"The fatigue actually is your first sign. There's no real clear way to measure that I know of beyond when you first start to feel tired, or drained really, and then when you've overdone it. At least until the headache starts, and sometimes your nose will bleed. Those are certain signs you've overused the muscle and also gone past your limits to drain yourself. At that point, you rest--sleep, and eat filling but gentle for your stomach foods. Because the body needs to recover from the injury and replenish itself."

No sugarcoating here for Eilonwy, as this was serious and Loki was treating it as such.