LOAD TRANSFER


The critical factor influencing whether a skier can establish a platform under the outside foot of a turn is the ability to transfer the load W from the central load-bearing axis to the head of the first metatarsal. The ability to establish a platform is essential to postural responses which are dependent on a contiguous source of ground reaction force.

The multi-axial joint system of the ankle complex resides between the lower end of the central load-bearing axis and the points of the foot that transfer the load to ground or surfaces that act as quasi-ground. Quasi-grounds are structures that extend ground reaction force to a plane above the ground. The fundamental role of the multi-axial joint system of the ankle complex is to enable the foot to simultaneously rotate in multiple planes so as to bring its load transfer points into firm contact with ground. The foot is specifically configured for this purpose. Despite the importance of load transfer to sound ski technique, like Rodney Dangerfield, “It don’t get no respect”.

The graphic below shows rear (L) and side views (R) of the central load-bearing axis.

Central Axis 3

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As noted on the graphic, the posture shown is independent of external sources of support. W is the weight of COM created by the attractive force of gravity that pulls COM towards the centre of the earth. In dynamic physical environments such as skiing and skating where forces are complex, the resultant force R creates the load W. In bipedal stance, the load W is transferred through the pelvis to the heads of the femurs. For the sake of clarity, COM is shown on the posterior aspect of the pelvis. It would normally not be visible. The head of the femur is the upper aspect of the central load-bearing axis. The lower aspect of the tibia is the lower aspect of the central load-bearing axis. The tibia articulates with the talus to form the ankle joint.

The lower aspect of the tibia is substantially fixed in position in relation to the base of support at the feet. It is the centre element of the COM/central load-bearing axis system. COM is limited in terms of fore aft movement to the COM Limits shown in the shaded area. The long bones of the leg transfer the load W to the ankle as WL and WR. The major muscles of the leg act to resist W by restraining the position of the joints in the central load-bearing axis.

Once the load W has been transferred by the central load-bearing axis to the ankle joint (WL-WR), the configuration of the skeleton as a whole determines how and where the load will be transferred to the load-bearing points of the foot (the calcaneus and the heads of the 5 metatarsals). The graphic below shows two stances with approximate knee angles noted.

Major Muscles2

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In quiet standing, the angles at the knee are relatively small. In this configuration, the load W is rotating COM forward about the ankle as shown in the left hand figure. Forward rotation of COM  is opposed by a bottom-up chain of muscles in eccentric contraction. These muscles act in concert to restrain the ankle, knee and hip joints. I refer to this as the Pelvic-Plantar Pull because the forward rotation of the pelvis is actually pulling on the heads of the 5 metatarsals through the Achilles/plantar aponeurosis bridge.

If the angle of the ankle is fixed, hip flexion requires a corresponding flexion of the knee joint. Past a knee angle of approximately 55 to 60 degrees, rotation of COM reverses direction as shown in the right hand figure. The activity of the gastrocnemius and hamstrings diminishes.  The angle of the knee is now restrained by the quads. Without intervention from the quads, COM would topple rearward. Many experts refer to the right hand stance as ‘athletic’ because it is typical of the ‘ready position’ of athletes like tennis foot ball players. This position prepares the athlete to accelerate COM by extending the ankle, knee and hip joints. But this configuration predisposes to weight on the heels and a rearward excursion of COM, aka as ‘skiing in the back seat’. Yet this stance is widely promoted in skiing based on the premise that it facilitates ‘knee angulation’.

The configuration of the leg muscles in the left hand figure enables lengthening beyond their normal range, allowing explosive peak force from the stretch reflex.

Once the load W has been transferred to the lower aspect of the tibia, muscles and joint configurations determine how and where W will be transferred to the load transfer points of the foot. In the graphic below, the left hand and centre images show the foot supported on a contiguous source of GRF.

Load Transfer Path

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Introducing a solid intermediate structure or Quasi Ground between the foot and ground as shown in the centre image, extends GRF from the ground to the foot. But without knowing the joint and muscle actions associated with the load transfer, it is not possible to determine where the Centre of Pressure will manifest.

In the right hand figure, the intermediate structure is only supported by GRF on its left hand or medial aspect.  The STJ is shown in neutral. In this configuration, it is certain that the load will be transferred to the base of the calcaneus. The misalignment of the opposing forces, load W and ground reaction force, creates an unbalanced lever or moment arm. Load W will cause the intermediate element and foot to rotate as a unit about the pivot formed by the point of GRF. Without a load to oppose the rotation, the intermediate element and foot will invert as a unit as the multi-axial joint system of the ankle complex attempts to transfer the load to the ground.