Postural control, or stance, can be defined as the control of our body position in space. This control depends on our ability, our actions’ requirements, and the environmental restrictions and possibilities. Proper postural control is essential for our skiing movements’ execution.
To achieve postural control, somatosensory, visual, and vestibular information take action on different areas of our brain. Also, postural control is affected by our volitional or emotional aspects.
By controlling our posture, we try to reduce the difference, if not coinciding, between our body setting and the balancing situation. Its functions are: to ensure balance, provide a stable platform for future actions, stabilize perception, and collaborate in skiing spatial representations processes.
Postural control is our body’s conscious stabilization and automatic balance maintenance by keeping the center of gravity on the base of support to prevent falls (Shaperd (2000). It is achieved through the coordination of sensory information, postural reactions, and feedback or feedforward mechanisms.
Stance control is based on two concepts that complement each other. One conceives control as an inverted pendulum in which we oscillate about a fixed point around our ankles within our ski boots attached to the skis. Oscillations cause our center of gravity to continuously move into multiple linear directions, and this is why we are never in equilibrium.
When adding rotational forces to linear oscillations, our postural control is then compared with an inverted cone, where we attempt to limit rotational oscillations. While skiing, we strive to constantly stabilize ourselves, minimizing these oscillations by keeping the center of gravity within the base of support, using our feet soles pressure centers as references. If the center of gravity moves away from the vertical axis, we move this center of pressure to locate it under the center of gravity in a bottom-up organization.
The second concept interprets that postural control is achieved through overlapping our body segments one above the other in a top-down organization. When body segments are misaligned, balance deteriorates and we increase muscle tension to keep standing, trying to find stability in our postural rigidity.
Posture control includes, according to Amblard (1985), two mechanisms: orientation and stabilization. Orientation refers to the maintenance of the relationship between our body segments and the body with the surroundings. Body orientation in space is the active alignment of our trunk and head to gravity, to the supporting surface, to the visual environment, and to our internal references. Orientation is part of our postural control and requires a representation that could be geocentric (in reference to gravity), egocentric (in reference to ourselves), or allocentric (in reference to space).
Stabilization indicates the conservation of our body position to align the center of mass on the base of support minimizing oscillations. Instability is a basic movement characteristic when skiing. As moving one body segment disturbs other segments producing instability, our stance control is achieved through the use of postural anticipatory adjustments, helping to maintain our postural stability by compensating for destabilizing forces associated with movements of one or more segments.
Berthoz (2013) proposes that to determine our posture and simplify its control, it is enough to define relations between our joint angles rather than the spatial position of each segment. Relating this to our skiing stance, the forming angles starting from flexing hips, knees, and ankles should conserve a certain relation. He suggests also that there are two sub-mechanisms of posture control: one ensuring the point of support or movement foundation, and the other, the muscle tone of the moving segments.
The beginner skier tries to control posture by focusing where the different body parts are and encourage the habit of pondering his next posture. The expert instead focuses on the space between body parts, reaching fluidity by occupying these spaces in which each part needs to be placed.
Posture control has external and internal constraints. External limits are imposed by environmental conditions as gravitational forces, accelerations, decelerations, and reactive forces of the supporting surface. Internal limits are originated from our body as internal forces (muscle contractions) and inertial characteristics of body segments. To control posture, we must take into account these limits, which require conscious learning.
Mechanisms collaborating in posture maintenance
Massion (1984) proposed the following mechanisms while maintaining posture:
- Muscle tone gives a certain rigidity that helps us maintain joints in a particular position.
- Postural fixation preserves the different body segments, balancing internal and external forces.
- Coordination between different body segments are involved in movements and postures.
- Anticipatory postural adjustments adapt to movement performance.
According to Horak (2006), our postural control consists of several sub-components that can be divided into the following categories. A disorder in any of these categories could lead to postural instability.
- Biomechanical constraints such as degrees of freedom, strength, and stability limits.
- Cognitive processes asattention and learning.
- Movement strategies that could be reactive, anticipatory or voluntary.
- Sensorial strategies that include sensory integration and readjustment.
- Space orientation as perception, gravity, terrain surface, vision, and verticality.
- Dynamic control as motion and pro-action.
Skiing postural control also depends on:
- Proprioception.
- The interaction of sensory information systems.
- Motor control functions.
- Adaptive and functional strategies.
- On ankles, hips, suspension, and step strategies.
Aspects that influence our postural control when we ski are:
- Gravity
- Speed
- Environmental conditions like weather and surface of support.
- Our emotional state.
- Injuries or pains.
Postural stability control can be classified in:
- Anteroposterior control, whereby stability is controlled mostly by ankles and hips.
- Medio-lateral control, performed generally by our hips.
- Multi-directional control,being the combination of ankles and hips activity adapted to disturbances in any direction.
Postural control development
Our skiing postural control is organized from two mechanisms in opposite directions: the top-down mechanism is appliedfrom our head towards our feet, and the bottom-up mechanism from our feet towards our head.
In the beginning of our skiing, postural control is achieved by a top-down mechanism since our head stabilization plays an essential role in posture acquisition through visual information. Then, the control is attained in a bottom-up order based on our feet’s sensory development in which podal and proprioceptive information organize our posture. Continues then a return to the top-down organization where our head has a vestibular predominance taking mobility on our shoulders. Later, the organization becomes progressively in a double sense: head-feet and vice versa including pelvic stabilization. It could be said that our feet collaborate in our head stabilization, and in return, our head guides our feet using vision.
