Understanding the energy demands of endurance sports like running and cycling can be complex. While heart rate (HR) is often used as a gauge of exertion, it doesn’t directly translate to power output in either discipline. However, exploring the relationship between energy expenditure and power can offer valuable insights into comparing “Running Vs Bike”.
Energy Expenditure in Running: The Basics
Studies analyzing exhaled gas during running have provided a foundational understanding of energy expenditure. Similar to metabolic assessments in cycling, measuring carbon dioxide output allows researchers to estimate the energy required to run at a specific speed. These studies reveal individual variations, but a useful rule of thumb emerges: for submaximal running on flat surfaces, energy expenditure approximates to 1 kcal/kg/km.
This principle, widely cited in exercise physiology texts, including those by McArdle, Katch, and Katch, means a 75kg runner expends roughly 75 kcal per kilometer. Interestingly, this estimation remains largely independent of speed for aerobic running, highlighting that it applies to endurance efforts rather than sprints.
Estimating Running Power: Watts and Kilojoules
To connect energy expenditure to power, we need to consider metabolic efficiency and units of measurement. We know that approximately 4.18 calories equate to one joule. Furthermore, human gross metabolic efficiency (GME) in running is around 20-25%. If we take an average GME of 23.9%, a convenient approximation arises: 1 kcal is roughly equivalent to 1 kilojoule.
Working backwards from the 1 kcal/kg/km rule and incorporating GME, we arrive at another practical rule of thumb: running speed in meters per second (m/s) is approximately equal to power output in watts per kilogram (watts/kg).
For instance, completing a marathon in approximately 2 hours suggests an average power output of just under 6 watts/kg. This figure aligns remarkably with the power output of elite cyclists, suggesting comparable physiological demands at the highest levels of endurance in both running and cycling.
Running vs. Cycling: A Power Comparison
Anecdotal evidence from athletes who participate in both running and cycling further supports this comparison. Many report that running a 10k in around 42 minutes feels subjectively similar in effort to completing a 40k cycling time trial in under an hour. Applying our rule of thumb, a 42-minute 10k (requiring roughly 4 m/s) translates to approximately 4 watts/kg, a figure consistent with the demands of a competitive 40k time trial.
This convergence in estimated power output underscores that while the modes of locomotion differ significantly between running and cycling, the underlying physiological challenge in terms of sustained power production at a high level is surprisingly similar. Both disciplines demand efficient energy utilization and a robust aerobic capacity.
Factors Affecting Individual Efficiency
It’s crucial to remember that these rules are estimations. Individual running economy can vary. A highly gifted runner might expend less than 1 kcal/kg/km due to superior biomechanics or physiological efficiency. Similarly, variations in metabolic efficiency will affect the precise conversion between energy expenditure and power output.
Furthermore, running terrain significantly impacts energy demands. Running uphill necessitates additional energy expenditure to overcome gravity, which must be factored into power estimations, as Phil Skiba’s running power models demonstrate.
Conclusion: Practical Rules for Endurance Athletes
While heart rate provides a readily available measure of exertion, understanding the principles of energy expenditure and power output offers a deeper insight into the physiological demands of running and cycling. The rules of thumb – 1 kcal/kg/km for energy expenditure and running speed (m/s) ≈ watts/kg for power – provide useful tools for athletes and coaches. These approximations allow for comparisons between running and cycling efforts and offer a framework for understanding training intensity and performance in both endurance disciplines. Keep in mind individual variability and terrain should be considered for more precise analyses.
