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Epidemiology
Epi stuff

An academic study found that the most common and prevalent injury was a hamstring strain (Orchard & Seward, 2002). The study further explains that hamstring strains represented 15% of all injuries per club per season also had a 34% chance of recurrence (Orchard & Seward, 2002). Another study showed that a previous hamstring injury is one of the most cited risks for future injury, with as many as one-third of active individuals experiencing a re-injury within 2 weeks of returning to activity (Sherry et al., 2015).

A meta-analysis article showed evidence that history of hamstring injury and older age were associated with increased risk of hamstring strains (Freckleton & Pizzari, 2013). One study found that men and master athletes (athletes older than forty) were at an increased risk of hamstring strains compared with women and younger athletes, respectively (Opar et al., 2014). This article adds that men are more likely to sustain hamstring strains in total than women, possibly due to men being less flexible than women (Opar et al., 2014). However, it is unclear whether flexibility serves as a risk factor and should be researched in the future (Dalton et al., 2015). Research has shown an increase in age can be a significant independent risk factor for hamstring strains (Opar et al., 2014). This can be due to an increase in body weight and a reduced flexibility in the individual's hip flexors (Opar et al., 2014). Another article found that static hamstring flexibility was not a related cause of a hamstring strain, however low hip flexor and quadriceps flexibility increased hamstring strain risk (Hrysomallis, 2013). However, a stronger explanation is that older active individuals may be at greater risk due to lower levels of eccentric knee flexor strength compared with their younger counterparts (Opar et al., 2014).

Muscle weakness has been implicated as a predisposing factor for both primary and recurring hamstring strain injuries (Elliott, Zarins, Powell, & Kenyon, 2011). Over a 10-year study more than 51.3% of hamstring strains occurred during the preseason (Elliott et al., 2011). In this study of 25 NCAA sports over four years also concluded that hamstring strain rates were higher in the preseason (Dalton et al., 2015). This study shows that hamstring strains account for 50% of muscle injuries in sprinters and are the most common injury in hurdling (Ahmad et al., 2013). The factors that have been implicated in this trend are the relative decondition and muscle weakness that occur during the off-season (Ahmad et al., 2013).