Exploration on the influence of menstrual cycle phase on biceps brachii tendon mechanics

Tendons are essential for force transmission, yet the influence of menstrual cycle phase on upper-limb positional tendon mechanics is unknown. This exploratory study investigated biceps brachii tendon mechanics across the menstrual cycle. Eight females (23.9 ± 4.3 years) underwent ultrasonographic assessment during the menses, follicular, and luteal phases. Between 0 and 20% of maximal voluntary contraction (MVC), tendon elongation was greater (Menses 5.4 ± 2.6 mm; Follicular 4.8 ± 2.7 mm; Luteal 3.3 ± 2.0 mm; p < 0.001), and stiffness lower (Menses 38.4 ± 31.7 N mm^-1; Follicular 43.09 ± 35.1 N mm^-1; Luteal 55.0 ± 32.4 N mm^-1; p = 0.009) during menses compared to the luteal phase. Young's modulus was lower (p = 0.02) and strain was higher (p = 0.005) during menses compared with the luteal phase. Tendon stress across force levels was greater (~ 15%) during menses compared to both the follicular (p = 0.002) and luteal (p = 0.002) phases. There was no significant difference for tendon stiffness from 20 to 100% MVC or hysteresis observed between phases (p > 0.1). The greater tendon elongation during menses compared to the luteal phase arises from the changes in the toe-initial region, which governs the initial mechanical response of the tendon. This emerging research highlights the importance of the distinct regions of elongation, and significance of the menstrual cycle on tendon mechanics.