Threshold effect of exercise frequency on comprehensive physical fitness with sex-specific mediation by body mass index in Chinese university students

Background University students represent a critical population for national health, yet the dose-response relationship between objectively measured exercise frequency and comprehensive physical fitness, along with the mediating role of body composition, remains insufficiently characterized in this demographic. Objective This study aimed to systematically investigate the associations between exercise frequency and multi-dimensional physical fitness indicators and to examine the sex-specific mediating role of body mass index (BMI) in a large cohort of Chinese undergraduates. Methods and results A cross-sectional analysis was conducted among 15,100 university students (7,653 males) using exercise frequency data automatically recorded by the Chuanggao-Sports mobile application. Participants were categorized into low (≤ 2 sessions/week), moderate (3–4 sessions/week), and high (≥5 sessions/week) frequency groups. Analysis of variance revealed a pronounced threshold-like associational pattern, wherein transitioning from low to moderate frequency yielded significant improvements in the overall fitness score and most specific indicators, while no statistically detectable additional fitness gains were observed in this sample when moving from moderate-to-high exercise frequency. To further objectively identify this threshold, we performed a restricted cubic spline (RCS) analysis. The RCS model demonstrated a significant non-linear relationship between weekly exercise frequency and predicted physical fitness score ( P for non-linearity < 0.0001). The bootstrap resampling (1,000 iterations) identified a threshold (inflection point) at 2.01 sessions/week (95% CI: 2.00 to 4.00), with a statistically significant change in slope. Sex-stratified path analysis further demonstrated distinct associational pathways: In males, exercise frequency showed both a significant positive association with physical fitness and an indirect associational pattern linked to BMI. In females, exercise frequency maintained a significant direct positive association with physical fitness, but the indirect association pattern through BMI was not supported. All reported relationships represent statistical associational patterns; reverse causality is plausible given the cross-sectional study design. Note that sex-specific associational-mediation results derived from path-analysis should be treated as purely exploratory given sub-optimal model-data fit. Discussion This study provides empirical evidence for a threshold-like associational pattern of exercise frequency on physical fitness and exploratory observations of sex-specific associational-mediation patterns based on BMI, a crude proxy of body composition.The findings underscore that pragmatic campus interventions should prioritize shifting inactive students to a moderate activity level, with sex-tailored strategies focusing on BMI management for males while exploring non-BMI pathways for females.