Does a Greater Hamstring Muscle Thickness Mean a Greater Aponeurosis Thickness?

The aim of the present study was to assess the association between muscle thickness and the superficial and deep aponeurosis thickness of the hamstrings. Ultrasound images were captured from the semitendinosus (ST) and biceps femoris long head (BFlh) of fifty young individuals (28 males and 22 females). Measurements were taken at six sites along the thigh in prone position. Superficial and deep aponeurosis and muscle thickness were evaluated across the entire length of the ST and BFlh using a computational image segmentation approach, which generated approximately ~360 data points per participant. There was significant interindividual variability in superficial (coefficient of variation, CV: 19.37–31.40%) and deep aponeurosis thickness (CV: 21.22–31.82%). Correlation analysis revealed a limited association between muscle and superficial aponeurosis thickness, with no significant correlations (r = −0.21 to 0.22, p > 0.05) observed across regions. However, moderate positive correlations were found for deep aponeurosis thickness in the proximal region of the BFlh (r = 0.43, p = 0.002) and in the middle (r = 0.34, p = 0.014) and distal–middle region of the ST (r = 0.35, p = 0.022). No consistent relation between muscle and aponeurosis thickness was observed, suggesting that aponeurosis morphology varies independently of muscle size. This variability may impact the mechanical strain distribution, emphasizing the need for individualized assessments in hamstring injury prediction.