To clarify, two things.
The brain does not directly drive the muscles. It does not have copper running to them, providing a charge. Instead, cell after cell transmits the signal, from brain to muscle, each cell providing its own charge. The brain expends more energy thinking about abstract ideas, than telling a muscle to move.
Second thing, healthy, fit people run for hours, bike for hours, without issue. As a teenager, I'd work on a farm for 10 hours, picking fruit, throwing bales of hay, digging, on and on without tire.
Yet just continually walking, uses more "brain telling muscles what to do", than any single heavy lift plus even perhaps 10 seconds of break.
Realistically, writing cursive with a pen, typing fast, should use as much brain energy, for fine, detailed control requires many different muscles all being used at once, hence more signaling.