In a Direction Change by Extension, the Center of Mass (located at the pelvis) actively moves up from the Center of Pressure (located at the feet). It is also known as a “cross-over”, “extended transition” or “eccentric turning” because the pelvis crosses over the Base of Support (BoS) after an extended body position.
Upon reaching a certain point in the extension, the CoM is projected laterally (lateral oscillation) toward the inside of the turn, determining an elliptical trajectory (from bottom to top, from back to front, and from side to side).
The peculiarity of this direction change is the tendency to shift weight from one foot to the other and then to change edges, i.e., the extension precedes the edge change as weight shifting begins on the “old” edge (little toe edge of the uphill ski).
Here, the strategy to change direction is engage-to-release, this is, engaging the uphill ski to then release the downhill ski. This strategy is mostly applied in wide turns when there is enough time and space availability.
We can say that it is effective but not efficient because we actively use the hamstring core of the uphill leg to support ourselves on the little toe edge, thus wasting muscle effort and failing to take advantage of the external forces generated by motion.
In addition, the other drawbacks of the Direction Change by Extension is that, as the CoM moves away from the CoP, there is a tendency for stability to decrease, as well as it presents a loss of snow contact sensation at the first part of the turn (the skis feel “floating”).
Likewise, due to the vertical extension, the CoM is first projected upwards and then laterally towards the center of the new turn, when it would be more efficient to project it diagonally forward.
NOTE: To clarify the designation of each ski/foot/knee/leg/hip/shoulder during a turn, we will define them as “outer” or “inner” when the skis are on or close to the fall line. When the skis are across the fall line, we will assign them as “downhill” or “uphill”. There will also be designations as “inner/uphill” or “outer/downhill” when the skis are between the fall line and across it.
The biomechanical phases in a Direction Change by Extension are as follows:
1. Generation Phase
The generating movement is the verging, which could be slight or moderate depending on skier’s and turn characteristics, of the CoM to the CoP prior to extension by flexing both legs.
2. Monopodal Phase
This occurs just before the direction change by tensing the leading uphill foot (the now ‘standing foot’). The entire body weight is transferred to the 5th metatarsal (little toe) of this foot and its corresponding edge, forming the CoP on which the active and precise extension of the uphill knee is performed using the gluteal and hamstring muscles. The extension of the uphill knee can, in certain cases, be full. At this point, the pelvis tends to sagittally align over the standing foot.
The extension of the uphill knee is active, while that of the downhill knee tends to be passive. In many cases, the extension of the uphill knee reaches its maximum and is not limited to the leg, but extends through the trunk, producing an extension of the hips.
The trunk, which in some cases becomes completely upright, then tends to lean toward downhill. Because of this, through the lengthening of the iliopsoas muscle of the external hip, the pelvis moves momentarily away from the center of the new turn.
The CoM tends to align itself over the little toe edge of the uphill foot and then move explosively, through the shortening of the iliopsoas muscle of the inner hip which contributes to its flexion, into a lateral trajectory towards the inside of the new direction during the Oscillation Phase.
This extension movement prior to the direction change is generated from the CoP situated on the uphill foot, causing an active and punctual pressure on the snow through the extension of the uphill knee. Both knee and hips extensions can be performed simultaneously or alternately: first the knee and then the hips, or vice versa. Due to the marked knee extension on the uphill foot, the downhill foot may lose contact with the snow.
3. Oscillation Phase
This phase is named after the oscillation of the CoM toward the new turning direction. As the extension of the uphill knee comes to an end, the ankle of the standing foot begins to evert along with the lateral oscillation of the pelvis toward the inside of the turn, while the ankle of the now leading foot, with a certain delay, begins to invert (supination of the foot towards the little toe edge). We say that it oscillates laterally because it is the only direction in which it can move, since the uphill supporting leg has exhausted all its joint range of motion.
This phase begins when the big toe edge of the uphill ski makes contact with the snow through the pronation of the uphill foot supported on the 1st metatarsal (ball of the foot).
The inner hip moves slightly or markedly forward together with the inner knee which flex passively. The now leading foot rests on the little toe edge.
Due to an explosive translation of the CoM first upward and then laterally toward the inside of the turn, the now inner ski tends to remain momentarily unloaded. The edge change of both skis tends to be alternating: first the uphill ski changes its edges then, immediately after, the downhill ski.
The drawback of this type of quick lateral translation of the CoM is that the initiation of the direction change is ‘floated’, i.e., no pressure is generated on the big toe edge of the uphill ski (no “early edging”), so the Centripetal Force is delayed. This is mainly due, in most cases, to a pause at the end of the extension (“up and waiting”), as the skier ‘waits’ to move the CoM.
4. Bipodal Phase
This is the longest stage of the turn in which the support between both feet tends to level out until the cycle is completed and repeated in a new direction change. There is a marked flexion of the inner ankle and knee. The hips continue their movement towards the inside of the turn, maintaining the necessary centripetal posture, mainly carried out by the gluteus and, especially, the piriformis muscle of the outer hip.
The pelvis as a whole tends to remain slightly behind the frontal plane of the ankles or the heels, depending on whether there is a sway-back posture due to an imbalance or if the skier intends to generate more pressure on the tails of the skis to accelerate. The shoulders tend to remain on the same frontal plane as the knees. The arms are forward in relation to the trunk, separated from each other, and are used to compensate imbalanced situations.
