How a loose boot steals your edge

Everything you do with your body reaches the ski through the boot. If your heel lifts or the shell is packed out, your foot moves before the boot does. Part of every move is lost, and the rest arrives late.

Slide the play up. The dashed line is the fore and aft movement you ask for through one turn. The solid line is what reaches the ski. With a race-tight boot, about 1% of your movement is lost and it arrives 32 milliseconds late, which you will never notice. At 4 mm of play, about 22% is lost and it is 240 milliseconds late. At 8 mm, about 44% is lost and it is 368 milliseconds late. At 12 mm, about two thirds of the movement never reaches the ski, and it is nearly half a second behind.

In the lab a turn takes 1.6 seconds, so 368 milliseconds is nearly a quarter of it. By the time the ski gets your message, you have already moved on. You feel the ski do less than you asked, so you ask harder, and the usual way to ask harder is to throw your hips back. That is how a loose boot ends up as a back seat, and how the back seat ends up as burning quads.

The millimetres here are a stand-in for fit, not a measurement of your boot. Play can come from a heel pocket that is too big, a liner that has packed out, a buckle that will not hold or a shell that is a size too large. The lab does not know which. It only knows how much of your movement goes missing, and when the rest turns up.

It also does not know how your boot feels. Pressure on the instep, a bruised shin or a heel that lifts when you flex are all things to tell a fitter, and they say more about fit than any number here. If a boot hurts rather than just feeling loose, that is a conversation for a fitter, and for real pain, a physio.

A quick check: flex forward in your buckled boots with a friend filming your heel from behind. If the heel clearly lifts or the foot slides, take the clip with you.