Posture and support surface
In our daily activities as human beings we are accustomed to walk noticing a particular friction between our shoes and the ground surface, so we adapt our posture according to this reference. While skiing, friction decreases causing slipping so because of this our walking posture must be adjusted.
To adapt to the snow surface our responses tend, initially, to be similar to those used in non-slipping conditions. With sensorial training on discriminating the snow’s slippery properties, our ability to regulate posture will improve. Even having a proper body posture perception on firm ground, in skiing we must re-educate it. From the first moments, our brain organizes movements and actions so posture can adapt to the sliding element.
Controlling the degrees of freedom
To control our posture while skiing we must control the degrees of freedom of our different body segments, which is a complex task due to the multiplicity of possible movements. There are two types of control: block control consists of blocking or restricting joints to minimize the number of degrees of freedom which need to be simultaneously controlled over the course of our movements’ execution, allowing in our initial skiing stages to reduce the number of variables to control. The articulated control consists of independently controlling the immediate limbs (anterior and posterior) of the corresponding joint.
Postural oscillations
We can define an oscillation as the measure of time and distance in which we remain away from our ideal balance posture. As it has been said, when skiing our body is considered as an inverted pendulum or cone that oscillates over our ankles and the control of these oscillations is used by us for the execution of other movements.
Skiing postural control is not only limited to minimize our postural oscillations, since some situations require that body oscillations should be reduced, like skiing in deep snow or bumps, and others are instead provided when our body, at the beginning of a turn, oscillates slightly forward and towards the inside, whereas at the end of the same turn oscillates back towards the center of our feet seeking a centered stance.
Postural tone and postural schema
The postural tone is the tonic activity of the antigravity muscles in order to keep our body in an upright position. It relates directly to our body axis and it should be sufficient enough to resist gravity but convenient to allow movement.
The postural schema is the perception of our body position in relation to its vertical and horizontal axes. As skiing postural control needs a mental representation or internal model control, we build it gradually and adjusts it actively by becoming conscious of it.
We obtain our postural schema representing the image of the external form of our own skiing stance, integrating sensory inputs and mobilizing motor forces. Also, we perceive forms and external lines of other skiers’ postures and, unconsciously, we partially integrate them in our postural schema. Body tilting will create asymmetric body tensions that induce perceptive changes in the vertical and horizontal planes, which will modify our body schema.
Influence of vibrations in posture
Vibration is the transmission of waves that cause postural strain and muscle tension. This spread depends on vibrations frequency and body posture.
Skiing is subjected to vibrations which if they are of low frequency will be attenuated by us but if they exceed a certain limit will lead to postural control disorders. This control depends on our nervous system to detect information that our feet are sending since they are the only points of contact with the ground.
Skis vibrations stimulates our feet soles promoting an illusory perception of imbalance. When significant vibratory interferences in feet soles or ankles occur, they can spread and affect our neck muscles then, our vestibular system will be disturbed, altering our postural control.
Neck muscle vibrations affects the accuracy of our vision and tends to deflect our perception of motion direction. It also alters our postural schema and motor performance in general. In addition, when vibrations reach our head, our vestibular system emits discordant information between the otoliths, that detect accelerations, and the semicircular canals, which sense angular accelerations.
Neck muscle vibrations may also induce a front tilt of our body due to the illusory perception of the elongation of these muscles. This forward tilting effect is produced by a reactive correction to the perception of the posterior tilt (Kavounoudias et al., 1998).
Body and spatial references for postural control
For skiing postural control, we may utilize the following references:
- Head reference, stabilizing it in space employing visual and vestibular information as well as neck muscles tone, which help us perceive the gravitational vertical in a top-down organization from head to toes.
- Feet reference,taking it from our feet support in a bottom-up organization from feet to head.
- Pelvis reference in terms of its stabilization in space to control our center of gravity in a double organization: from pelvis to head and from pelvis to feet.
In relation to spatial references in an urban environment, our postural orientation consists mostly of vertical and horizontal visual references. In the mountains, to these references, we should add the oblique references, so we must integrate them into our body schema for postural control.
Righting reflex as a reaction to slants
As human beings, we are used to maintain an upright posture on horizontal and flat surfaces. In our first experiences in the mountains at placing laterally on a slope, we perceive a tilted posture.
This generates a disturbance in our vestibular system that triggers a righting reflex with the following characteristics:
- Our neck will flex uphill and against gravity, getting our head away from down the hill.
- Almost at the same time there will be a lateral flexion of our upper body up the hill.
- Our downhill leg will get away from our body and our downhill arm will get away from our upper body by extending it.
- If slope tilt increases, our upper body will tend to rotate uphill, our downhill arm will cross in front of our body, and our downhill leg will also move forward.
