Energy Expenditure during Physical and Mental Activity

What makes up total energy expenditure

The daily energy expenditure of the body is formed from four basic components:

1. Basal metabolic rate (BMR) — the energy required to maintain minimal vital activity in a state of complete rest (in a calmly lying person, this indicator is approximately 35 kcal/m²).
2. Specific dynamic action of food — the energy expended on the digestion and assimilation of nutrients.
3. Thermoregulation — maintaining a constant body temperature.
4. Physical and mental activity.

The energy that a person spends during the day in excess of the basal metabolism directly for work and any activity is called the working increment. If we add the basal metabolism and the working increment, we get the value of gross energy metabolism.

The impact of muscular work and body position

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 a state of rest:

• Raising and leaning on the back of the bed: +15%.
• Sitting position: +42%.
• Standing position: +70% (postural muscles are engaged).
• Calm, unhurried walking: +180%.

Absolute values can vary within huge ranges. While a person lying down spends about 1,265 kcal per day, active house cleaning requires 2,800 kcal, competitive rowing 4,080 kcal, and extremely heavy labor (such as bricklaying) up to 5,850 kcal.

The choice of energy substrates for oxidation depends directly on the load parameters. During short-term work, the body burns primarily carbohydrates. If the workload becomes prolonged, fats come into play (they provide up to 80% of the energy requirement). Interestingly, in well-trained professional athletes, muscle contractions can be fueled exclusively by the oxidation of fats.

Labor groups and physical activity level (PAL)

To objectively assess how adequate energy expenditure is among workers in various professions, the World Health Organization introduced the physical activity level (PAL). This is the ratio of total energy expenditure for all types of life activities to the basal metabolic rate.

Note: PAL values are the same for men and women. However, the absolute energy expenditure in groups with identical PAL is always lower in women due to a smaller body mass and, consequently, a lower basal metabolic rate. There is also an important hygienic standard: the severity of routinely performed work should not require energy expenditure exceeding the basal metabolic rate by more than 3 times.

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

1. Group I (PAL 1.4) — predominantly mental workers with very light activity. This includes researchers, teachers, students, and computer operators. They expend 1,800 to 2,450 kcal/day.
2. Group II (PAL 1.6) — light physical labor. This includes nurses, orderlies, non-food retail clerks, and urban transport drivers. Expenditure: 2,100–2,800 kcal/day.
3. Group III (PAL 1.9) — moderate physical labor. Includes surgeons, machine operators, textile workers, bus and excavator drivers. Expenditure: 2,500–3,300 kcal/day.
4. Group IV (PAL 2.2) — heavy physical labor with high activity. Metallurgists, construction workers, and the bulk of agricultural workers. Expenditure: 2,800–3,850 kcal/day.
5. Group V (PAL 2.5) — exceptionally heavy physical labor (very high activity). This group includes only men: miners, lumberjacks, navvies, and loaders engaged in unmechanized labor. Expenditure reaches 3,750–4,200 kcal/day.

Energy expenditure during mental and emotional activity

Contrary to popular myth, intensive brain work in itself hardly burns any calories. If a person performs purely mental activity while remaining physically stationary, the increase in metabolism is only 2–3% compared to the resting state.

However, the picture changes when the emotional factor is added:

• Intense emotional activity (a public speech, a student taking an exam) increases energy expenditure by 11–20%.
• Pre-start state, i.e., the psychological setup for upcoming exertion, causes a pronounced increase in metabolism. Such spikes are characteristic of athletes before an important competition, scientists before defending a dissertation, or pilots before a critical flight.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Definition of basal metabolic rate and influencing factors
• Specific dynamic action of food
• Mechanisms of body thermoregulation
• Biochemical pathways of carbohydrate and fat oxidation in muscles

Other topics in the "Metabolism and Energy" section

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