This static hold is great for building a base of strength and endurance in the legs (particularly the quads.) With your back pressed into a wall slide down until your knees and hips are at an approximate 90 degree angle. Your shins should be vertical. Hold for as long as possible. To make the exercise harder simply hold weights in each hand. Since this is a very quad dominant move try to balance it out with some hip and hamstring dominant exercises e.g. bridges.