The significance of sensorial information in postural control
Each skiing situation requires a specific posture originated by sensors’ activation distributed throughout our body, triggering a series of reactions. The first sensation perceived when skiing is our muscle tone degree while opposing gravity. While moving through changing environments, our nervous system constantly updates the sensory information process on our posture estimation.
Stance control requires sensory coordination, including the combination of visual, vestibular, and somatosensory systems. When standing on a firm surface, we generally rely on 70% on somatosensory, 20% in vestibular and 10% on the visual system. If the supporting surface changes to an unstable, the importance of our vestibular and visual systems rapidly increases (Peterka, 2002).
Our somatosensory system makes use of information coming from pressure receptors in our feet (plantar receptors) reporting how support is distributed. In addition, proprioceptive information is essential because it gives us all the necessary information about our joints and body segments positioning. If somatosensory information of our feet and legs is reduced or insufficient, it alters our postural responses. If we suppress this information, we will risk imbalance. In foggy or flat light conditions, visual information is not accurate so we compensate it with somatosensory and vestibular information.
Head function in postural control
Proper head placement stabilizes our posture since it provides an inertial unit to monitor the gravity. Head stabilization allows simplifying considerably sensory information processing (Pozzo et al., 1995). Our references to head stabilization comes from our gaze, the gravity or our trunk’s longitudinal axis.
There are three types of sensors in our head: the vestibular system, sensible to the force of gravity and the inertial forces; the visual system that allows stabilizing head and body related to space; and the neck proprioceptive muscles that report head position associated to our trunk. As our eyes are postural detectors, it is essential that our head is kept stable to maintain our gaze horizontal stability.
Stoffregen (1986) indicates that head stabilization facilitates optical flow applied in motion perception. Pozzo et al. (1995) propose that head stabilization facilitates the combination between visual information of position and movement and the information from the otoliths on gravitational and inertial forces; also, head stabilization improves visual perception and vestibular information. Head tilting alters gravity receptors’ sensitivity, which detects body verticality (Schöne & Udo of Haes, 1968), and otoliths sensitivity decreases.
Keeping a stable head in skiing collaborates in balance maintenance and provides a reference to the perceptual systems; instead,tilting it modifies our visual references. When these references are not sufficient, or when they are not properly used, we tend to keep our head in the same inclined direction of our body. Research indicates that we experience an error increase in the subjective vertical perception when our head is tilted. It can be concluded that it is more complicated to ski when our head is tilted because it provides less reliable information and modifies our visual scene.
Perception of posture verticality
Skiing involves continuously deviating from the vertical posture to which we are used to, so the perception of verticality is essential in our posture control. We have an internal model of gravity, which is based on information from the visual, vestibular and somatosensory systems, contributing to the construction of references applied to represent verticality. Gravity is an external reference during skiing motion since it allows defining the absolute vertical.
When we move, we suffer accelerations and decelerations so the vestibular system, especially the otoliths, may not correctly detect the orientation of gravity in each situation. To compensate for this drawback, our brain elaborates vertical representations to help control spatial orientation and posture. These representations are the following:
- On the basis of the gravitational vertical as the only absolute vertical, we use as references the perception of the behavioral vertical, also called physiological or spontaneous postural vertical, which is body position in relation to the anti-gravity posture reaction, i.e., to have an upright sensation, we adapt our stance against gravity.
- The subjective or apparent vertical is the estimation of gravity direction and concerns all our body. Several authors suggest that we build the subjective vertical as a longitudinal body axis or ‘idiotropic vector’, taking as reference the appreciation of the gravitational vertical of the place we are and the orientation of our body related to that vertical.
- In the visual vertical reference, we maintain our posture based on spatial visual orientation and establishes an imaginary vertical line comparing it with the gravitational vertical. Tactile sensations are utilized to determine the haptic vertical (plantar sensations derived from our feet) when the visual vertical is not accessible or it is disturbed because of poor visibility or flat light effects. The beginner skier adopts posture taking as a reference the gravitational vertical axis (upright trunk posture), while the expert does it around his longitudinal body axis that is perpendicular to the plane of support (inclined trunk posture).
Postural restrictions due to ski boots characteristics
Postural restrictions due to ski boots are disturbances that restrict or alter our skiing posture. Using ski boots and skis expand our base of support, facilitating skiing posture maintenance, but ski boots have restrictions affecting postural control because of their height and hardness.
On the one hand, they benefit ankles lateral support but on the other, they restrict their anteroposterior mobility. In the beginner, this rigidity makes it difficult for ankles’ strategy application to balance recovery, forcing him to rely on hips’ strategy. This makes him assume inappropriate postural tactics to compensate for ankles’ mobility limitations. In addition, boots weaken pressure sensitivity coming from the ground, which is an important source of information for movements’ appropriate execution (Noe & Paillard, 2005).
The podal function
Our feet are an essential reference for our postural control. The tactile feedback of our feet soles are used by the posture control system to control balance (Kavounoudias et al., 1998). Certain plantar areas inform the nervous system about our body position in relation to the vertical reference, and this is considered the base of our postural system because of its function as a nexus between ground and body.
With 80% of the legs sensory receptors, our feet are considered as postural regulators since they detect displacements of the center of pressure. When feet and legs are not participating in postural control, we increase the reactions of head, trunk, and arms.
Thanks to plantar mechanoreceptors, we perceive the limits of our body oscillations in an anteroposterior as well as in a lateral sense. By detecting modifications of our plantar center of pressure, these mechanoreceptors act as a protective postural reflex (righting reflex), activating mainly lower leg muscles, controlling postural oscillations to return to a stable stance, preventing a potential fall.
Also, our feet are essential postural captors and the interface between our body and the ground. They are the mechanical sensors that provide information such as snow consistency, terrain inclination, pressure control and edge grip.
