary to quickly accelerate from a stationary position or slow running (Ishøi et al., 2019; Morin et al., 2015). Thus, preventing potential HSI in the initial sprinting acceleration phase may play an important role in soccer player performance. Nevertheless, because few studies have analyzed potential mechanisms for HSI in the initial acceleration phase, only limited evidence exists to define and predict the potential HSI mechanisms within it.
Previous studies found that HSI is caused by high levels of muscle activation and great muscle strain. Specifically, HSI occurs when the hamstring undergoes eccentric loading with peak musculotendon stretch and force at the late swing in a running gait cycle during the maximum speed phase (Chumanov et al., 2007, 2012; Schache et al., 2012; Thelen et al., 2005a), or late stance and late swing during the mid-acceleration phase of sprinting (Higashihara et al., 2015; Yu et al., 2008). A high level of hamstring activation was found to be related to the occurrence of HSI during these phases of the running gait cycle (Higashihara et al., 2018; Schache et al., 2009, 2012; Yu et al., 2008). The early stance of the running gait cycle during the maximum sprinting speed phase also was reported relating to HSI, the combination of the greatest knee flexion and hip extension overload the hamstring muscle, which was determined to be a high-risk factor for HSI (Kenneally-Dabrowski et al., 2019). For more detail in the mid-acceleration phase (after 10 or 15 m from the starting position) of sprinting, the results showed a significant rise in the forward pelvis tilt and hip flexion, which led to a sharp increase in hamstring muscle length. Additionally, the hamstring muscle strain during the mid-acceleration phase was significantly greater than that during the maximum speed phase, specifically during the foot strike and toe-off (Higashihara et al., 2015; Yu et al., 2008). However, these studies focused on the mid-acceleration phase of sprinting, so the potential mechanism of HSI may be different during the initial sprinting acceleration phase in soccer players due to the nonuniformity in sprinting kinematics (Bezodis et al., 2014; Macadam et al., 2019). To the best of our knowledge, no studies have investigated the potential mechanism of HSI during the initial sprinting acceleration of soccer players such as in the first two or three steps. Thus, clear insight into hamstring muscle kinematics and activation during the initial acceleration phase is essential for the definition of the injury mechanism, which could be useful for the development of effective HSI prevention strategies.
Therefore, this study aimed to characterize hamstring muscle kinematics and activation during the initial acceleration phase of sprinting in soccer players to define the potential mechanisms of HSI during this phase. We hypothesized that the hamstring muscle would undergo the mechanisms of potential hamstring strain injury, which are great strain and a high level of activation during the late swing and early stance of the running gait cycle.
Musculoskeletal modeling was employed to investigate hamstring kinematics during the initial acceleration phase of sprinting. This is a useful and common platform for quantifying the movement patterns of players in sports (Bulat et al., 2019; Delp et al., 2007; Hirashima, 2011; Maniar et al., 2020; Schache et al., 2012; Thelen et al., 2005a; Thelen et al., 2005b). Specifically, it facilitates the analysis of various parameters of muscle kinematics and mechanics without invasive procedures.