In my last post, I said that I would offer an explanation for the historically consistent 67 mm (+ or -) ski waist dimension of skis. From a mechanical and biomechanical perspective, the innermost inside edge limit of the outside ski of a turn of the human foot is the ball of the great toe. The anatomical centre-line of the human foot runs through the centre of the heel and the centre of the ball of the 2nd toe. Therefor, it follows that if the foot is oriented on the proximate transverse running centre of a ski, the width under the ball of the foot should be in the order of 2 times the centre-to-centre dimension between the balls of the great and 2nd toes. The graphic below shows these relationships with X indicating the centre-to-centre dimension.
It is easy to measure this dimension on your own or someone else’s foot using an out-to-out micrometer (with points) or a protractor. The foot should be weighted when the measurement is taken. Although the balls of the feet (metatarsals) spread in a fan-like manner when the forefoot is weighted, the centre-to-centre dimension between the balls of the great and 2nd toes does not increase significantly. In addition, as foot sizes and widths vary, the variation in the centre-to-centre dimension, X is small, in the order of 1-2 mm per full size. The centre-to-centre dimension between the balls of the great and 2nd toes of my US Men’s size 12 foot is in the order of 35 mm. My spouses’ US Ladie’s size 8 foot (US Men’s size 7) centre-to-centre dimension is in the order of 30 mm. The table below shows an extrapolation of these numbers based in studies of specific foot references for a range of sizes. I am going to do a study that measures dimension X for a cross section of foot sizes, both male and female. As far as I am aware, such data does not exist. Nor, am I aware of any studies or teachings pertaining to the mechanics and biomechanics of skiing where this issue is discussed.
As far back as 1978, I was very much aware of the importance of positioning the ball of the foot as close as possible to the inner sidewall of the boot shell as well as maintaining the normal alignment of the big toe, something the DIN standard toe shape makes extremely difficult. Ideally, the Centre of Pressure should be over or slightly on the inside turn aspect of of the inside edge of the outside ski of a turn. Although edging mechanics that are driven by external forces require more than the aforementioned alignment of the centre-to-centre dimension between the balls of the great and 2nd toes, as the dimension of the waist of the ski becomes increasingly unfavourable a point is reached where edging mechanics driven by external forces are no longer an option.
As far as I know, the current low end of waist width in skis is 63 mm. It is not unusual for female racers to have feet as small as US Men’s size 5 or even 4. The implications are that a female racer with feet in this size range could need a ski with a waist dimension as small as 54 or 55 mm in order to be competitive. There are significant implications for females with small feet that are skiing on skis with waist dimensions that are significantly in excess of these dimensions. It is worth noting that the design of ski boots is typically based on a US Men’s size 9 foot and then scaled up and down for other sizes.