Although all the ski instructors in both studies applied high pressure COP to the heel and head of the first metatarsal, it is noteworthy that the magnitude of the pressures varied significantly between ski instructors. It is also noteworthy that much larger pressures were seen on the right foot of most ski instructors than on the left foot. One possible explanation is that the left foot is typically up to half an inch (1.2 cm) longer than the right foot and usually has a larger volume. As such, the function of the left foot tends to be more inhibited by the structures of a ski boot than the right foot. Foot function affects stance and stance affects COP, especially the ability to create and maintain high pressure COP under the head of the first metatarsal throughout a turn.
PRESSURES UNDER THE HEEL
My observation is that the majority of skiers tend to ski with their weight (pressure) on the center axis of the ski, with COP on their heels and with varying degrees of pressure gradient towards a center between the 2nd and 3rd metatarsals. Whether they realize or not, those who advocate a centred balance or mid stance position are actually advocating a heel COP only stance. So I will limit this post to high pressure COP on the heel and discuss high pressure COP on the first metatarsal and heel-first metatarsal COP pressure differential in my next post.
The graphic below shows changes in the 3-dimensional pressure gradient from heel strike into the early stance phase of the gait cycle. The pinnacle of the pressure gradient over the heel is the actual point center of ground reaction force (GRF). Knowing the location of the point centre of GRF force is important to understanding the mechanical influence of COP on the ski, especially where COP is positioned in relation to the inside edge. Not all pressure data shows the point center of pressure. Note that as COM advances on the foot, COP moves toward the forefoot and the pressure under the heel decreases. Technically, only one COP can be present in the foot. But for the sake of discussing the distinct areas of pressure under the heel and head of the first metatarsal and the ability for a skier to establish and manage the pressure differential between the two main pressure areas, I will refer to them as two high pressure COPs.
In the foot load sequence in the above graphic, the load W from the weight of the body, is transferred to the heel bone from the tibia. The angle of the tibia with the ground and the forward movement of the body in the right hand image is pushing the forefoot towards the ground causing it to rotate about its contact point at the heel with the ground. This causes the ankle joint to plantarflex. In the plantarflexed state, the foot maintains the looseness in its joints that it had in the unweighted state so the foot can absorb the force of impact with the ground. COM is where the body is in relation to the foot. At heel strike, in the left image, the foot is like a loose sack of bones. When the weight W, is on the heel on one or both feet on skis, the position of COM can only be guesstimated. In order for the pressure of COP to be high under the heel, COM must be close to the heel in terms of its front to back or sagittal plane location. Since the shaft of the boot can create what is called a phantom foot, in concert with the ski, COM can be behind the heel in skiing, something that is not possible in normal stance foot balance scenarios.
As the sole of the foot comes into full contact with the ground (centre image), and COM advances towards the balls of the feet, the load from W begins to compress the medial arch, causing it to decrease in height. As the arch decreases in height, the foot elongates and increases in width across the metatarsals. This creates tension in the sheet-like ligament called the plantar aponeurosis and in the intrinsic muscles that support the arch.
I refer to the tension resulting from compression as Intrinsic Dynamic Tension or IDT. When the load points of the foot at the heel and the heads of the five metatarsals are supported on a monoplanar (flat) surface, the height of the arch is directly proportional to the degree of IDT in its 3 arches. Intrinsic Dynamic Tension is the relationship between the magnitude of compressive force acting on the 3 arches, but especially the medial or inner arch, and the resulting magnitude of horizontal shear force. The greater the IDT, the more quasi-rigid the foot becomes. As IDT progressively increases, the medial arch is transformed into a structural truss or ‘bridge’ between the 2 principle load points under the heel and head of the first metatarsal as shown in the greyed portions of the skeleton foot in the graphic below.
This structural bow or beam has important implications for skiing in terms of the ability to establish COPs on the both the heel and head of the first metatarsal and to manage the pressure differential between them. The lowering of the height of the arch is a normal and necessary response to compression loading and the creation of IDT. But this process has been represented in skiing as an indication that the arch has collapsed and failed and requires external support in the form of a supportive insole, usually one that is shaped to the form of the arch in a neutral (STJ) or partially pronated position.
An analogy that is often used to rationalize custom insoles is that, like a house, the foot needs a strong foundation to support it. Some claim that putting insoles under the foot is needed to create even pressure over the entire surface and that this will result in better ski feel. But the foot is nothing like a house. It is closer to the 3 dimensional truss shown the the graphic above. Proponents of the foundation model conveniently fail to mention that in order for a foundation to support a house, the foundation must be founded on solid ground. It is well-established in principles of truss mechanics that introducing structures within the span of a truss, between the load points, can compromise structural integrity and potentially cause failure. In skiing, structures introduced between the sole of the foot and the boot board that forms the base of a ski boot, can limit or prevent high pressure COP from being established under the end of the bow truss opposite the heel; under the head of the first metatarsal or cause it to be lost as loads peak in the bottom of a high load turn. This may explain why some instructors in the studies moved pressure from the head of the first metatarsal to the heel in some or all turn types.
The forward advance of COP from the heel to the head of the first metatarsal is not linear, but occurs in a sweeping arc as shown in the graphic below.
Even when COP is only under the heel, pressure can be pushed towards the forefoot along the centre axis of a ski, even when the ski is on edge, by moving COM forward with stance adjustments. But for COP to be established under the head of the first metatarsal, the foot must be flat on the snow so it can pronate. Even a brief interval of ski flat at the end of the transition or in the fall line can be enough for the foot to pronate if the ski boot is set up to allow the required biomechanics. Due to the lack of tension in the foot when COP is only under the heel, the pressure applied by the foot is localized under the heel with nebulous pressure present under the forefoot. When COP is localized under the heel, the structure of the foot is loose and mechanical influence on the ski poor.
IDT peaks in the late stance phase of the gait cycle as shown in the graphic below.
In his book, Ultimate Skiing (2010), LeMaster, uses the position of the blade on ice skate in relation to the ankle to attempt to explain how torque for the ankle resulting from an offset of the ankle with the inside edge of a ski makes the ski flatten and slip. LeMaster’s is correct in stating that torque results from the offset. But it is the load W from the weight of the body transferred to the lower end of the tibia and from there to the heel and through the boot-binding-ski interface to the base of the ski, where the offset of W with the inside edge creates a moment arm that causes the ski to rotate about a pivot formed by the inside edge.
The graphic below shows what is really happening when W is offset from the snow reaction force S in the plane of the base of the ski. The vertical arm, emanating from S at the inside edge, represents the inner aspect of the shaft of the boot. The boot and ski have been removed in the right hand image to show the moment arm configuration.
The graphic below shows what happens when the load W causes the ski system as a unit to torque about the inside edge.
The force from the load W causes the ski system to rotate (invert) as a unit transferring the load from W to the inner aspect of the lower leg which acts as a source of ground reaction force for the GRF that is not present under the portion of the ski outboard of the inside edge. This creates a false perception of balance because the edge holds. But the load transferred to the leg is acting to further unbalance the skier.
While the Ottawa researchers did not explore this aspect, they identified equipment, including custom insoles, technical skills and technique as potential reasons that might explain why the pressures seen under the heel and first metataral of some instructors was much higher than the pressures seen in the same locations in other instructors. Bad technique and boot problems tend to ensure that COP remains under the heel. A skier who is only able to create high pressure COP under the heel is literally without a leg to stand on. The central issue, one that defines the world’s best skiers and racers, is the ability to create high pressure COP under the head of the first metatarsal and the heel and manage the pressure differential between them. This will be the subject of my next post.