Imaginary chair

Wall sit Skiing holds you in a flexed stance for whole runs, and a wall sit trains exactly that: the thighs working hard without the knee moving. Start with holds of about 20 seconds and add a few seconds each week until you can hold for a full minute. Keep your knees in line with your toes rather than letting them cave inward. Change the depth from session to session rather than always holding the same angle, and stay higher, short of a right angle at the knee, if your knees complain.