Where Does All the Energy Go? Understanding Human Mechanical Efficiency and Heat in Sport - truefuels

Where Does All the Energy Go? Understanding Human Mechanical Efficiency and Heat in Sport

The Speed Read

Human beings are remarkably adaptable but surprisingly inefficient at converting food energy into athletic performance. During exercise, only about 20–25% of the energy produced by the muscles is transformed into movement; the rest is lost as heat. This inefficiency is not a design flaw but an evolutionary adaptation with important implications for athletes. Understanding the science behind mechanical efficiency and heat management empowers us to optimise training, enhance performance, and reduce the risk of overheating (Davis, 2020; Périard et al. 2021).

What Is Mechanical Efficiency?

Mechanical efficiency is the ratio of useful work performed to the total energy expended. For humans, this typically ranges from 20-25%. In other words, for every 100 calories burned during exercise, only 20-25 calories are converted into physical work; the remainder dissipates as heat (Davis, 2020).

A person pedalling at 200W is doing 720kJ of mechanical or “external” work per hour, but 3-4 times this amount of internal work. That is in the region of 2160-2880kJ of heat energy, which has to be either absorbed by the body tissue or lost. Meaning that the total muscular energy production is in the region of 3000kJ per hour. Most of us understand Kcal as the unit of energy intake (on our food packaging), and 1 kJ = 0.24 kcal.

Compared to electric motors, which convert up to 90% of energy into mechanical work, humans are less efficient, with only 20–25% of metabolic energy used for movement and 75–80% released as heat. This inefficiency is an evolutionary adaptation, as the heat produced during exercise maintains core body temperature, optimising enzymatic and metabolic functions critical for survival across diverse climates (Sawka et al., 2011).

Can Mechanical Efficiency Change?

Mechanical efficiency varies with activity and individual factors. Cycling, for example, is more efficient (22–25%) than running (18–22%) due to lower weight-bearing demands. Efficiency can also improve with skill, training, and technique—elite cyclists may increase efficiency from 20% to 24% through optimised pedalling and muscle coordination. However, most athletes and activities remain within the 20–25% range, constrained by biological limits (Wilmore & Costill, 2004; Coyle, 1995).

Heat Production and Thermoregulation

As covered above, 75–80% of the energy not used for movement becomes heat. This heat production, while necessary for maintaining internal body temperature, can become a limiting factor, particularly in endurance sports or hot environments. During endurance activities or in hot environments, managing this heat is critical.

Our bodies rely on various mechanisms to regulate this heat, mainly 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 (Périard et al. 2021).

Evolutionary Trade-Offs

Why aren’t humans more mechanically efficient? The answer lies in evolution. Only about 20–25% of metabolic energy is converted into movement during exercise, with 75–80% released as heat—a trade-off that supports survival. This heat maintains core body temperature, optimising enzymatic and metabolic functions critical for life in varied climates (Sawka et al., 2011). Unlike machines built for high mechanical output, humans evolved as endurance athletes, adapted for prolonged activity like persistence hunting in savannahs. Our low mechanical efficiency enabled long-distance pursuits by leveraging efficient heat dissipation through sweating, ensuring adaptability across diverse environments (Bramble & Lieberman, 2004).

Practical Implications for Athletes

Key Takeaways

  1. Humans are not very thermodynamically efficient!
  2. Movement Efficiency: Refining technique (e.g., running form, cycling posture) can reduce wasted energy.
  3. As our core temperature increases, our movement efficiency decreases as more energy is spent on removing heat.
  4. Hydration & Cooling: Staying hydrated and using strategies to increase body heat loss are key to sustaining performance.
  5. Heat Acclimatisation: Gradual exposure to heat improves sweat response and cardiovascular efficiency, enhancing performance in hot conditions.
  6. Training Adjustments: On hot days, reduce intensity or duration to minimise overheating risk.

Ongoing research explores ways to optimise the balance between energy use, mechanical efficiency, and heat management. Advances in clothing technology, hydration strategies, and cooling systems are helping athletes push the limits of performance. Meanwhile, biomechanical studies continue to refine our understanding of movement efficiency.

Conclusion

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.

References

  1. Davis, L. (2020). Efficiency of the Human Body. Physics LibreTexts.  https://phys.libretexts.org/Bookshelves/Conceptual_Physics/Body_Physics_-_Motion_to_Metabolism_(Davis)/10:_Powering_the_Body/10.09:_Efficiency_of_the_Human_Body
  2. Davis, L. (n.d.). Human Efficiency and Heat Loss. OpenOregon - Body Physics 2.0.  https://openoregon.pressbooks.pub/bodyphysics2ed/chapter/locomotive-efficiency/
  3. Périard, J. D., Eijsvogels, T. M. H., & Daanen, H. A. M. (2021). Exercise under heat stress: Thermoregulation, hydration, performance implications, and mitigation strategies. Physiological Reviews, 101(4), 1453–1492. doi:10.1152/physrev.00038.2020journals.physiology.org
  4. Wilmore, J. H., & Costill, D. L. (2004). Physiology of Sport and Exercise. Human Kinetics. https://www.human-kinetics.co.uk/9781718228429/physiology-of-sport-and-exercise/
  5. Coyle, E. F. (1995). Integration of the physiological factors determining endurance performance ability. Exercise and Sport Sciences Reviews, 23, 25–63. https://pubmed.ncbi.nlm.nih.gov/7556353/
  6. Bramble, D. M., & Lieberman, D. E. (2004). Endurance running and the evolution of Homo. Nature, 432(7015), 345–352. doi:10.1038/nature03052
  7. Sawka, M. N., Wenger, C. B., & Pandolf, K. B. (2011). Thermoregulatory responses to acute exercise-heat stress and heat acclimation. In Handbook of Physiology: Environmental Physiology (pp. 157–185). Oxford University Press.

About the Author

Alistair Brownlee is a two-time Olympic gold medallist, Ironman Champion, and co-founder of Truefuels. He is driven by a belief in science-backed training, clear structure, and removing friction from performance.

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