This study examines how different phases of the menstrual cycle are associated with selected performance and musculoskeletal parameters in physically active women aged 18-35 years.

Participants will track their menstrual cycle using a mobile app for approximately 2 months and attend 3 laboratory visits scheduled to specific cycle phases.

At each visit, participants will complete non-invasive assessments including lower-limb strength/performance testing, musculoskeletal measurements (muscle and tendon properties), body composition assessment, and symptom-related questionnaires.

The goal is to improve understanding of physiological variability across the menstrual cycle and support evidence-based planning of training, testing, and clinical practice in women.

Background and rationale:

Female physiology changes across menstrual cycle phases, which may influence neuromuscular performance, perceived symptoms, and selected tissue/mechanical properties. Current evidence is heterogeneous, partly due to methodological differences in cycle-phase identification and measurement protocols. This study aims to provide standardized data from repeated within-subject assessments across the menstrual cycle.

Objectives:

Primary objective is to evaluate associations between menstrual cycle phase and selected performance-related outcomes in physically active women.

Secondary objectives are to explore phase-related differences in musculoskeletal parameters, body composition outputs, and self-reported symptoms.

Study design:

Prospective, repeated-measures observational study. Each participant is measured repeatedly across predefined menstrual cycle phases.

Participants:

Women from the general population, 18-35 years, with occasional to recreational physical activity level.

Procedures:

Screening and eligibility confirmation according to protocol criteria.

Menstrual cycle tracking via mobile application for approximately 2 months.

Three laboratory visits timed to predefined cycle phases.

At each visit, non-invasive assessments are performed, including:

lower-limb strength/performance testing,

assessment of muscle and tendon mechanical properties,

body composition measurement,

recording of subjectively perceived menstrual-related symptoms.

Outcomes:

Main outcomes include phase-specific differences/associations in selected performance and musculoskeletal variables; additional outcomes include symptom profiles and selected body-composition variables.

Duration and setting:

Individual participation lasts approximately 2 months including cycle tracking and 3 lab visits (approximately 60-90 minutes per visit). Study procedures are conducted at FTVS UK (Prague).

Ethics and data protection:

Participation is voluntary, and participants may withdraw at any time without giving a reason. Data are processed in compliance with applicable data-protection regulations (including GDPR), pseudonymized for analysis, and reported in aggregate/anonymized form. The study is conducted according to institutional ethics approval and the Declaration of Helsinki.

Inclusion Criteria:

• Female, age 18-35 years

Premenopausal

Occasional to recreational level of physical activity

Willing and able to track menstrual cycle using a mobile application for approximately 2 months

Willing and able to attend 3 laboratory visits scheduled to predefined menstrual cycle phases

Able to provide written informed consent

Exclusion Criteria:

• Irregular menstrual cycle (outside protocol-defined regularity)

Current pregnancy

Use of hormonal medication for therapeutic purposes (per protocol)

Current acute illness at time of testing

Musculoskeletal injury of the lower limb limiting safe performance testing

Contraindication to body composition assessment methods used in the study (e.g., BIA/DXA, per site protocol)

Any other condition that, in the investigator's judgment, may interfere with safe participation or data quality

Premenopausal women aged 18-35 years from the general population with occasional to recreational physical activity, recruited for repeated within-subject assessments across predefined menstrual cycle phases.