Friday, October 15, 2010

Anticipatory postural adjustment (APA) & Posture



Objective of this review: To provide the reader basic idea of the anticipatory postural adjustments with spinal disorders. Understanding this topic will lead to appreciate the kinetic chain concepts through understanding of basic postural system operation.

Posture & Poise: Posture is a term to describe shape whether good or bad. Poise is either present or absent at any moment so to describe poise as good or bad is to misunderstand its meaning. The term posture is generally accepted to relate to the dynamic relationship of the body segments in activity. Poise is a state; an ability to maintain appropriate muscle tension at all times in both movement and static positions.

A well-balanced structure is supported and mobilised by gravitational forces with minimal effort. Correct posture is considered vital for health and functioning of the internal organs and all bodily functions. A poorly balanced structure requires inappropriate muscular activity to maintain position and initiate movement. This constant state of activity leads to unnecessary contraction to hold a position and impedes functioning. For example, tight muscles around the torso restrict movement of the ribcage and prevent natural breathing.

Ideal ‘plumb-line’ correct posture can also have poor movement patterns. This is because it is not the shape that is important but how it is maintained. A 'correct posture', i.e, one that looks right, can be achieved with totally inappropriate muscular activity. Physiologist Charles Sherrington once described our systems for maintaining posture as the ‘most uncertain and untrustworthy of all’. This is not encouraging when all corrective methods for improving posture rely on the very systems that are the cause of the problem. The fact that posture can deteriorate suggests the fallibility of the controlling mechanisms.

Core muscles (muscles towards the axis of the movement) have significant impact on postural stability on which movements are superimposed by the dynamic movers that lie in a layer(s) over the core muscle(s).
Anticipatory postural adjustments are a part of any movement, hence let us understand APA.

APA- Anticipatory postural adjustment

When a task to perform a fast, focal voluntary movement coexists with a task to maintain equilibrium in the field of gravity or posture of a limb, feedforward adjustments in the activity of apparently postural muscles are used to counteract the expected perturbing forces. These reactions are generated by the central nervous system in anticipation of a perturbation, and, therefore, they have been termed "anticipatory postural adjustments" (for a review see Massion 1992).

Anticipatory adjustments have been studied in a variety of experimental procedures, including voluntary foot movements (Alekseev et al. 1979; Dietz et al. 1980), trunk movements (Oddsson and Thorstensson 1986), and arm movements in standing subjects (Belenkiy et al. 1967; Bouisset and Zattara 1981; Cordo and Nashner 1982; Friedli et al. 1984, 1988; Riach et al. 1992; Aruin and Latash 1995), as well as in tasks restricted to upper extremities that did not involve maintenance of the vertical posture (Dufosse et al. 1985; Struppler et al. 1994).

Preceding all voluntary movements, no matter how small, a pre-emptive adjustment occurs to prepare for a change of position, known as postural preparation or anticipatory postural adjustment.

We do not start to attempt the act until we have first set up the conditions we associate with the movement, even if unsuitable. You will be generally unaware of these preparatory actions and continue to apply them for every move. The affect of unsuitable preparations on movement can be likened to applying the brake on a car before driving off. Your habit of getting set to lift, run or play a shot could be reducing performance. If you are already in a state of unbalance before you move due to poor employment of muscle, the ensuing preparations for movement will be inappropriate leading to inefficient actions.

Study 1:

CNS uses the same organization of the motor command for the control of both APA (segmental control) & CPA - corrective postural adjustments (whole body posture). But APA can be non-efficient in segmental postural control. CNS use anticipatory postural muscle activities (APMA) in synergy where gravity plays a crucial role.

Chabran et al (2001) in one study concluded central nervous system (CNS) uses the same organization of the motor command for the control of both segmental and whole-body posture: APA and corrective postural adjustments (CPA), which are based on well-organized anticipatory postural muscle activities (APMA), except that APA can be non-efficient in segmental postural control.

Further this study suggests that the CNS uses a sequence of APMA: a postural muscle synergy which is predetermined as a function of the intended direction of the movements and modulates the gain towards certain muscles, in accordance with the gravitational effects, and supports reaction changes.

Study 2:

Separate control of abdominals.

A study by Hodges et al (1999) reveal that transverse abdominal (a core muscle) is separately controlled than other abdominals. APA of Tr Abd do not vary with lower limb movement but co-vary with the other abdominal muscles. Hence TrA may be controlled independently of the motor command for limb movement in contrast to the other abdominal muscles.

Study 3:

Inertia associated with APA, when the time comes, balance the inertia forces due to the movement of the mobile limb therefore counteracting the disturbance to postural equilibrium.

