Generally, balance has priority over any other motor task that we perform while skiing. This is known as posture-first strategy, being used in those situations where the risk of falling is high (Shumway-cook et al. 1997).
Basically, balance control has two mechanisms. One is resisting perturbations (imbalances) with the objective of maintaining balance, being the mechanism commonly observed at beginner levels. The other is to induce imbalance to facilitate movements, as expert skiers do.
In the beginner, balance is controlled through constant muscle contraction, causing premature fatigue and making skiing more difficult. Instead, in expert levels, balance control is done through specific contractions that counteract external forces that provoke unbalance.
Posture is essential in balance control. Ideally, posture should be kept central, or better still, slightly forward so the projection of our CoM falls over the anterior part of the feet (metatarsus) and by this, we create the necessary imbalance to disengage from the previous turn.
Physiology of balance
Balance control works based on sensory inputs by the integration of three systems: vestibular, visual, and somatosensory (touch and proprioception). They provide us with information to correct body deviations by sending signals to nerve centers, transmitting them to motor effectors to create responses for imbalance correction.
Balance regulation is a series of reflexes acting when our body is away from the gravitational vertical. Joint, muscle, visual and vestibular receptors detect inclination and cause muscle contractions to straighten it. Peterka (2002) suggested that each system detects an error that indicates a body deviation related to the posture of reference.
The vestibular system detects head deviations relative to the gravitational vertical. The visual system detects head orientations related to the visual world. The somatosensory system, through tactile and proprioceptive receptors, detects body orientation in relation to the supporting surface. Some authors propose that when a system fails or does not apply in certain situations, others compensate.
Bottom-up and top-down control
There are two mechanisms for balance control: bottom-up control works via information coming from receptors located in our muscles, tendons, and joints (proprioceptive system), and tactile plantar information (exteroceptive system). Top-down control is based on our vestibular system. Our central nervous system compares both controls and if it detects that the balance situation is stable, privileges the proprioceptive system, otherwise will rely on the vestibular system.
Predictive and reactive control mechanisms
Regularly, we experience unpredictable sliding variations where friction increases or decreases and we must react as when passing over an ice patch, a bump with loose snow or the suffered acceleration when pressing a ski tail. In these situations, our ability to quickly readjust balance is essential to avoid falling.
Balance control mechanisms are also classified in predictive control (anticipatory) and reactive control (compensatory). In the predictive control, balance is regulated through proactive procedures under anticipatory postural adjustments performed before the destabilizing event. This control is based on the visual system; from which we receive constant information about the environmental changing conditions. This type of control also considers the external forces that affect us, which should be contemplated in advance. We apply this control when inclining our body slightly backward to anticipate the imbalance produced by our skis’ deceleration because of greater snow friction.
In the reactive control, we employ compensatory adjustments after a sudden destabilizing event where we must compensate for the imbalance. This control mode is then reactive because we did not have the opportunity or failed to anticipate the imbalance. Reactive balance control is used because of the failure of the predictive control (Patla, 1997), when we do not perceive or consider the environment and the external forces, omitting the necessary postural adjustments and, as a result, we suffer an unexpected destabilization.
Visual and podal references
To maintain balance while skiing, we may take our furthest body parts as references, like our eyes (visual reference) or our feet (podal reference).
Visual reference is used for orienting posture while podal reference is for regulating it. Specifically, our peripheral vision has a more significant balancing function, being more sensitive to our movements and orientation in relation to the surroundings. Our visual system contributes particularly in correcting lateral imbalances because the threshold is lower in lateral oscillations than in fore-aft oscillations (Paulus et al., 1984).
When we ski in poor visibility conditions, visual information is not sufficient and the vestibular, tactile or proprioceptive information may fail to compensate. For this reason, we lose balance or feel unstable, triggering the fear of falling. In addition, under these conditions, visual field, peripheral vision and depth perception affect our visual control of balance.
If we prioritize our posture control by visual information, we lose stability in fog or flat light because of the difficulty to distinguish between vertical and horizontal environmental references, as well as slope inclination references. Instead, we should use our somatosensory system for balance maintenance, and rely on the information coming from this system.
In podal reference, which is frequently misestimated, our feet set the single area of body contact with the snow through our boots and skis. Our feet have a very important purpose because the sensory activity they generate has a direct effect on our postural balance.
Our feet have a double effect: they are effectors and at the same time detectors. They are effectors because they produce the desired effect (edging control, pressure control, and steering actions); and they are detectors because of obtaining sensations through sensory receptors. The lack of conscious plantar contact gives origin to a sensorial deficit affecting our stability, as commonly occurs at the initial learning stages.
We can conclude then that the primary inputs contributing to our balance maintenance while skiing, in addition to global proprioception, are found in the extremities of our body.
