Joint range of motion (ROM) is an important parameter for athletic performance and muscular injury risk. Nonetheless, a complete description of muscular factors influencing ROM among individuals and between men and women is lacking. We examined whether passive muscle stiffness (evaluated by angle-specific muscle shear modulus), tolerance to muscle stretch (evaluated by muscle shear modulus at end-ROM), and muscle slack angle of the triceps surae are associated with the individual variability and sex difference in dorsiflexion ROM, using ultrasound shear wave elastography. For men, ROM was negatively correlated to passive muscle stiffness of the medial and lateral gastrocnemius in a tensioned state and positively to tolerance to muscle stretch in the medial gastrocnemius. For women, ROM was only positively correlated to tolerance to muscle stretch in all muscles but not correlated to passive muscle stiffness. Muscle slack angle was not correlated to ROM in men and women. Significant sex differences were observed only for dorsiflexion ROM and passive muscle stiffness in a tensioned state. These findings suggest that muscular factors associated with ROM are different between men and women. Furthermore, the sex difference in dorsiflexion ROM might be attributed partly to that in passive muscle stiffness of plantar flexors.

The passive straight-leg-raise (PSLR) and the sit-and-reach (SR) tests have been widely used to assess hamstring extensibility. However, it remains unclear to what extent hamstring stiffness (a measure of material properties) contributes to PSLR and SR test scores. Therefore, we aimed to clarify the relationship between hamstring stiffness and PSLR and SR scores using ultrasound shear wave elastography. Ninety-eight healthy subjects completed the study. Each subject completed PSLR testing, and classic and modified SR testing of the right leg. Muscle shear modulus of the biceps femoris, semitendinosus, and semimembranosus was quantified as an index of muscle stiffness. The relationships between shear modulus of each muscle and PSLR or SR scores were calculated using Pearson’s product-moment correlation coefficients. Shear modulus of the semitendinosus and semimembranosus showed negative correlations with the two PSLR and two SR scores (absolute r value≤0.484). Shear modulus of the biceps femoris was significantly correlated with the PSLR score determined by the examiner and the modified SR score (absolute r value≤0.308). The present findings suggest that PSLR and SR test scores are strongly influenced by factors other than hamstring stiffness and therefore might not accurately evaluate hamstring stiffness.

Purpose: The stiffness of muscle–tendon units and of tendons in the lower legs plays important roles in sprinting and long-distance running. However, the association of muscle stiffness with sprinting and running remains unknown. This study aimed to identify the characteristics of muscle stiffness in sprinters and long-distance runners, and to determine how muscle stiffness is related to the performance of these athletes. Methods: In 22 male sprinters (SPR group), 22 male long-distance runners (LDR group), and 19 healthy untrained control male subjects (CON group), the muscle shear wave speed (a proxy for stiffness) of the vastus lateralis (VL) was measured under passive (resting) and active (contracting the knee extensors at 50% of maximal voluntary contraction) conditions, by using ultrasound shear wave elastography. Results: The passive VL shear wave speed in SPR group was significantly lower than that in LDR group (P = 0.039). The active VL shear wave speed in LDR group was significantly higher than that in SPR (P = 0.022) and CON (P < 0.001) groups. In SPR group, the 100-m race time was negatively correlated to the passive VL shear wave speed (r = −0.483, P = 0.023) and positively correlated to the active VL shear wave speed (r = 0.522, P = 0.013). In the LDR group, the 5000-m race time was positively correlated to the passive VL shear wave speed (r = 0.438, P = 0.047) but not to the active VL shear wave speed. Conclusion: The muscles of sprinters and long-distance runners exhibit characteristic stiffness that can be beneficial to their athletic performance. Passive and active muscle stiffness may play different roles in human locomotion, depending on locomotion speeds.