The Balance Axis
The two University of Ottawa pressure studies that used elite ski instructors found that, without exception, every ski instructor started their turns with pressure under the head of the first metatarsal (ball of the foot) and that in some, but not all turns, the pressure moved back to the heel in the last part of the turn. What the studies did not make clear is that when the Center of Pressure (COP) is under either the heel or the head of the first metatarsal, there will always be a secondary center of pressure under some other aspect of the foot. When COP is under the heel, there will be a diffused, but weak, area of pressure in the forefoot across the heads of metatarsals 1-2-3. But when COP is under the head of the first metatarsal, there will always be a well-defined, localized secondary area of pressure under the heel. The relationship of the two COPs (heel or head of the first metatarsal), affects the relationship of the balance axis of the foot and the balance axis of the ski.
The ski has two balance axes. These are the theoretical points where a ski would balance on the head of a pin if the mass of the ski were perfectly distributed. The graphic below shows the two balance axes of a ski. A section below the top view of the ski shows the minimum profile width or waist of the ski in black. The limits of the sidecut are shown in grey. A triangle indicates the Balance Axis.
(click on graphics to enlarge them)
When COP is under the heel, the load or pressure, will lie on the transverse Balance Axis of the ski where it will be distributed outward from its center as shown in the graphic below. Because of the weak, diffused secondary area of pressure across metatarsals 1-2-3, pressure under the heel will extend more forward, towards the shovel, than rearward, towards the tail. The load on the heel lies on the anatomical center axis of the foot that runs through center of the heel and the head of the second metatarsal. As with the Balance Axis of the ski, the anatomical center axis of the foot is the theoretical transverse balance point in bipedal stance. This is the reason why skate blades are mounted on this axis. LeMaster and others have used the skate example to explain why a ski is hard to hold on edge compared to an ice skate. But the balanced load across the transverse aspect of the ski will keep the ski level on flat terrain and resist forces that attempt to tip it onto one of its edges.
What has not been recognised in skiing, is that the transverse balance axis of foot becomes re-aligned in monopedal stance so that it runs through the center of the heel and the center of the head of the first metatarsal as shown in the graphic below. By placing the highest load on the ski under the head of the first metatarsal, the re-alignment of the anatomic balance axis that occurs in monopedal stance, unbalances the load on the outside ski causing it to tip on edge. This unbalanced load causes the limits of the ski at the shovel and tail to displace away from the center of the load (COP) on the portion of the inside edge under the head of the first metatarsal. The diagonal load axis across the Balance Axis of the ski sets up components of force that are perpendicular and parallel to the Balance Axis of the ski.
The re-alignment of the balance axis of the foot that occurs in monopedal stance can be clearly seen in images below of speed skaters and Mikaela Shiffrin balancing on a slack line.
The re-alignment of the anatomical balance axis of the foot is the end result of the transition to the inside ski while it is still on its current edge and the creation of pressure from extending the leg. This mechanism enables racers such as Shiffrin to Get Over it at ski flat and initiate edge and turning effects simply by balancing on the new outside ski and managing the heel/first metatarsal pressure differential.
The actual mechanics are more complicated than what I have described in this post. But as the saying goes, “One step at a time.”