Reference Intervals and Prediction Equation for Sustained Maximal Inspiratory Pressure in Healthy Adults

Introduction Sustained maximal inspiratory pressure (SMIP), obtained using the test of incremental respiratory endurance, has been proposed as an index of inspiratory muscle performance reflecting both force generation and the ability to sustain inspiratory effort. However, reference intervals and prediction equations for SMIP in healthy adults have not been sufficiently established. This study aimed to establish sex- and age-specific reference intervals for SMIP, identify factors associated with SMIP, and develop a practical prediction equation. Methods This cross-sectional study included healthy adults aged 18 years or older. After exclusion of participants with respiratory or neuromuscular disease and those with missing data, 439 participants were included and stratified by sex and age (<35 and ≥35 years). Reference intervals were calculated for each partition using a nonparametric percentile method. Univariate linear regression analyses were performed to examine factors associated with SMIP, followed by multiple linear regression analyses to develop a prediction equation using age, sex, height, and weight. Results SMIP was highest in younger males and lowest in older females. The 95% reference intervals were 383.2-1209.9 pressure time units (PTU) in males aged <35 years, 141.0-1222.0 PTU in males aged ≥35 years, 109.5-667.8 PTU in females aged <35 years, and 97.3-617.1 PTU in females aged ≥35 years. In multivariable analysis, age, sex, and height were independent predictors of SMIP, whereas weight was not independently associated with SMIP after adjustment. The final model explained 56.9% of the variance in SMIP (adjusted R² = 0.565; SEE = 171.20 PTU). Conclusions This study established sex- and age-specific reference intervals and a simple and practical prediction equation for SMIP in healthy adults. These findings provide a basis for the quantitative interpretation of SMIP and may support its broader clinical application.