Posture
Posture is defined as actively creating internal forces to hold the body’s segments against external forces of everyday life. In general, gravitational force has the greatest significance3.
Posture is frequently misconstrued and used as a synonym to “erect standing on two extremities” or to sitting, but this is not the case3.
Movement Prerequisite
Posture is the main component required for movement3.
We can divide any movement into “freeze frames” where the individual is holding different static postures3.
We can divide each of these “freeze frames” into 3 postural categories:3
Ideal Posture
| Segment | Positioning |
|---|---|
| Head | Neutral |
| Cervical spine | Curvature slightly lordotic (convex anteriorly) |
| Shoulder blades | Retraction and depression (Adhere to chest wall) |
| Thoracic spine | Slightly kyphotic (Curvature slightly convex posterior) |
| Lumbar spine | Slightly lordotic (Curvature slightly convex anterior) |
| Pelvis | Neutral (anterior superior iliac spines lie in same vertical plane as the symphysis pubis) |
| Hip joints | Neutral |
| Knee | Neutral |
| Ankle | Neutral (shins perpendicular to the plane of the feet) |
Categories
Postural Stability
In a “static position,” the body as a whole is static and does not change its position, but each joint is undergoing dynamic processes3. When holding a static position, we are in a constant state of losing the fight to external forces then overcoming those forces to return to the static position3.
You can view this in two ways:
In a therapeutic environment, when holding a static position, we can still work on dynamic motions of a joint.
In a pathological sense, an individual can perform a dynamic movement of the whole body, but a dysfunctional joint can be static and not allowing for dynamic processes.
Postural stability is influenced by both biomechanical and neurophysiologic factors3.
Biomechanical factors:
- Base of support: The part of the surface that is in direct
- Center of mass (COM):
- Support base: The whole area bordered by the farthest margins of the support area
Stability is directly proportion to the size of the support base and weight. Stability is indirectly proportional to the height of the center of gravity above the support base, the horizontal distance between center of gravity and the support base and the angle of base of support3.
\[ S \propto \frac{(B \cdot W)}{(H \cdot D \cdot \theta)} \]
where:
- ( S ) = Stability
- ( B ) = Base of support size (width, length, or area)
- ( W ) = Weight (mass)
- ( H ) = Height of the center of gravity (CoG) above the base of support
- ( D ) = Distance from the projection of CoG to the center of the support base
- ( ) = Angle of the base of support from the horizontal plane
\[ \sum F_{\text{external}} = W + M + F_f + R \]
where:
- ( F_{} ) = Total external force acting on the body
- ( W ) = Gravitational force (weight)
- ( M ) = Momentum force
- ( F_f ) = Friction force
- ( R ) = Ground reaction force
For stability during locomotion:
\[ \sum F_{\text{external}} \text{ must project into the base of support.} \]
Postural Stabilization
“Postural stabilization is understood to be an active (muscular) holding of body segments against the activity of external forces controlled by the CNS. This muscle activity holds body segments (active segment holding) against the action of external forces (especially gravitational force). During static conditions (in standing, sitting, etc.), a relative tightness of joints is achieved via muscular activity, which is coordinated by the activity of agonists and antagonists (co-activation activity). This activity also allows for resisting gravitational force in a given position. The tightening of body segments enables the achievement of an erect posture and locomotion of the body as a whole (an analogy can be seen in an experiment where we are trying to erect a wooden wand and a chain or a board and a net). Without coordinated muscle activity our skeleton would collapse - hence postural stabilization. Postural stabilization does not only act against gravity, but it participates in all movements, even movements involving only the lower or upper extremities.”3
Postural reactability
Interventions
- Endurance parameters for posture should be performed -msk
- Best to start at the base/bottom up -msk
Automatic posture
- Reticulospinal tracts functions in automatic posture4