This year I have returned to alpine skiing and found myself relatively fit for quite intensive skiing session (up to 10 runs per hour for 3-4 hours straight twice a day for 5 days) except for two things where my body failed.

First one - on the fourth day the muscle which goes from buttocks to the waist (left one) started to hurt a lot in its upper part at the beginning of every right turn I did. There is a very specific movement at the beginning of a ski turn when you sit down and at the same moment bend your knees inside the turn. Except for this specific movement this muscle has never troubled me and I do bends touching my toes every day and squats every other day.

So there is a question - what exercise I could add to my routine to train this muscle?