Postural adjustments are regulations prior to movements execution carried out in order to stabilize our body by activating various muscles, initiating their activity before moving a body segment.
They are muscle contractions and de-contractions made to remain stable and for this, we face two antagonist restrictions: one is to move a body segment or segments involved in the action towards the goal, and the other is to stabilize the segments that are not directly involved in the action. These settings are used to prevent or minimize oscillations of the center of gravity with the aim of achieving efficient movements (Frank & Earl, 1990).
Our nervous system promotes the center of gravity to move automatically when we decide to move an arm or leg in a protective attitude to support a suited stance to the action we want to perform.
Classification of postural adjustments
The compensatory postural adjustments, or reactive mechanisms, are employed through feedback mechanisms which are activated by sensory information caused by the loss of the desired posture. In this case, we recover postural control after a not expected disturbance.
The anticipatory postural adjustments are proactive or feedforward mechanisms caused by predicted disturbances that generate muscle responses to aid stability maintenance. Such is the case when we modify our posture ahead of time in relation to a potentially destabilizing situation, as the initiation of a change of direction or turning over a bump.
These settings are generated prior to the onset of voluntary or automatic actions, with the aim of reducing postural disturbances produced by the action. They allow an action initiation creating an initial imbalance as, for example, bringing the body forward to allow turn initiation.
The proactive mechanism relies on visual information to predict trajectories, estimating disturbance anticipation forces. In general, the beginner makes use of reactive postural control or compensatory mechanisms, while the expert skier employs proactive or anticipatory mechanisms. As an example, when we raise our arm to prepare our pole plant, our muscles in the trunk are activated in advance to the muscular activity of the shoulder and arm.
Muscle contractions in the trunk and legs are an anticipatory postural adjustment because they precede the main movement of the arm. Another common example of the compensatory postural adjustment occurs when at turn initiation, we release the pressure of the downhill foot, automatically compensating our stance by locating the center of gravity above the uphill foot without changing the edge of the uphill ski. This strategy makes it difficult to start turning since the upper body is placed vertically over the center of pressure (the uphill edge of the uphill ski), delaying edge changing. Because of this, we tend to rotate our shoulders to start guiding our skis. In the same situation, the anticipatory postural adjustment occurs when, while shifting our support from one foot to the other, we project our pelvis in advance towards the inside of the future turn, assuming a proper centripetal posture.
To change our stance, we must take into account the following factors:
- Mechanical factors describe the limits determined by external forces that are generated while skiing, related to joint mobility and movement amplitude.
- Sensory factors refer to, as posture modification causes sensation changes. This information contributes to posture organization we are looking for.
- Energy factors getinvolvedwhen our posture is not efficient because it gets away from stability and generates energy translated into more muscular effort.
- Bodily factors refer to when we must be conscious of body segments to be modified, so we should get to know our body first (corporeal consciousness).
- Balance factors suggest thatthe goal of our posture modification is aimed at improving balance.
- Muscle tone factors relate to situationswhere muscle tension increases, sensitivity is reduced. This makes us not being aware of minor postural deviations, tensing up, and not noticing the extra effort.
