Associations between physical fitness components and metabolic syndrome in middle-aged adults: a cross-sectional study using relative strength indicators and ROC analysis

Metabolic syndrome (MetS) is a major risk factor for cardiovascular disease (CVD) and type 2 diabetes, particularly in middle-aged populations. Physical fitness, especially muscular strength and cardiorespiratory capacity, has emerged as a pragmatic indicator of metabolic health. In this cross-sectional study, we investigated the association between multiple fitness components and the presence of MetS in middle-aged adults, with emphasis on weight-normalized indices.

A total of 570 middle-aged adults (369 male, 201 female) were enrolled in this cross-sectional study. Assessments included body composition measured by bioelectrical impedance analysis (appendicular skeletal muscle mass, ASM), handgrip strength, isokinetic lower-limb strength at 60°/s using the HUMAC NORM system, and cardiorespiratory fitness (CRF) assessed by maximal treadmill testing (VO₂max, mL·kg^-1·min^-1, Bruce protocol). MetS was defined according to the International Diabetes Federation criteria. Within each sex, logistic regression models [scaled per 1 SD decrease in each exposure; Model 1 adjusted for age; Model 2 adjusted for age and body mass index (BMI)] were used to quantify associations. Receiver operating characteristic analyses with the Youden index were used to identify sex-specific cutoffs.

The participants with MetS showed significantly lower relative grip strength, relative lower-limb strength, and VO₂max than their non-MetS counterparts. A lower ASM ratio was also characteristic of MetS. In age-adjusted models, the decreased ASM ratio and lower relative grip strength were associated with higher odds of MetS, with attenuation after additional adjustment for BMI. ROC analysis yielded pragmatic thresholds for identifying MetS: ASM ratio < 24.0% in male individuals and < 20.0% in female individuals; relative grip strength < 53.3 in male individuals and < 38.0 kg/BW in female individuals; relative lower-limb strength < 5.94 in male individuals and < 5.03 Nm/BW in female individuals; and VO₂max < 24.0 in male individuals and < 19.3 mL·kg^-1·min^-1 in female individuals.

Weight-normalized fitness indices, particularly the ASM ratio and relative grip strength, are informative for identifying prevalent MetS in middle-aged adults and support sex-specific screening thresholds suitable for clinical and community practice.