The purpose of this study was to investigate differences in the acute effects of passive knee extension (PKE) and active knee extension (AKE) stretching on the shear elastic modulus of the hamstrings. In 20 healthy men, maximum knee extension (maximum range of motion [ROM]) and shear elastic modulus of the hamstrings (biceps femoris long head, semitendinosus, semimembranosus) were measured before (Pre) and after (Post) AKE and PKE stretching. The maximum ROM during stretching was measured. In both stretching methods, maximum ROM increased and the shear elastic modulus decreased (p<0.01), but no difference was found between the procedures. No significant difference was observed in the maximum ROM during stretching between the procedures (p=0.06). The shear elastic modulus was significantly lower in the biceps femoris long head and semimembranosus than in the semitendinosus muscle (p<0.05). Static stretching with PKE and AKE stretching showed an increase in maximum ROM and a decrease in hamstring elasticity, but no difference was found between the methods. Both stretching protocols were effective for reducing elasticity of the biceps femoris long head and semimembranosus.

Although force transfer during elongation occurs longitudinally and transversely, the influence of transverse force transfer between the biceps femoris long head and short head remains unclear. This study aimed to investigate whether separating the intermuscular connections between the biceps femoris long head and short head alters tension in the biceps femoris long head. Eight human cadaver legs were used, and ultrasonic shear wave elastography measurements were performed under four conditions: (1) intact, (2) removal of all tissues from the skin to the deep fascia, (3) intermuscular dissection, and (4) biceps femoris short head detachment. Measurements were taken in four limb positions, defined by hip and knee joint angles, under each tissue condition. The shear modulus of the biceps femoris long head significantly increased by 62.2% after intermuscular dissection compared to fascia removal, and further increased by 174.7% after biceps femoris short head detachment. In contrast, the shear modulus of the biceps femoris short head significantly decreased by 36.0% following intermuscular dissection and by 75.1% after detachment. In conclusion, reducing biceps femoris short head tension while increasing biceps femoris long head tension may influence muscle stress distribution, particularly during movement.

Dynamic stretching (DS) uses a controlled movement through the range of motion (ROM) of the active joint(s) by contracting the agonist muscles without being held in an end position. In contrast, ballistic stretching (BS) typically uses uncontrolled movements of a higher velocity with bouncing actions. However, BS is often considered to be a form of DS. When considered together, DS and BS reportedly increase flexibility, evidenced by single- and multiple-joint ROM improvements and other measurements. However, a meta-analysis with subgroup analyses revealing the acute effects of DS and BS on flexibility in detail, independently of other stretching methods, has yet to be conducted. The purpose of this meta-analysis was to investigate the acute effects of DS and BS on flexibility in healthy participants. The PubMed, Web of Science, and Scopus databases were searched for eligible papers published before September 9, 2024; 17 papers were included in the meta-analysis. The main meta-analysis was performed with a random-effect model, and subgroup analyses were performed to examine the effects of age (young vs. middle-aged and older), sex (male vs. mixed sex), stretching methods (DS vs. BS), stretched muscles (hamstrings vs. plantar flexors vs. multiple muscles), and flexibility outcomes (single-joint ROM vs. straight-leg raise test vs. sit-and-reach test). A small increase in flexibility was found following DS and BS (considered together) (effect size = 0.372, Z = 3.936, 95% confidence interval = 0.187–0.557, p < 0.001, I2 = 27%). Subgroup analyses revealed no significant differences between age (p = 0.24), sex (p = 0.76), stretching method (p = 0.83), stretched muscle (p = 0.20), or flexibility outcome (p = 0.34) groups. Our results suggest that DS and BS effectively provide acute, small-magnitude improvements in flexibility that are not significantly affected by individual characteristics, stretching methods, stretched muscles, or flexibility outcomes.