Anticipatory adjustments are feedforward motor controls to predictable imbalances based on experiences of similar situations. They are executed by displacing our feet forward, backward or sideways as the palm of a hand would do at keeping a pole vertically in the air. For example, to move from compact to deep snow, we push our feet forward or pull our body backward, to anticipate sudden speed decreasing caused by the increased snow friction. The same example can be taken in reverse: going from deep to compact snow where skis accelerate because of friction decrease. In this situation, we will slightly pull back our feet or move our body forward. The purpose of these resources is to limit and compensate our body oscillatory excesses.
As our balance is unstable, we must constantly compensate imbalances, when failing in anticipating them, so at trying to reset our center of mass after these imbalances, sensory signals emerge from the central nervous system, triggering compensatory adjustments (Horak & Nasher, 1986). In unexpected imbalances, different postural adjustments, or motor strategies, are normally applied to relocate our CoM over our BoS. According to these authors, to maintain balance, our nervous system selects and plans adjustments or strategies.
After experiencing an imbalance, a sudden and unexpected movement is generated, causing a reactive control of the postural system based on two types of compensatory adjustments: change of support and feet in place. In the first one, our feet are consolidated to the ground to recover the CoM by body oscillation. These are divided in turn into the ankle, hip, and suspension strategies.
Compensatory adjustment of the ankles or Ankle strategy
In scientific investigations on reactions about balance maintenance on a mobile platform, it was noticed that the majority of people reacted the same way: first with an initial reaction of feet muscles, then with thighs musculature, and finally with the muscles of the trunk. This adjustment compensates for low amplitude imbalances, being the initial response of the postural system against imbalance, causing ankle muscles to resist movement so the CoM returns over the BoS (Kuo & Zajac, 1993).
We are prone to tense our ankles when perceiving unreliable signals coming from unstable balance situations. This type of adjustment requires the activation of the body’s rear musculature for anterior destabilizations, and front musculature for posterior ones. Due to ski boots stiffness, the beginner does not know how to employ ankles strategy yet so tends to use hip strategy to stabilize himself.
Compensatory adjustment of the hips or Hip strategy
On a narrow support surface as the top of a mogul, we tend to activate first the pelvis with thighs and buttocks. This adjustment compensates bigger amplitude imbalances. It is performed if ankles adjustment is not possible or insufficient and in dynamic imbalances on a reduced BoS where movements of several joints are used to carry the CoM within the BoS (Nasher et al., 1989).
In the case of the beginner skier, who is in the adaptation process of ground inclination and the slippery element, he compensates for the lack of plantar sensitivity and boots’ restriction with hip movements to control imbalances. This strategy is effective when the CoM is quickly shifted (Winter et al., 1998).
Compensatory adjustment by flexing or Suspension strategy
Nashner & McCollum (1985) observed an additional strategy, calling it suspension strategy which is performed by flexing ankles, knees, and hips with the aim of bringing the CoM towards the BoS.
Compensatory step or Change of Support strategy
This strategy is used when our balance falls out of the BoS, then we recover it by taking a step, moving a ski or supporting over one or both poles. It is used when ankles or hips strategies are insufficient to recover balance. The step or displacement of one ski could be sideways, backward or forwards and could be applied to compensate little or large imbalances. A very common example is when we lean too much towards the inside of the turn or the uphill side of the slope when skiing at low speed: our CoM is projected outside our BoS and noticing that our balance is compromised, we make a step or lateral displacement with the inner or uphill foot or lean on our pole.
Lateral stability
In lateral or medial-lateral imbalances, due to our limitation of lateral movement at the ankles because of boots’ height, we recover stability using hips and upper body in large imbalances, and knees in reduced imbalances. Studying transversal imbalances, Winter et al. (1998) noted that organizational responses are descending (top-down processes), i.e., we first adjust head movements, then hips, and finally our ankles.
Final considerations
Strategy refers to a planning process and movements execution oriented towards a goal, which in our case is the recovery of equilibrium after a perturbation. Ankle, hip or suspension strategies are different ways to achieve that goal, maintaining the CoM location in relation to our feet.
Ankle strategy is associated with all of our body moving around our ankles (pendulum or inverted cone mechanisms) and it is aimed at maintaining ourselves stable from below all the way up, according to a compensatory bottom-up process. Hips strategy, flexing or extending the hips in response to fore-aft body oscillations, is likely to stabilize ourselves from up towards down in a top-down compensatory process. To recover stability, the beginner uses mainly hips or change of support strategies, while the expert skier employs them all.
In reduced sliding imbalance in spatiotemporal conditions, we tend to use ankle compensatory adjustment, otherwise we apply hips compensatory adjustment. In relation to the BoS, if it is reduced, we will likely use hips compensatory strategy; instead, if it is wider, we can compensate the imbalance with our ankles.
Selecting strategies usage is influenced by our previous experiences and/or by current sensorial information, so we may employ the combination of both strategies in particular surface conditions.
