What powered Leonardo da Vinci’s proposed armored vehicle without an engine?

Leonardo da Vinci’s proposed armored vehicle was powered by human-powered cranks.

The fighting vehicle drawing shows a conical armored shell mounted on wheels. Inside, several men would turn cranks connected through gears to drive the wheels. The design belongs to the late fifteenth century and was conceived as a protected platform from which soldiers could operate weapons.

The cranks are often discussed because the surviving design appears to contain a gearing problem that would make the vehicle difficult or impossible to move as drawn. Some historians have suggested that the apparent error may have been deliberate, since Leonardo could have wanted to keep a military idea from being immediately usable. Whatever the reason, the vehicle had no steam engine or internal-combustion engine.

This question concerns the vehicle’s power source, not its shape or crew. Its conical shell and eight-person crew are separate identifying details often used in descriptions of Leonardo’s fighting machine.