The head
We begin our analysis with the head, given that it plays an important role by orienting itself in advance toward the next destination point, confirming its initial inertial function during a direction change, mainly with visual anticipation.
The upper body
The upper body oscillates slightly or markedly forward to move the Center of Mass (CoM) outside the Base of Support (BoS) when seeking pressure on the front of the outer ski.
Upper body muscles include the torso flexors (abdominals) and the extensors, which counteract the abdominals. The extensors provide the main force to complete trunk’s flexion and extension movements. Both muscle groups control upper body posture.
The pelvis
The pelvis serves to orient the lower limbs in all directions, making our body resemble an inverted cone.
The displacement of the pelvis has two functions. From the Inflection Point (I.P.) at the start of the direction change it moves toward downhill, i.e., depending on the turning conditions, somewhere between the center of the turn and the apex of the actual curvilinear trayectory, facilitating the edge change while generating the centripetal posture. From the apex (fall line) to the end of the turn, the pelvis moves uphill maintaining the centripetal posture.
During a direction change, the pelvis, through both hips, performs a rotational movement around the femurs. The inner hip is brought slightly or markedly forward during the Oscillation Phase.
The pelvis is also used to counteract the centrifugal effect generated by the centripetal force through muscle activation in the core area (centripetal posture). The action of the gluteus maximus, hamstrings and piriformis of the outer leg resist the centrifugal effect that tends to flex and rotate the hips outward; in other words, these muscles allow maintaining the centripetal posture throughout the turn.
During the Neutral Direction Change (which will be developed in a future article), the hip flexor muscles help to pull the femur and knee upward, lightening the weight, or facilitating its reduction, on the standing foot prior to the direction change allowing the downhill leg to flex. The iliopsoas acts in the Oscillation Phase to initiate internal hip flexion, while its contraction shortens the inner leg and propels it forward. The hip adductors (inner leg muscles) are used to keep the skis stable during motion.
Hip extension and flexion through the coxofemoral joint are movements that allow for a wide range of motion. Inner hip flexion brings the front of the inner thigh closer to the trunk, bringing the knee closer to the abdomen, while outer hip extension allows the outer leg to be extended while stretching the hip forward.
The main hip flexors are the iliopsoas, rectus femoris, and sartorius. The main hip extensors are the gluteus maximus and hamstrings (biceps femoris, semitendinosus, and semimembranosus). The tensor fascia latae assists in maintaining the transversal position of the pelvis. The piriformis muscle helps maintain the centripetal posture. In addition, all these muscles assist in stabilizing the pelvis.
The knees
As for the knees, during the initiation of the Neutral Direction Change, the upward movement of the femur initiates the flexion of the inner knee during the Generation Phase, which is brought forward, while the foot remains in contact with the snow on its little toe edge. The quadriceps contracts to flex the inner knee in the Bipodal Phase of the turn. In the active extension Monopodal Phase during the Neutral Direction Change, the future outer knee does not fully extend.
While extending the outer leg to initiate a turn, the quadriceps and sartorius contract together with the vastus medialis, generating a feeling of tension in the medial part of the knee (the inner side closest to the opposite knee), which is still confused with “knee angulation”. This is a visual illusion of a tucked outer knee which, in reality, is not an angulated knee, but rather the outer femur rotating internally.
Feet and ankles
Proper use of both feet and ankles is essential when skiing. The movements of the rest of the kinetic chain depends on the proper utilization of the feet.
To achieve efficient skiing, it is necessary to take advantage and precisely control all the movements available to the foot-ankle system. The ankles are considered indispensable shock absorbers for skiing flexibility.
The feet play a key role as pressure detectors and pressure effectors. Variations in support relocation generate different degrees of pressure on the feet, which suffer deformations in the anterior and inside arches.
Feet’s sole consists of three points of support: the head of the 1st metatarsal (big toe/ball of the foot), the head of the 5th metatarsal (little toe), and the calcaneus (the heel).
The feet, considered as the Center of Pressure (CoP) and through the arches, allow external forces to be received from the ground through joints reaction and body weight.
During the Monopodal Phase of the Neutral Direction Change, the outer/downhill foot, named “standing foot”, supporting itself on the big toe edge, has the task of holding most of the body’s weight and regulating pressure, while the inner/uphill foot increases, maintains, or decreases the necessary edge angle. This is noticed when standing on the new outer ski, the pelvis is moving forward and this supporting foot is immediately behind the hips.
This inner/uphill foot, named “leading foot”, which receives less pressure by supporting itself on little toe edge, is in supination, the ankle in dorsal flexion (contraction of the tibialis anterior and quadriceps muscles) and inversion, i.e., it is in constant tension. All this is facilitated if the inner/uphill knee is flexed and in external rotation, depending on the degree of body inclination required for the turning situation.
The supporting foot-ankle complex (external to the turn) is in eversion, pronation, and plantar extension. At the moment of the direction change, the CoP moves, through an oscillatory ‘rocking chair’ movement of the body, from the outer border of the calcaneus, crossing the inner arch and reaching the first metatarsal (the ball of the foot), through which the monopodal postural function and simultaneous diagonal oscillation are experienced, facilitating the start of the new direction change by taking advantage of the tangential inertial force.
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.
