The aim of this study was to identify the main components that determine firefighters' level of physical fitness using a stair-climbing test. The age, weight, height, body fat, and VO(2max) of the firefighters were recorded before the trial, and percentage of heart rate reserve (%HRR) was recorded during the stair climbing. Nonlinear modeling of HRR time series and Principal Component Analysis (PCA) was applied to the data to isolate a small number of variables that quantify overall individual physical fitness. The HRR was represented as a function of time using the sum of linear and trigonometric functions. Four main factors that influence performance, obtained from PCA analysis, emerged (78.2% of total explained variance): the capacity to carry the extra load (22.8% of total variance); the effect of body fat (19.6% of total variance); the influence of age in the task (19.3% of total variance); and the overall fitness level (16.4% of total variance). This approach allowed us to make a rapid assessment of each subject's fitness level. Such an assessment could be used in planning individualized functional training programs to improve each firefighter's job performance and reduce injuries and hence save time, energy, and financial resources.