It is well known that the incidence of biceps femoris injury, and that of the long head in particular, is much greater than that of the other hamstring muscles (Gibbs et al. 2004; Cohen et al. 2011). The long head of the biceps femoris has a unique architecture consisting of a short fascicle, muscle tendon junction, and aponeurosis (Woodley and Mercer 2005). Longer fascicles enable greater muscle lengthening and reduce the risk of over-lengthening during eccentric contractions (Brockett et al. 2004; Butterfield 2010). However, the long head of the biceps femoris has short fascicles.

Magnetic resonance imaging has shown that muscle strain often occurs at the muscle tendon junction (De Smet and Best 2000). The biceps femoris muscle has a larger muscle tendon junction (Woodley and Mercer 2005). A three-dimensional model showed that the area near the proximal muscle tendon junction was the most stretched during muscle lengthening (Rehorn and Blemker 2010). In this chapter we report that the proximal aponeurosis of the biceps femoris is narrower than the distal aponeurosis. These unique anatomical features of the long head of the biceps femoris most likely contribute to the high incidence of strain injury.

Hamstring muscle strength is thought to be a risk factor for strain (Agre 1985). Many investigators have investigated the correlation between muscle strength and muscle strain. To date, they have suggested possible causes of hamstring strain that include imbalance of the bilateral leg (Croisier et al. 2008) as well as a lower strength compared to that of the quadriceps (H:Q ratio) (Yeung et al. 2009).

Some prospective studies have shown that in the presence of muscle imbalances between the right and left legs, hamstring strain occurred in the weaker leg. These findings suggested that an 8–15 % difference in hamstring strength between the two sides was a cause of increased hamstring injury rates in a number of sports (Orchard et al. 1997; Croisier et al. 2008). One causal factor could be that hamstring strength asymmetry alters running biomechanics and thus affects hamstring loading during the terminal swing phase (Opar et al. 2012).

Investigators commonly state that a hamstrings:quadriceps (H:Q) ratio ≥0.6 reduces the hamstring strain rate (Yeung et al. 2009). H:Q ratio has been often measured in concentric contraction. However, conventional H:Q ratios do not reflect the role of the hamstring during the terminal swing phase of running, the point at which hamstring strain occurs frequently (Verrall et al. 2001). As such, some investigators have recently measured the functional H:Q ratio (Fousekis et al. 2011; Sugiura et al. 2008). The functional H:Q ratio compares the function of the eccentric hamstrings to the concentric quadriceps. A study of professional soccer players found that players with a strength imbalance (functional H:Q ratio ≤0.89) had a significantly higher hamstring strain rate than players without strength imbalances (Croisier et al. 2008). However, the correlation between the isokinetic H:Q ratio and hamstring strain is weak if the measured speed and contraction type are not considered. Orchard et al. (1997) report that although H:Q ratios measured at 180 and 300°/s did not differ between the injured and non-injured groups, that measured at 60°/s was significantly different between groups . Thus, both contraction type and measuring speed should be considered in future studies.

Some studies have reported that hamstring strain occurs at a higher rate in the latter half of matches (Woods et al. 2004; Brooks et al. 2006). Due to such reports, fatigue has been proposed as a risk factor for hamstring strain. Repeated over-lengthening of the muscles causes microscopic damage which may lead to muscle strain (Morgan 1990). Another study showed that in muscles fatigued by repeated eccentric contraction, the capacity of absorbing energy to resist overstretching was decreased compared with that of non-fatigued muscles (Mair et al. 1996). These studies used animal muscles.

In humans, fatigued hamstring muscles lead to an increased magnitude of knee extension angle during the terminal swing phase of running (Pinniger et al. 2000). An increasing knee extension angle may lead to overstretching during the running phase.

Problems involving muscle fatigue may emanate from other organ systems such as the nervous control network (Opar et al. 2012). Fatigued athletes display reduced motor control, which may lead to mistimed contractions of the biceps femoris (Agre 1985). Fatigue also decreases an athlete’s ability to concentrate, which increases their risk of injury in many regards (Devlin 2000). However, further studies examining the correlation between fatigue and hamstring strain are needed.

Poor hamstring flexibility may increase the risk of strain injury due to a reduced capacity of the muscle–tendon unit to absorb lengthening forces (Clark 2008). Some studies have reported that poor hamstring flexibility is related to muscle injury (Jonhagen et al. 1994; Witvrouw et al. 2003), while other studies found no correlation between poor hamstring flexibility and strain risk (Arnason et al. 2004; Gabbe et al. 2006a). Accordingly, it is unclear whether flexibility is a risk factor of hamstring strain. Unfortunately, earlier reports used different methods to assess flexibility, there is as of yet no “gold-standard” method for precisely measuring either hamstring flexibility or length (Foreman et al. 2006). Tests such as the sit-and-reach or the passive/active knee extension are influenced by hip or pelvic motion, so they cannot precisely measure hamstring flexibility. Thus, a more quantitative, or at a minimum, standardized method of determining flexibility is required if its potential role as a risk factor is to be determined.

Increasing age is considered a risk factor for hamstring strain by many investigators (Woods et al. 2004; Gabbe et al. 2006a). Unlike other risk factors, many studies have demonstrated that increasing age is related to hamstring strain prevalence. In fact, using multivariate analysis, a number of studies have clearly indicated that increasing age is a definite risk factor (Verrall et al. 2001; Orchard 2001).

Freckleton et al. (2013) performed a meta-analysis to evaluate the correlation between increasing age and hamstring strain. They reported that increasing age increases the risk of hamstring injury (odds ratio, 2.46; 95 % confidence interval, 0.98–6.14; p = 0.06)***Assuming a relatively linear time course, an rough estimate of the rate of increase in hamstring strain probability over time, say over a 10 or 20 year increase in age, would be of interest to most readers. Ie: “Their data indicate that a 10 year increase in age increases the likelihood of hamstring strain by a factor of x.x.”***

The reason for the increase in injury rate with increasing age is unclear. One study suggested that the lumbar nerve roots of L5/S1 tend to be more affected by age-related spinal degeneration than other lumbar nerve roots (Orchard et al. 2004). The lumbar nerve roots of L5/S1 control the hamstring muscles, so nerve degeneration caused by aging is thought to be a reason for the increasing hamstring injury rate. However, there is no scientific evidence of this suggestion. As such, further studies investigating why older age increases the occurrence rate of hamstring strain are required to elucidate the correlation between age and hamstring strain rate.

A number of studies have suggested that a previous muscle strain increases the muscle strain recurrence rate (Orchard 2001; Croisier et al. 2008). In fact, meta-analyses indicate that athletes with a previous hamstring injury are 4.06 times more likely to experience repeat injury (Freckleton and Pizzari 2013). Previous injuries induce a change in knee angle produced peak torque (Brockett et al. 2004), and decreased tendon tissue compliance (Silder et al. 2010). Neuromuscular control is also altered by previous injury. Opar et al. (2013) suggest that the affected limb of athletes with previous unilateral hamstring strain has a lower surface electromyogram level than that of the contralateral limb. From this result, they state that previous muscle strain alters neuromuscular function. Thus, adequate rehabilitation is needed to prevent future injury.