Regional changes in shear modulus of the biceps femoris long head following load application to the biceps femoris short head.

Understanding the mechanical behavior of the biceps femoris long head (BFlh) may be insightful due to its high susceptibility to strain injuries, particularly during high-speed running in sports, such as soccer and track and field. While prior research has focused on intrinsic muscle properties, emerging evidence suggests that the biceps femoris short head (BFsh) may influence BFlh tension. Thus, we examined the effects of BFsh load application on the tensile strength and regional shear modulus of the BFlh. Seven legs from four cadaveric specimens (mean age: 83.2 ± 7.4 years) embalmed using the Thiel method were used. BFlh was secured to a mechanical testing device equipped with a load cell, whereas BFsh was connected to a custom-built mechanical apparatus. A tensile strain of 8 % was applied to the BFlh, whereas incremental loads (0, 150, 300, 450, 600, and 750 g) were gradually added to the BFsh. The tensile force and shear modulus in the three BFlh regions (proximal, central, and distal) were recorded using shear wave elastography. The results demonstrated that BFsh loading notably reduced BFlh tensile strength, with the lowest tension at 750 g (P < 0.01). The shear modulus decreased in the proximal and distal regions at loads > 450 g (P < 0.01), with no change in the central region. The distal region exhibited a greater decrease in shear modulus compared with the proximal and central regions (P < 0.01). These findings suggest that BFsh loading reduces BFlh tensile strength and alters its mechanical properties, particularly in the distal region.