Energy Expenditure in Physical and Mental Activity

Components of Total Energy Expenditure

Daily energy expenditure in the body consists of four basic components:

1. Basal metabolic rate (BMR) — the energy required to maintain minimum vital functions in a state of complete rest (in a quietly lying person, this is approximately 35 kcal/m²).
2. Thermic effect of food (specific dynamic action) — the energy cost of digesting and metabolizing nutrients.
3. Thermoregulation — maintaining a constant core body temperature.
4. Physical and mental activity.

The energy that a person expends throughout the day above the basal metabolic rate directly for work and activity is called the working increment. Adding the basal metabolic rate to the working increment gives the gross energy expenditure.

Impact of Muscular Work and Body Posture

Any muscular activity requires significant energy expenditure and is accompanied by increased heat production. There is a direct correlation: the more intense the activity, the higher the daily expenditure. Even a simple change in posture radically alters metabolic demands compared to rest:

• Raising the head and leaning against a hospital bed backrest: +15%.
• Sitting position: +42%.
• Standing position: +70% (due to postural muscle recruitment).
• Calm, leisurely walking: +180%.

Absolute values can vary within immense limits. While a person lying down expends about 1,265 kcal per day, active household cleaning requires 2,800 kcal, competitive rowing requires 4,080 kcal, and extremely heavy work (such as bricklaying) requires up to 5,850 kcal.

The choice of energy substrates for oxidation depends directly on the parameters of physical load. During short-term exertion, the body burns primarily carbohydrates. As exertion becomes prolonged, fats are utilized (providing up to 80% of energy requirements). Interestingly, in highly trained professional athletes, muscle contractions can be fueled exclusively via fat oxidation.

Occupational Groups and Physical Activity Ratio (PAR)

To objectively assess whether energy expenditure is adequate across different professions, international health organizations utilize the Physical Activity Ratio (PAR) or physical activity level (PAL). This is the ratio of total energy expenditure across all daily activities to the basal metabolic rate.

Note: PAR values are identical for males and females. However, absolute energy expenditure in groups with identical PARs is always lower in females due to a lower body mass and consequently a lower basal metabolic rate. Furthermore, an important hygienic standard dictates that the physical strain of regular work should not require energy expenditures exceeding the basal metabolic rate by more than 3-fold.

Depending on daily energy expenditure, occupations are divided into five groups:

1. Group I (PAR 1.4) — predominantly sedentary workers with very light activity. Includes researchers, educators, students, and office workers. Expends 1,800 to 2,450 kcal/day.
2. Group II (PAR 1.6) — light physical labor. Includes nurses, orderlies, retail sales associates, and public transit drivers. Expenditure: 2,100–2,800 kcal/day.
3. Group III (PAR 1.9) — moderate physical labor. Includes surgeons, machine operators, textile workers, and bus or excavator drivers. Expenditure: 2,500–3,300 kcal/day.
4. Group IV (PAR 2.2) — heavy physical labor with high activity. Metallurgists, construction workers, and most agricultural workers. Expenditure: 2,800–3,850 kcal/day.
5. Group V (PAR 2.5) — very heavy labor with extremely high activity. This group includes males only: miners, lumberjacks, manual ditchdiggers, and unmechanized loaders. Expenditure reaches 3,750–4,200 kcal/day.

Energy Expenditure in Mental and Emotional Activity

Contrary to popular myth, intensive brain activity by itself burns almost no extra calories. If a person performs pure mental work while remaining physically immobile, the increase in metabolism is only 2–3% compared to the resting state.

However, the picture changes when an emotional factor is introduced:

• Intense emotional activity (public speaking, taking an exam) increases energy expenditure by 11–20%.
• Pre-start state, meaning psychological anticipation of upcoming exertion, triggers a pronounced rise in metabolism. Such metabolic surges are typical for athletes before important competitions, scientists before a thesis defense, or pilots before a critical flight.