The body's generation of heat is known as thermogenesis and it can be measured to determine the amount of energy expended.<unk>  generally decreases with age, and with the decrease in lean body mass (as may happen with aging). Increasing muscle mass has the effect of increasing<unk> . Aerobic (resistance) fitness level, a product of cardiovascular exercise, while previously thought to have effect on<unk> , has been shown in the 1990s not to correlate with<unk>  when adjusted for fat-free body mass. But anaerobic exercise does increase resting energy consumption (see "aerobic vs. anaerobic exercise"). Illness, previously consumed food and beverages, environmental temperature, and stress levels can affect one's overall energy expenditure as well as one's<unk> . Indirect calorimetry laboratory with canopy hood (dilution technique)<unk>  is measured under very restrictive circumstances when a person is awake. An accurate<unk>  measurement requires that the person's sympathetic nervous system not be stimulated, a condition which requires complete rest. A more common measurement, which uses less strict criteria, is resting<unk> <unk>  (RMR).<unk>  may be measured by gas analysis through either direct or indirect calorimetry, though a rough estimation can be acquired through an equation using age, sex, height, and weight. Studies of energy<unk> m using both methods provide convincing evidence for the validity of the respiratory quotient (RQ), which measures the inherent composition and utilization of carbohydrates, fats and proteins as they are converted to energy substrate units that can be used by the body as energy.
Basal metabolic rate
Basal body temperature
Heart rate variability
Basal rate
Electrodermal activity
Glycemic index
Cardiac reserve
Renal threshold