# Does a Greater Hamstring Muscle Thickness Mean a Greater Aponeurosis Thickness?
## 1 Introduction  
Hamstring strain injuries (HSIs) are among the most frequent injuries in running-based and high-speed sports [1, 2]. These injuries often lead to extended periods away from training and competition, diminishing athletic performance and causing lasting physical and structural changes [3, 4]. The biceps femoris long head (BFIh) is susceptible to these injuries, often occurring at or near the musculotendinous junction (MTI)) during high-intensity eccentric contractions [1, 2, 5]. These contractions are especially prominent during the terminal swing phase of running gait, where rapid muscle lengthening under load places extreme mechanical stress on the muscle-tendon unit [1, 2]. Aponeurosis morphology has a critical role in muscle function [6, 7]. The aponeurosis is a connective tissue structure responsible for transmitting force from the muscle fibers to the tendon and ultimately to the skeletal system [6, 7]. Its morphology (thickness, volume, and width) significantly influences the mechanical strain distribution within the muscle-tendon unit during contraction [6, 7]. Abnormalities in these structural properties can lead to localized strain concentrations, particularly near the MTJ, which has been linked with greater risk of muscle injury [6, 7]. Specifically, it has been hypothesized that disproportionate dimensions of the aponeurosis, such as a thinner or narrower aponeurosis, exacerbatestrain concentrations, predisposing the muscle to injury during activities involving high mechanical loads [8; 9]. Computational modeling and imaging-based assessments have indicated that strain maldistribution is most pronounced in regions adjacent to the MT) [10; 11; 12]. Thus, understanding aponeurosis geometry is pivotal for identifying individuals at greater risk of HSIs. The relationship between muscle and aponeurosis size is complex and characterized by significant variability [13; 14; 15; 16]. Research has shown that the size of the vastus lateralis aponeurosis is directly related to the overall volume of the quadriceps [17]. In contrast, the size of the BFlh aponeurosis did not consistently correlate with muscle size, suggesting that aponeurosis dimensions are not proportionally scaled to the muscle's anatomical cross-sectional area or volume [13; 15]. For instance, [PERSON] et al. [13] reported no relationship between aponeurosis area and knee flexor strength or muscle size, highlighting the high variability in aponeurosis-to-muscle area ratios. In contrast, [PERSON] et al. [14] reported moderate correlations (\(r=0.53\)) between the BFlh muscle volume and BFlh aponeurosis volume. Additionally, [PERSON] et al. [15] observed a significant positive moderate correlation between BFlh aponeurosis and muscle volume (\(r=0.64\)) in a previously injured limb, but no correlation in uninjured or healthy limbs. These findings emphasize the complex relationship between muscle and aponeurosis dimensions, which is influenced by individual anatomical differences and by structural adaptations that may occur in response to mechanical loading or injury. Investigations of aponeurosis dimensions have relied predominantly on magnetic resonance imaging (MRI) [13; 14; 15; 16]. While MRI remains the gold standard for aponeurosis morphological assessments, its high cost and limited accessibility have constrained its application. However, recently, it has been found that ultrasound (US) is as a reliable and cost-effective alternative for evaluating aponeurosis thickness with relatively low measurement error (standard error of measurement 0.88 to 2.72%) [18]. While previous research has provided insights into the role of aponeurosis morphology in HSIs, several critical gaps persist. Prior studies have primarily focused on male populations [13; 14; 15; 16], neglecting potential differences across genders. Furthermore, these investigations have been limited to assessing the BFlh, without considering other hamstring muscles such as the semitendinosus (ST), which may also play a pivotal role in injury risk and prevention.