How efficient are human beings during exercise?

Speed Read: Where does all the energy go?

Human beings are far from perfectly efficient when it comes to converting energy into work during exercise. On average, humans have a mechanical efficiency of about 20-25%, meaning that for every 100 units of energy we consume, only 20-25 units are converted into useful movement, while the rest is lost as heat. This heat production affects not just performance but also thermoregulation and endurance, which have important implications for athletes. Understanding the science behind mechanical efficiency and heat management helps to optimize training, improve performance, and reduce the risk of overheating [1][2][3].

How Efficient Are Human Beings During Exercise? Mechanical Efficiency, Heat Production, and What This Means for Us

A few blogs ago we talked about mechanical efficiency and used the figure of 25% for a simple calculation. That is not perfectly accurate (of course, its the human body!) and the story is a bit more complicated. When you exercise—whether running, cycling, or lifting weights—your body converts the chemical energy from food into mechanical energy that powers your movement. But just how efficient is this process? Research indicates that humans typically have a mechanical efficiency of around 20-25%, meaning that out of every 100 calories burned during exercise, only about 20-25 calories contribute directly to physical work, while the rest is dissipated as heat [1][2].

Compared to machines, such as electric motors, which can achieve efficiency rates of up to 90%, human beings are relatively inefficient. However, this inefficiency serves an evolutionary purpose. The heat generated during exercise plays a crucial role in maintaining internal body temperature, supporting the metabolic processes necessary for survival [2].

For comparison a (very naughty!) internal combustion engine is 20-40% efficient. In the same way as an engine (or any organic fuel derived energy making process) human bodies produce carbon dioxide as a byproduct of energy production. However if we compare the energy usage of a human on a bike to an electric car, it is quite incredible! First the human, lets say that an output power of 200w allows someone to travel 30km in an hour. At 20% mechanical efficiency, they are producing 1000w of power for that hour, which is 1kWh of total energy. My electric car can do around 6km for each kWh of energy, so with 5 people in, its about the same energy consumption per kilometer.

This lead to one of my favourite facts; cycling on an Ebike results in less release of carbon dioxide per kilometre than a solely human-powered bike. Okay, I know… I can hear you from here, to know for sure it depends on what food the human has eaten (plant-based or not) and where the energy used to charge the battery has come from. But let's not get bogged down in the details!

Mechanical Efficiency: What It Is and How It’s Measured

Mechanical efficiency is the ratio of useful work done to the total energy expended. For humans, this efficiency sits between 20-25%. For example, if you burn 500 calories during a run, only about 100-125 calories are converted into the energy required for muscle contraction, while the rest is dissipated as heat [1][2].

Interestingly, the type of exercise affects mechanical efficiency. Cycling tends to be more efficient than running because of the lower weight-bearing demands. Despite the varying efficiencies between activities, the range of 20-25% holds true across most physical tasks [1][2]. Moreover, mechanical efficiency can vary slightly depending on factors like skill level, training, and technique, with trained athletes potentially displaying better movement economy, although they still fall within the general efficiency range [3].

Heat Production and Thermoregulation: The Cost of Efficiency

The 75-80% of energy that doesn’t go into mechanical work is converted into heat, which the body must manage (dissipate) to avoid overheating. This heat production, while necessary for maintaining internal body temperature, can become a limiting factor, particularly in endurance sports or hot environments. During exercise, the body’s muscles generate heat as a byproduct of the chemical reactions that produce energy [1]. In fact, humans are often compared to combustion engines in this respect, as both systems lose a significant portion of energy to heat [2].

Our bodies rely on various mechanisms to regulate this heat, the most well-known being sweating and increased blood flow to the skin’s surface. However, these mechanisms are not always sufficient. On hot and humid days, for instance, sweat doesn’t evaporate as efficiently, which hinders cooling and can lead to overheating [3]. Efficient heat management is especially critical for endurance athletes like marathoners and cyclists, who generate large amounts of heat over extended periods [3].

Why Humans Aren’t More Efficient

Given that so much energy is lost as heat, you might wonder why humans aren’t more mechanically efficient. The answer lies in evolution. Over time, our bodies have developed in ways that prioritize survival over efficiency. The heat generated during physical activity helps maintain a stable internal environment, supporting enzymes and metabolic reactions that are sensitive to temperature changes [2].

The human body wasn’t designed for maximum mechanical output like a machine. Instead, humans evolved as endurance athletes, capable of long bouts of physical activity at moderate efficiency. Our relatively low efficiency is a trade-off that allows for adaptability across different environments and activities [2]. On the savannahs, along with having two legs, this gave us the advantage to allow us to hunt prey over many hours.

Implications for Athletes and Fitness Enthusiasts

For athletes and those looking to optimize their exercise performance, understanding mechanical efficiency and heat production can lead to more informed training strategies and better performance outcomes. Here are a few key takeaways:

1. Focus on Efficiency: While we can’t change our inherent mechanical efficiency, we can improve how efficiently we move by refining our technique. For instance, improving running form or cycling posture can reduce energy waste and improve overall performance.

2. Hydration and Cooling: Managing heat during exercise is crucial, particularly during intense or long-duration activities. Staying hydrated and wearing breathable, moisture-wicking fabrics can help your body dissipate heat more effectively, reducing the risk of overheating and maintaining performance [3].

3. Acclimatization to Heat: For athletes training or competing in hot climates, heat acclimatization is key. Gradually exposing the body to higher temperatures allows it to adapt, improving sweat response and cardiovascular efficiency. This can result in better heat tolerance and improved performance in hot conditions [2].

4. Training Smart: Understanding that a significant portion of energy is lost as heat can also help athletes adjust their training intensity. On hot days, reducing the duration or intensity of exercise can help prevent the body from overheating and reduce strain on the cardiovascular system [3].

What This Means for the Future of Sports Science

Sports scientists continue to explore ways to optimize human performance by understanding the balance between energy use, mechanical efficiency, and heat production. Innovations in clothing technology, hydration strategies, and cooling systems are helping athletes manage heat more effectively, while research into biomechanics and training methods offers opportunities to maximize mechanical efficiency where possible [3].

In the end, while humans may not be as mechanically efficient as machines, our ability to adapt, endure, and regulate heat has allowed us to excel in a wide range of physical activities. By better understanding these processes, we can train smarter, recover better, and continue pushing the limits of human performance [2][3].

Sources

1. Efficiency of the Human Body

“The mechanical efficiency of humans is approximately 20-25%, with most of the energy lost as heat.”

Physics LibreTexts. Efficiency of the Human Body. Available at: https://phys.libretexts.org/Bookshelves/Conceptual_Physics/Book%3A_Conceptual_Physics_(Crowell)/10%3A_Energy_and_Thermodynamics/10.09%3A_Efficiency_of_the_Human_Body

2. Human Efficiency and Heat Loss

“The body is around 25% efficient at converting food energy into mechanical energy, with the rest lost as heat.”

OpenOregon. Human Efficiency and Heat Loss. In: Body Physics 2.0. Available at: https://openoregon.pressbooks.pub/bodyphysics/chapter/human-efficiency/

3. Energy Expenditure and Thermoregulation

“Humans produce a significant amount of heat during exercise, which must be managed through mechanisms like sweating and increased blood flow.”

PM&R KnowledgeNow. Energy Expenditure and Thermoregulation: Energy Expenditure During Basic Mobility and Approaches to Energy Conservation. Available at: https://now.aapmr.org/energy-expenditure-during-basic-mobility-and-approaches-to-energy-conservation/