Hamstring strain injury (HSI) remains the most common muscle injury in high-intensity running in humans. The majority of acute HSI occur specifically within the proximal region of the long head of biceps femoris and there is a sustained interest among researchers in understanding the factors that predispose to HSI. The present critical review describes the current understanding of biceps femoris long head (BFlh) structural features that might influence strain injury risk. Inter-individual differences in muscle-tendon architecture and interactions, muscle fiber type and region-specific innervation are likely to influence biceps femoris long head injury risk and might inform why some individuals are at an increased risk of sustaining a HSI during running. However, more research is needed, with future studies focusing on prospective data acquisition, improved computer simulations and direct imaging techniques to better understand the relationship between structural features, hamstring muscle function, and injury risk. Key Points Fiber/fascicle lengthening might be greater in the long head of biceps femoris (BFlh) during running, thereby increasing injury risk. However, architectural gearing allows fibers/fascicles to rotate as they shorten/lengthen during contractions, allowing for a greater length change in the muscle relative to the fibers/fascicles, which should provide significant protection against strain injury. The intramuscular architectural structure of BFlh is non-uniform, with BFlh pennation being greater in the proximal-middle sections compared to the distal and extreme proximal sections. These variations likely result in variations in both stress and strain across the muscle, which increases injury risk. Depending on the task, different hamstring muscle activation patterns may occur that could lead to shear forces being generated between compartments within BFlh, which could increase injury risk.
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