Self-rated physical fitness and measured cardiorespiratory fitness, muscular strength, and body composition.

The purpose was to examine the correlation and association between a single-item question of self-rated physical fitness and objective measures of fitness and cardiometabolic risk factors in a large population-based study. Participants were 3441 men and women aged 18-85 years who filled in a questionnaire and participated in a clinical health examination in the Danish Health Examination Survey 2007-2008. Cardiorespiratory fitness was estimated by an indirect maximal exercise test. Muscle strength was measured by (a) sit-to-stand test, (b) handgrip strength, and (c) bent arm strength. Body mass index and fat percentage were used as measures for body composition. Associations were derived from regression analyses, correlations were calculated using Spearman's correlation test, and agreement was tested by kappa statistics. Within categories of self-rated physical fitness moving from lowest to highest, objectively measured cardiorespiratory fitness increased. Self-rated physical fitness was strongly correlated to cardiorespiratory fitness (rmen  = 0.69 and rwomen  = 0.65) and moderately correlated to the sit-to-stand test (rmen  = 0.49 and rwomen  = 0.48), bent arm strength (r = 0.45) and fat percentage (r = 0.46) among men, and handgrip strength among women (r =0.41). Mutually adjusted analysis showed a significant association between self-reported physical fitness and cardiorespiratory fitness for men and women and bent arm strength for women. The single-item question of physical fitness was correlated to cardiorespiratory fitness, muscle strength, and body composition. However, this study suggests that it mainly captures cardiorespiratory fitness. At the population level, the single-item question could be a useful tool to identify and monitor variation in fitness levels.