"Gravity is pushing you down as you use more muscle to push up," Rekhviashvili explains. "We’re not [as] used to walking uphill or climbing stairs as we are to just walking. When you’re doing stairs you’re pretty much doing a lunge. So already you’re working against gravity, and on top of that, activating a lot more muscle, stabilizers, and your glutes to actually push off. In general everyday life, these are just not muscles we activate a lot.”
And if you do happen to still be carrying some holiday weight, well, that's just like adding more dumbbells to this hellish process.
"Weight obviously plays a factor here," says Rekhviashvili. "You’re lifting a lot more weight as it is, so the more you weigh, the more you’re lifting."