1. Foot Placement

First, your feet must be hip width apart. With feet separated the correct amount, your legs will be parallel with knees straight over ankles from the front view. If you place feet too wide, knees pinch in.  If you place feet too close,  your knees flare out.  Both can cause knee discomfort and change the distribution of weight in your leg muscles, making it less challenging.

This is the correct foot placement/leg spacing for every single O15 exercise, whether standing, seated, lying on your back, bent over, etc.  When doing the chest press or crucifixion with the legs in the air for example, it will look like this:

2. Knee Angle

In the wall sit, your knees must always be a straight line down to your ankles from the side view as well.  When you are at the lowest position (a 90 degree angle) this is easy to eyeball:

If it gets too difficult from this low position and you must slide up along the wall, you can’t keep your feet in the same spot or they will now be further forward than your knees.  You will eventually feel the exercise in your knees like this.  As you move up the wall, you must walk your feet backwards until there is again a straight line from knees down to ankles:

Side View : Knees Over Ankles When >90°

3. Foot Movement

Once you have your feet placed at the proper width for your body & straight under your knees, you need to keep your feet flat and level on the ground to maintain this position. There are two ways people mistakenly move the feet: changing the angle of the foot or placing weight unevenly on sides of feet.

a.) Angle : Moving the angle of the foot by rotating the toes outward or inward moves the knees in the same way as a wrong foot placement. Standing  pigeon toed pinches knees in while standing duck footed flares knees out.

Foot Angle

b.) Weight Placement : You want to distribute your weight evenly through the width of your foot.  If you overpronate, you push more weight towards your instep, which pinches your knees together.  If you supinate, you push too much weight to the outer edge of your feet, flaring your knees out.

Many people make this mistake when lifting the toes.  When you are instructed to “lift your toes” you should only lift the toes, not the ball of the foot (metatarsus) as well.  An advanced client can lift the ball of the foot and keep the feet level, but most people move or rotate the feet when they do this. If you imagine your foot making a triangle where the outermost bones (1st & 5th metatarsals) in the ball of the foot are the base of a triangle and your heel is the point of the triangle, you will have an easy time keeping your feet flat and knees stable before lifting your toes.