Sex Differences and Patterns of Muscle Stiffness in the Knee Flexor and Extensor Musculature Through Analysis of Isolated Bellies.

ABSTRACT: Martín-San Agustín, R, Benítez-Martínez, JC, Medina-Mirapeix, F, and Casaña-Granell, J. Sex differences and patterns of muscle stiffness in the knee flexor and extensor musculature through analysis of isolated bellies. J Strength Cond Res 35(4): 1044-1049, 2021-Muscle stiffness (MS) is one of the key factors in joint control. The purpose of this study was to determine sex differences in the MS of 5 isolated muscle bellies (biceps femoris [BF], semitendinosus [ST], rectus femoris [RF], vastus medialis [VM], and vastus lateralis [VL]) and in the pattern of differences among their respective MS. Twenty female and 20 male recreational athletes participated. Muscle stiffness was measured by tensiomyography using maximum radial deformation (Dm) as an indirect indicator of MS. Sex differences were observed only in the Dm of RF (mean difference = 2.07 mm, p < 0.05) when values were adjusted by body mass and stature. Males and females showed a similar pattern in the Dm between the muscle bellies: within the hamstrings, ST had a significantly higher Dm than BF in females (3.02 mm) and males (4.28 mm); within the quadriceps, RF also had a significantly higher value than VL and VM in females (6.50 and 7.38 mm, respectively) and males (4.87 and 4.82 mm, respectively). Sex differences in patterns were found between BF and the vastus muscles: the BF of females had a significantly higher Dm than VL (3.78 mm) and VM (4.51 mm), but this was not observed in males. Differences may imply different involvement of the bellies in countering the movements of the lower extremities. Our results can help to direct exercises to improve the MS in certain muscular bellies.