7.8 Work, Energy, and Power in Humans

Energy Conversion and Usage in the Human Body

The human body is an energy conversion system. It takes chemical energy stored in food and transforms it into forms that power everything from breathing to sprinting. Understanding how this conversion works, and how to calculate the energy involved, connects the physics of work and power directly to human physiology.

Energy Conversion in the Human Body

Your body breaks down food through cellular respiration, converting glucose into ATP (adenosine triphosphate), the molecule your cells actually use as fuel.

• Aerobic respiration occurs when oxygen is available and yields 36–38 ATP molecules per glucose molecule. This is the primary energy pathway during rest and moderate activity.
• Anaerobic respiration occurs without sufficient oxygen and yields only 2 ATP molecules per glucose molecule. It produces lactic acid as a byproduct, which is why intense exercise causes that burning feeling in your muscles.

That ATP then gets spent on a range of bodily functions:

• Basal metabolic rate (BMR): The minimum energy your body needs just to stay alive at rest. This covers breathing, blood circulation, cell repair, and brain function.
• Physical activity: Muscle fibers consume ATP to generate force and motion through a process called myosin-actin cross-bridge cycling. The more intense the activity, the faster ATP is used.
• Thermoregulation: Your body spends energy maintaining a stable internal temperature, whether through shivering (generating heat) or sweating (dissipating heat).
• Digestion and absorption: Processing food itself requires energy for processes like peristalsis (moving food through the gut) and active transport of nutrients across cell membranes.

Calculation of Energy Output

Energy output can be measured in calories (cal) or joules (J). The conversion is:

$1 \text{ calorie} = 4.184 \text{ joules}$

Note that the "Calories" on food labels are actually kilocalories (kcal), where $1 \text{ kcal} = 1000 \text{ cal} = 4184 \text{ J}$.

Three key equations tie work, energy, and power together for the human body:

Work is the product of force and displacement in the direction of the force:

$W = F \times d$

Power is the rate at which work is done:

$P = \frac{W}{t}$

Efficiency is the ratio of useful work output to total energy input:

$\text{Efficiency} = \frac{W_{\text{out}}}{E_{\text{in}}} \times 100\%$

Here's how to apply these in practice:

1. Lifting a 10 kg object 1 meter upward:

   • The force needed equals the object's weight: $F = mg = 10 \text{ kg} \times 9.81 \text{ m/s}^2 = 98.1 \text{ N}$
   • Work done: $W = 98.1 \text{ N} \times 1 \text{ m} = 98.1 \text{ J}$

2. Running at a power output of 200 W for 30 minutes:

   • Convert time to seconds: $30 \text{ min} \times 60 = 1800 \text{ s}$
   • Energy expended: $E = P \times t = 200 \text{ W} \times 1800 \text{ s} = 360{,}000 \text{ J} = 86 \text{ kcal}$

3. Calculating body efficiency:

   • If a person consumes 100 kcal of food energy and performs 25 kcal of useful mechanical work:
   • $\text{Efficiency} = \frac{25 \text{ kcal}}{100 \text{ kcal}} \times 100\% = 25\%$
   • The remaining 75% is released as heat. The human body typically operates at about 20–25% efficiency for physical tasks, which is why you warm up during exercise.

Energy Forms and Conservation

The human body involves constant conversion between energy forms, and the conservation of energy principle applies throughout.

• Kinetic energy is the energy of motion. When you run or throw a ball, your body converts chemical energy (ATP) into kinetic energy.
• Potential energy is stored energy due to position or configuration. A raised arm has gravitational potential energy; a stretched tendon stores elastic potential energy.

Energy can change forms but is never created or destroyed. When you eat food, chemical energy enters the system. That energy either does useful mechanical work, gets stored (as fat or glycogen), or leaves the body as heat. This is the foundation of energy balance: if energy input exceeds output, the excess is stored, and if output exceeds input, stored energy is used up.

Factors Affecting Energy Consumption

Factors Affecting Metabolic Rate

Your BMR accounts for roughly 60–75% of your total daily energy expenditure, so the factors that influence it have a big impact on how much energy you use overall.

• Body size and composition: Larger individuals generally have higher BMRs because they have more metabolically active tissue. Muscle tissue burns more energy at rest than fat tissue does, so two people at the same weight can have different BMRs depending on their ratio of muscle to fat.
• Age: BMR tends to decrease with age, largely because of gradual muscle loss and hormonal changes.
• Sex: Men typically have higher BMRs than women, mainly due to greater average muscle mass.
• Genetics: Variations in mitochondrial efficiency and other genetic factors mean some people naturally burn energy faster or slower than others.
• Hormonal factors: Thyroid hormones (T3 and T4) are major regulators of metabolic rate. Hyperthyroidism raises BMR, while hypothyroidism lowers it.

A higher BMR means greater daily energy expenditure even at rest. This directly affects energy balance, which is the relationship between calories consumed and calories burned. When energy intake consistently exceeds expenditure, the excess is stored (weight gain). When expenditure exceeds intake, stored energy is used (weight loss).

From a physics perspective, the laws of thermodynamics govern all of these energy transformations. The first law (conservation of energy) tells you that all energy entering the body must go somewhere. The second law tells you that no conversion is perfectly efficient, which is why the body always produces waste heat. Maintaining homeostasis, a stable internal environment, requires continuous energy expenditure, and that cost is built into your BMR.