Ageing, Sex Differences, and REDs Risk in Endurance Runners: An Integrated Cross-Sectional Study Protocol

TL;DR

This cross-sectional study protocol will examine 112 participants stratified by sex, age (20–35 vs. 65–80 years), and training status (endurance runners vs. inactive controls) to assess cardiorespiratory fitness and REDs-related risk. It aims to clarify how ageing and sex influence endurance physiology, musculoskeletal and bone health, and REDs susceptibility, but reports no empirical results.

Key findings

• No results are reported; the paper is a protocol. The primary outcome is estimated VO2max, with secondary outcomes including body composition, musculoskeletal function, biochemical and hormonal markers, and REDs-related screening variables.
• The a priori sample size is n = 112 total, corresponding to 14 participants per each of eight sex × age × training-status groups, calculated for an omnibus factorial ANOVA (α = 0.05, power = 0.90, Cohen’s f ≈ 0.42).
• Assessments include incremental cycling VO2max, DXA bone/body composition, isometric knee dynamometry, Y-Balance testing, LEAF-Q/LEAM-Q/IOC REDs CAT2 screening, seven-day dietary/training monitoring, actigraphy, venous blood sampling, and vastus lateralis muscle biopsies.
• The working hypothesis is that master endurance runners may show equal or lower prevalence of “high-risk” REDs screening profiles than younger endurance runners, while still potentially exhibiting specific REDs-related indicators such as low fat-free mass or altered endocrine/bone markers.

Methods

• Cross-sectional protocol developed in accordance with SPIRIT guidelines; 112 participants will be enrolled across eight groups by sex, age (young adult 20–35 years; master/elderly 65–80 years), and athletic status (endurance runners vs. inactive controls). Endurance runners include elite-level Slovak athletes actively competing nationally/internationally, with performance benchmarks of ≤35 min (young) and ≤55 min (masters) in a 10 km run.
• Primary outcome: estimated VO2max from a continuous incremental cycling test using the ACSM metabolic equation; secondary/exploratory outcomes include DXA body composition and bone parameters, isometric knee dynamometry, mobility, REDs screening tools (LEAF-Q, LEAM-Q, IOC REDs CAT2), seven-day dietary and training monitoring, wrist actigraphy, blood biomarkers, and muscle biopsy analyses (immunohistochemistry, electron microscopy, Western blot, qPCR, glycogen).
• Statistical analysis: factorial ANOVA with sex, age category, and training status as fixed factors; post hoc comparisons with correction where significant; effect sizes reported as partial eta squared and Cohen’s d; exploratory outcomes interpreted as hypothesis-generating; significance set at p < 0.05.

Limitations

• Stated: the cross-sectional design and single one-week monitoring period limit the ability to capture long-term physiological and health changes; selection bias is acknowledged because master endurance runners represent a highly motivated, health-conscious “healthy survivor” subgroup, limiting generalisability to the broader ageing population.
• Stated: VO2max is estimated indirectly from maximal cycling power output, which may systematically underestimate aerobic capacity in endurance runners compared with treadmill running, and cycling efficiency may vary by age and sex; REDs screening tools rely on self-reported symptoms and may be vulnerable to recall bias.
• Stated: the large number of assessed outcomes increases the likelihood that some findings will remain primarily descriptive, and the design does not allow longitudinal monitoring of training load, dietary patterns, hormonal fluctuations, or menstrual-cycle variability. Evident: as a protocol, no outcome data or conclusions can yet be drawn.