Spontaneous physical activity in a respiratory chamber is correlated to habitual physical activity

During a stay in a respiratory chamber without an exercise protocol, physical activity is limited to activities of daily living, change of posture and 'fidgeting,' collectively referred to as spontaneous physical activity (SPA). SPA is quite variable among individuals and is a heritable trait. A low SPA during a chamber stay is a predictor of weight gain in men. However, it remains to be established whether physical activity in a respiratory chamber relates to physical activity under habitual, free-living conditions. The purpose of the present study was to determine whether physical activity in a chamber is correlated to habitual, free-living physical activity. Fifty healthy, non-diabetic Pima Indians (30 M/20 F, 30+/-6 y; 37+/-10% body fat; means+/-s.d.) completed a 24 h stay in the respiratory chamber followed by a 7 day measurement of habitual, free-living energy expenditure by doubly labeled water. Free-living physical activity was expressed as activity energy expenditure (AEE(FL); daily energy expenditure-(sleeping metabolic rate+thermic effect of food)), physical activity level (PAL(FL); daily energy expenditure/sleeping metabolic rate) and body-size independent activity units. Activity during the chamber stay was expressed as PAL(Ch), AEE(Ch), and based on radar sensor measurements, as percentage of time with activity (SPA(Radar)). AEE(FL) (averaging 930+/-310 kcal/day (3.89+/-1.30 MJ/day)) was correlated to AEE(CH) (averaging 440+/-160 kcal/day (1.84+/-0.67 MJ/day)) and higher in men than in women (r=0.53, P=0.003) and r=0.53, P=0.02, respectively). Likewise, PAL(FL) (averaging 1.75+/-0.21) was correlated to PAL(Ch) (averaging 1.42+/-0.10) and higher in men than in women (r=0.49, P=0.006 and r=0.42, P=0.02, respectively). Free-living activity expressed in body-size independent activity units (averaging 17.8+/-7.0) was correlated to SPA(Radar) (averaging 6.4+/-1.7) with no effect of sex (r=0.30, P=0.03). Physical activity in a respiratory chamber was correlated to habitual physical activity, whether expressed as AEE, PAL or body-size independent activity units, providing a plausible explanation for the demonstrated association between a low SPA in the chamber and weight gain. The study encourages further studies of the genetic and non-genetic determinants of SPA and non-exercise activity thermogenesis (NEAT).