The effect of menstrual cycle timing on female elbow flexion force steadiness

This study examined female elbow flexion force steadiness (n = 12) across menses, late follicular, and luteal phases of the menstrual cycle. To control for repeated testing effects unrelated to menstrual phase a male comparison group (n = 12) completed the same protocol over three equally spaced sessions as females. Maximal voluntary contractions (MVC) and force steadiness were assessed in neutral and pronated positions. Elbow flexion force tasks were performed at 2.5%, 5%, 10%, 25%, 50%, 75% MVC, and steadiness quantified as the coefficient of variation of force (CV force). Males were stronger than females (p < 0.001), and MVC did not differ between the self-reported menstrual cycle phases (p > 0.14) or between days in males (p > 0.56). CV of force did not differ across sessions for males (p < 0.36). However, in females, CV of force was greater during the luteal phase compared to menses (Neutral p = 0.02; Pronated p = 0.001); late follicular phase did not differ (Neutral p = 0.71; Pronated p = 0.10). During the luteal phase, females showed greater CV of force than males in the pronated position (p = 0.05). This study identifies reduced upper-limb force steadiness during the luteal phase (24.5 $\pm$ 5.6 menstrual cycle days). This underscores the importance of accounting for menstrual cycle phase when conducting sex-related comparisons.