Newtonian Mechanics – Why Does Running Spend More Energy Than Walking?

Question

The study energy expenditure of walking and running concludes that running spends more energy than walking.

My understanding is that although running makes one feel more tired, that only indicates that the power was higher (since the time of displacement was shorter), but at the end of the day the total energy dispensed to move oneself forward by friction should be the same.

Given the study shows otherwise, what could be the flaw in my reasoning?

Answer 1

The basic reason is that an ideal object does work in the physics sense, but a biological entity has so many, many, ways it doesn't behave like an ideal object.

One can estimate and calculate, but other answers attempt that. I'm just going to try and summarise some of these big not-ideal-object behaviours

• Running involves more and varied movements, it's a very different gait. It is definitely not just the same movement as walking but faster. Some of those movements are vertical, or relate to jumping, some have shock absorption components and relate to landing. Much of that extra energy is dissipated both ways - we use energy both to jump and to cushion and come to a halt on landing. We also accelerate our feet to match our ground speed and must slow them to zero each stride, then speed and lift the other way as well, not just rely on gravity and pendulum activity. The fact this is at extension and not at ground impact doesn't change anything. Again, energy is lost both ways. I'm also going to guess that it's harder to be efficient across a wider compared to narrower range of motions, therefore the wider range of movements and systems used in running means it's much more likely that efficiency varies considerably, according to biological subsystem or type of movement.
• Human gross muscle motion energy handling/metabolism is not efficient and doesn't behave like an ideal object. We have multiple energy pathways, and switch between them according to need. This happens less with walking, more with vigorous exercise like running. The "emergency" or "sustained activity" energy cycle our bodies switch to, when running, is less efficient - if it was more efficient it would probably have evolved as our primary not our fallback. And of course many biochemical reactions and body responses just aren't linear; they also may have min/max rates or durations.
• Other systems may be activated as a result, that aren't activated, or aren't activated the same, when just walking. For example, there may be changes to brain bloodflow, and blood pressure, and energy use purely due to higher metabolic rate. We may have to activate heat dispersion systems when running but not walking (sweating?), change some neurotransmitter production/uptake (stimulants? Or others affecting other bodily systems' activity levels and energy use as a byproduct), or change the tone of our blood system (constricted or relaxed to control flow differently). We may de-activate entire systems - digestion, intestinal activity. The rate of metabolic breakdown product production (lactic acid?) is higher and perhaps needs extra activity to try and deal with it. More water must be channeled to hold all these extra metabolites and products in solution in the bloodstream, or to replace other water lost or used. If other products are needed for the pathways activated (no idea - enzymes for example?) then the processes for producing these in the body must be activated too. Running, like any vigorous physical activity, also causes more muscle fibre tearing and other microscopic damage/wear, so there may be extra repair afterwards. In other words in the background our body may start or stop many other processes.
• Running is rarely optimised. We do other movements as well, from habit or need. We breathe differently, we may clench fists or other muscles, or grit teeth a bit (jaw muscles). Related, we may find that we need to hold some skeletal muscles stiffer, not relaxed, to prevent body parts "flopping around" (jerky motion and perhaps damage as we run), which wouldn't be needed with walking.