I have seen this too, and thought it was really interesting. I would have thought that muscle relaxation was controlled almost completely by neural mechanisms, but I guess not.
One idea I had was maybe that, endurance sports have a large relaxation component, so that the athletes use as little energy as possible. Top endurance athletes would be very good at relaxation from all of their background training. As well, these athletes would have the most mitochondria compared to others as a result of their training. So maybe it wasn’t the mitochondria necessarily involved in the relaxation, but the previous training that the athletes in the study were doing?
That is probably not right at all, it was just an idea.
If anyone could tell me the mechanism on how mitochondria speeds up relaxation, that would be great.