Bouisset et al tried to find out whether APAs are programmed or not & biomechanical consequence of APA. They found APA like others (Chabran et al) corresponds to dynamic phenomena which are centrally preprogrammed. The inertia forces associated with APA may, when the time comes, balance the inertia forces due to the movement of the mobile limb therefore counteracting the disturbance to postural equilibrium.

Hence it is suggested that posturo-kinetic programming could result from a differential sensitivity of the CNS to two biomechanical factors, a linear and a rotational one, which can characterize the perturbation associated with voluntary movement.

Study 4:

Chabran et al (2002) studied whether fatigue of postural muscles influence the coordination between segmental posture and movement or not. They found postural fatigue induced by a low-level isometric contraction has no effect on voluntary movement and requires no dramatic adaptation in postural control.

Study 5:

CNS can also suppress APA

Alexander et al (1998) found in their study in conditions of high stability demands; the central nervous system may suppress APAs as a protection against their possible destabilizing effects. These effects are more pronounced when the direction of an expected perturbation is in the plane of instability.

Study 6:

APAs are less efficient in chr. low back pain (LBA)

According to Mosley & Hodges (2006) sustained decrease in the variability of anticipatory postural adjustments (APAs) occurs when performing cued arm raises following acute, experimentally induced low back pain (LBP). Jesse et al (2009) found people with chronic LBP may be less capable of adapting their APAs to ensure postural stability during movement.

Attempts to postural improvement:

Postural correction is one of the primary interventions of many physiotherapeutic interventions.
By now we are aware that postural muscles reactions start in anticipation of movement whose inertia helps to counteract the perturbing force to equilibrium. One may see the poised & coordinated segmental posture & movements notoriously in the presence of fatigue of the postural muscles because CNS can suppress APAs.

At this point of our discussion, famous physiologist Charles Sherrington’s description our systems for maintaining posture as the ‘most uncertain and untrustworthy of all’ sounds very true. This is not encouraging when all corrective methods for improving posture rely on the very systems that are the cause of the problem.

The fact that posture can deteriorate suggests the unreliability of the controlling mechanisms. Hence to take corrective methods following problems need to be understood & addressed:

(1) functional organization of the postural system
(2) localization of postural functions in the mammalian CNS
(3) postural networks
(4) impairment of postural control caused by different deficits such as vestibular, prorioceptive, kinesthetic etc.

Core stability & posture:

Core stability is the ability of the lumbopelvic hip complex to prevent buckling and to return to equilibrium after perturbation. Although static elements (bone and soft tissue) contribute to some degree, core stability is predominantly maintained by the dynamic function of muscular elements.

Dangers of not having proper core stability:

There is a clear relationship between trunk muscle activity and lower extremity movement. Current evidence suggests that decreased core stability may predispose to injury and that appropriate training may reduce injury. Core stability can be tested using isometric, isokinetic, and isoinertial methods.

A unique approach: Core integration to lengthen myofascia to improve posture

Postural organization is controlled by the central nervous system in conjunction with the skeletal, muscular, and fascial systems. DellaGrotte et al (2008) described a neuromotor re-education intervention for static and dynamic postural misalignment. This DellaGrotte et al (2008) approach is leads to lengthening of myofascia & core integration that leads to improved postural organization.

Appropriate intervention directed for core stability & integration may not only result in decreased rates of back and lower extremity injury but also in postural improvement.

References:

1. Chabran E et al; J Electromyogr Kinesiol. 2002 Feb;12(1):67-79.
2. Chabran E et al; Exp Brain Res. 2001 Nov;141(2):133-45.
3. DellaGrotte J et al; J Bodyw Mov Ther. 2008 Jul;12(3):231-45. Epub 2008 Jul 9.
4. Bouisset et al; Journal of Biomechanics, Volume 20, Issue 8, 1987, Pages 735-742 (Biomechanical study of the programming of anticipatory postural adjustments associated with voluntary movement)
5. Alexander et al; Electroencephalography and Clinical Neurophysiology/Electromyography and Motor Control Volume 109, Issue 4, August 1998, Pages 350-359 (Anticipatory postural adjustments in conditions of postural instability )
6. Willson JD et al; J Am Acad Orthop Surg, Vol 13, No 5, September 2005, 316-325.
© 2005 the American Academy of Orthopaedic Surgeons. (Core Stability and Its Relationship to Lower Extremity Function and Injury)
7. Hodges et al; Neuroscience Letters Volume 265, Issue 2, 16 April 1999, Pages 91-94 (Transversus abdominis and the superficial abdominal muscles are controlled independently in a postural task).
8. Jesse V et al; Behav Neurosci. 2009 April; 123(2): 455–458 (People with chronic low back pain exhibit decreased variability in the timing of their anticipatory postural adjustments).
9. Deliagina TG et al; Physiol Behav. 2007 Sep 10;92(1-2):148-54. Epub 2007 May 21. (Nervous mechanisms controlling body posture).





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