There are people who train their entire bodies to function on different biochemical processes, generally long distance endurance athletes training to perform in

There are people who train their entire bodies to function on different biochemical processes, generally long distance endurance athletes training to perform in a fat adapted state for ultramarathons and the like. The research here just seems to suggest that the soleus muscle itself has a lower "barrier to entry" before utilizing different energy sources (blood glucose and fat oxidation) which allows it to sustain activity for a longer time duration. This makes sense, as the soleus is highly involved in walking, and humans basically evolved to walk more than we've evolved to do anything else.