2024-07-26 06:00:06
By Clément Lemineur, Clément Naveilhan and François Dernoncourt – Université Côte d’Azur
It’s Monday morning, the alarm goes off and it’s already 7:30, you’re 30 minutes late. Normally it takes you 45 minutes to walk the 3 kilometers to your workplace, but this morning it will be 20 minutes running. Yes, but at lunchtime you feel more tired and you feel like you’ve used up more energy than usual on the journey. However, you’ve covered the same distance as on other days, how is that possible?
The caloric expenditure associated with an activity is called “metabolic cost”, and corresponds to the energy consumed by our organs to travel a given distance. It is possible to determine this metabolic cost by analyzing gas exchanges. Depending on the oxygen consumed and the carbon dioxide product, we estimate the quantity of energy expended by our body, and thus the metabolic cost. It is thanks to this method that, since the 70s, the researchers had already answered our question.
So, and this may not surprise you that much, running consumes more energy than walking for the same distance covered. How can this be explained?
Energy lost while running
Imagine you are watching someone running. Now, look carefully at the shift vertical (up and down) of his pelvis and head. As can be seen in the figure, when running, our body oscillates more vertically than when we walk.
This implies that the muscles of the lower limbs must generate more force to produce this vertical displacement, which consumes more energy without getting us closer to our destination. Thus, when running, a part of the energy expended is used to move upwards rather than forwardsThe energy required to cover these 3 km is therefore higher for running than for walking.
Running involves a much greater vertical oscillation of the center of mass than walking. This is the main reason why running uses more energy than walking for the same distance covered.
François Dernoncourt, Provided by the author
This difference between walking and running is not limited to what happens during the activity itself. In fact, each physical exercise causes a delayed energy expenditure over time, which is added to the expenditure during the activity.
Taking this parameter into account, it is once again running that is more energy-intensive than walking. Just after running your 3 km, the increased energy consumption (compared to rest) continues for several minutes due in particular to the rise in body temperature and the reconstitution of energy reserves. This additional expenditure after the race would be more than twice that observed after walkingdue to the difference in intensity between the two exercises.
It all depends on the speed
Running therefore involves a higher caloric expenditure than walking for the same distance covered. But this is on condition that the walking speed considered is “normal”, that is to say approximately 5 km/h. Thus, if we walk very slowly, we will take so much time to cover these 3 km that the caloric expenditure will be greater in the end. This is explained by the fact that the body expends a certain amount of energy per unit of time anyway, regardless of the activity carried out (this is what we call the “metabolism (baseline).
Same observation if the walking speed is very fast (more than 8 km/h): running is more energy efficient. In this case, the coordination required to walk at such speed involves a activation increased elasticity of our muscles without being able to take advantage of the elasticity of our tendons as is the case in running.
Moreover, we have a very precise intuitive perception of the energy efficiency of a particular style of locomotion. If we move on a treadmill whose speed gradually increases, the speed of the treadmill at which we spontaneously switch from walking to running coincides with the speed at which it becomes more energy-intensive to walk than to run !
Modeling of metabolic cost (kilocalories expended per kilogram per kilometer traveled) as a function of speed (kilometers per hour) for walking and running. The curves cross at a certain speed (purple line; around 8 km/h): this means that beyond this speed, walking becomes more energy-intensive than running. It is at approximately this threshold speed that individuals spontaneously switch from walking to running.
François Dernoncourt, Adapted from Summerside et al, Provided by the author
In conclusion, due to a greater oscillation of the center of mass as well as an increased energy expenditure after exercise, running to work is more energy-intensive than walking the same distance. But remember, whether you choose to walk or run to work, the most important thing is that you are already saving energy!
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