Predicting low muscle strength from routine fitness-monitoring data in hypertensive older adults living at high altitude: development and internal validation of interpretable models.

Introduction: Low muscle strength based on grip-strength thresholds is a readily measurable marker of impaired muscle function, yet it is under-recognized among hypertensive older adults at high altitude. We developed a concurrent clinical prediction model using routine fitness-monitoring data and tested whether the full feature set added information beyond sex, age, and site/altitude. Methods: This cross-sectional secondary analysis included 433 hypertensive adults aged 60-69 years from three sites in the National Physical Fitness Monitoring program in the Tibet Autonomous Region. Low muscle strength was defined using Asian Working Group for Sarcopenia 2019 grip-strength thresholds (men <28 kg; women <18 kg). Fifteen predictors were prespecified for routine availability and plausible relevance to body composition, cardiometabolic status, respiratory or neuromotor function, physical activity, and site/altitude. Elastic-net logistic regression, random forest, and histogram gradient boosting were evaluated by repeated nested cross-validation. Results: Low muscle strength was present in 171/433 participants (39.5%). Random forest had the highest AUC point estimate (0.788, 95% CI 0.742-0.831), but its interval overlapped with elastic-net logistic regression (0.781, 0.732-0.825) and histogram gradient boosting (0.774, 0.729-0.819). Elastic-net was designated as the candidate model because calibration was closest to ideal (slope 0.99, 95% CI 0.78-1.24; intercept -0.00, -0.22 to 0.24) and it provides a transportable equation. Its PR-AUC was 0.725 versus a no-skill baseline of 0.395. Incremental discrimination over sex plus age plus site/altitude was small (ΔAUC 0.016, 95% CI - 0.009 to 0.041). AUC was 0.705 without sex, 0.653 under a sex-specific tertile outcome, and 0.626-0.759 when each site was held out. Conclusion: Routine fitness-monitoring variables contain information about concurrent low muscle strength, but much of the apparent performance is explained by the sex-specific outcome definition and site/altitude structure. Direct grip-strength measurement remains preferable whenever feasible. The elastic-net model should be regarded only as a candidate for opportunistic triage when a valid grip measurement is unavailable or unusable; independent external validation, recalibration, and prospective impact evaluation are required before implementation.