Menstrual cycle does not change sagittal plane segments coordination variability during deadlift, a nonlinear dynamical analysis approach.

The menstrual cycle is a key biological rhythm in women that influences multiple physiological systems, including neuromuscular control. However, its impact on inter-segmental coordination during strength-based tasks remains unclear. This study investigated the effect of menstrual cycle phase on sagittal-plane coordination variability during deadlifts under different loading conditions in healthy, recreationally active women. Ten eumenorrheic females (20–27 years) performed conventional deadlifts during three self-reported menstrual phases: early follicular (days 1–3), late follicular (days 11–13), and luteal (days 21–23). Kinematic data were captured with a 3D inertial motion system (200 Hz), and inter-segmental coordination between trunk, pelvis, thigh, shank, and foot was assessed using modified vector coding. Coordination patterns (in-phase and anti-phase, proximal/distal dominance) and coordination variability (CV) were quantified across repetitions. Segmental range of motion (ROM) was also calculated. Segmental ROM tended to increase with external loading (50% bodyweight vs. bodyweight) and was slightly higher in the luteal phase compared with the follicular phases, but no statistically significant differences were found across phases or loads (p > 0.05). Similarly, no significant differences were detected in coordination pattern frequencies or CV waveforms across phases or loading conditions (p > 0.05). Contrary to the hypothesis, menstrual cycle phase did not significantly affect sagittal-plane coordination or its variability during deadlifts. These findings suggest that the deadlift, as a bilateral and strength-dominant movement, is biomechanically stable across the menstrual cycle. Coaches and clinicians may not need to adjust deadlift training or rehabilitation protocols based on menstrual timing. However, further studies with larger cohorts, direct hormonal assays, and multi-planar analyses are warranted to clarify potential cycle-related effects in more dynamic or unstable tasks.