Although the term “neutral” may be interpreted as something passive, controlled flexion and extension movements are performed since the vertical separation between the CoM and the CoP tends to vary slightly, unlike the marked displacements caused by the extension or the flexion direction changes. The Neutral Direction Change is considered a combination of the extension change and the flexion change, or an advanced version of the latter.
The purpose of this type of direction change, also called “Middle Transition”, “Cross-through”, or “Direct Edge Change”, is to absorb the Inflexion Point (I.P.), i.e., the point at which the curvature of a curvilinear trajectory changes sense by means of a precise and quick edge change.
In this form of direction change there is no gradual sagittal alignment between the CoM and the CoP, like in the Direction Change by Flexion, since at the end of the turn, approximately ¾ of the way along the curved trajectory, after passing the Critical Point (the point where the speed in the turn is maximum and therefore the external forces supported by the skier are also maximum), the CoM moves explosively towards the new turn (as a reference, towards the tip of the future inner ski).
We are talking about a fore-diagonally translation of the CoM, which is tangential to the curved trajectory, in order to exert pressure on the front part of the outer ski. This translation is oriented towards a combination of the frontal and sagittal body planes, i.e., an intermediate direction between the two which, depending on the skier’s intention or needs, will tend towards one or the other plane.
In the Neutral Direction Change, the skier’s intention is to change the edges just before weight shifting, that is, trying to extend the uphill supporting leg over the big toe edge already on the snow (active pressure on the ‘new’ edge known as “early edging”) to compensate for the tendency of the pressure to decrease in the Depression Phase, where the trajectory of the CoM splits in the sagittal plane from the trajectory of the CoP (from the I.P. to the fall line or apex).
Here, the strategy to change direction is release-to-engage, this is, releasing the downhill ski to engage the uphill ski.
This type of direction change is effective and also efficient because, by absorbing the I.P., a quick transition is achieved at the end of the turn to economize muscular effort by taking advantage of the generated inertial forces, especially the tangential force, to simultaneously produce a new centripetal force as early as possible.
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.
Biomechanical phases
Generation phase
This is the exact moment when a rotational retraction of the supporting leg occurs to disengage the edges and initiate the new centripetal force in the opposite direction. In other words, the support on the big toe edge of the standing ski at the turn’s ending is instantly removed, allowing the CoM to move towards the new turn, while generating a new edge-snow reaction that is completely opposite to the previous one.
The objective of this phase is the active absorption of the Inflection Point (I.P.), releasing the pelvis from the present centripetal posture, so that it is projected fore-diagonally towards the new direction. From neurophysiological and biomechanical points of view, this refers to the precise moment when the nerve impulse is triggered, resulting in a specific muscle contraction of the supporting downhill leg, to trigger the direction change through a quick and precise edge change.
Neurophysiologically, this nerve impulse, as an action potential, is generated in a motor neuron and travels along the axon until it reaches the neuromuscular junction where it connects with the muscle fiber, resulting in its shortening (the downhill leg shortens slightly or markedly depending on the skier’s intention or needs), which produces the necessary movement for the intended action, that is, to release the current centripetal posture in order to generate a new one in the opposite direction.
Biomechanically, this occurs at the intentional moment of the direction change through an active and timely relaxation of the downhill leg extensor muscles -glutes and hamstrings- at the end of the turn, followed by an active flexion of the knee through the main control of the quadriceps, the inversion of the ankle, and supination of the foot with the corresponding dorsal flexion through the activation of the tibial muscles of the leg and the dorsal muscles of the foot. At the same time that the downhill leg contracts and shortens, the uphill leg starts contracting and lengthening through an auxotonic contraction.
Monopodal phase
This phase begins when the ball of the uphill foot makes contact with the snow through ankle eversion and foot pronation. In this phase, the pelvis begins to move fore-diagonally in the direction of the new turn, going over the feet and generating a new centripetal posture thanks to an auxotonic uphill leg extension.
An auxotonic contraction is a type of muscle contraction where both the tension and the length of the muscle change simultaneously as it encounters increasing resistance. It combines characteristics of isometric contractions (the muscle contracts without changing length) and isotonic contractions (the muscle contracts with a change in length). In this type of extension, the muscle contracts generating force, slightly changing its length to adapt to the demands of the movement.
The auxotonic leg contraction is functional because it provides a more accurate model for real muscle function, as pure isometric or isotonic contractions are rare in daily skiing.
Oscillation phase
In this phase, there is an active but gradual oblique extension of the uphill knee, mainly controlled by the hamstrings, and a specific support on the first metatarsal (big toe/ball of the foot) of the now standing foot with a slight or marked extension of the ankle (plantar extension of the foot).
Continuing with the oscillation, pressure is transferred through the plantar arch (internal arch) of the standing foot, which deforms by lengthening. The pelvis continues its fore-diagonally displacement generating the centripetal posture, which is carried out mainly by the outer hip piriformis muscle. The inner hip moves slightly or markedly on the transverse plane with respect to the external hip.
While oscillating, or ‘rocking chair’ movement, the CoP is initially located on the outer heel of the uphill foot (toward the tail of the ski), which is slightly supinated. Pressure then moves quickly toward the medial area of the foot (over the middle of the ski) and continues forward until it reaches the forefoot, i.e., the first metatarsal or ‘ball’ of the foot (toward the tip of the ski).
Bipodal phase
This is the longest phase of the turn, during which the support on both feet tends to level out until the cycle is complete and the generation phase is repeated in a new direction change.
There is a slight or marked flexion of the ankle and knee of the inner/uphill leg, depending on the situation. The leading foot remains slightly or markedly pressured on the little toe edge, depending on snow conditions, skier’s intention or needs, or movement tendencies, and in a marked dorsal flexion.
The inner/uphill hip is slightly or markedly forward in relation to the outer/downhill hip. Both hips tend to remain on the same frontal plane as the ankles or heels, depending on the situation. The pelvis accentuates the centripetal posture through the work of the glutes and, mainly, the piriformis muscle. The shoulders are kept on the same frontal plane as the knees. The arms are forward in relation to the trunk and functionally separated from each other.
