Some feedback loops between the brain and the rest of the body are more mysterious to me than others. F.e. the placebo effect is something I find hard to ration

Some feedback loops between the brain and the rest of the body are more mysterious to me than others. F.e. the placebo effect is something I find hard to rationalize in many cases. This particular connection really seems kind of obvious. My amateurish explanation would be that every voluntary muscle movement has to be actuated by brain activity, and hard workouts require a lot of muscle steering. If the brain is tired (which means its stores of energy or some other chemicals/transmitters are lowered/exhausted) it becomes harder for it to induce muscle activity.