Changes in Posture Control Across the Life Span- A Systems Approach
In this article, a systems approach to the development of posture control across the life span and its integration with voluntary tasks such as walking is described.
Research shows a clear cephalocaudal gradient in the development of postural responses. Postural muscle synergies develop appropriate temporal organization through experience in each new level of postural skill development. Sensory inputs contributing to posture control influence postural responses very early in development, with responses being elicited by vision alone, or by somatosensoy and vestibular cues in isolation. Studies of older adults indicate small, but significant, increases in onset latencies and disruptions in the temporal organization of postural muscle responses when subjects are given external threats to balance. In addition, older adults, like young children, use antagonist muscles more open in coactivation with agonist muscles. Older adults also have more difficulty balanc-
ing when sensory inputs are reduced experimentally or pathologically. Ankle dorsiflexor muscle weakness is also a factor in balance dysfunction in the older adult.
Summary and Conclusions
These studies on the development of posture control across the life span and its integration with voluntary tasks
such as walking show a number of interesting principles of developmental progression. First, infants show a clear cephalocaudal gradient in the development of postural responses, with control first appearing in the muscles of the neck, then the trunk, and finally the legs. Data show that postural muscle synergies develop appropriate temporal organization through experience in each new level of postural skill development. Muscle strength changes may also contribute to the development of postural control, but few data are available on this aspect of postural development. We believe sensory inputs contributing to posture control may be able to influence postural responses very early in development, with postural responses being evident if influenced by vision alone, or by somatosensory and vestibular cues in isolation. Studies on the older adult indicate small, but significant, increases in the onset latencies and disruptions in the temporal organization of postural muscle responses when subjects are given external threats to balance. In addition, older adults, like young children, use antagonist muscles more often in coactivation with agonist muscles when balancing. Older adults also have more difficulty balancing when sensory inputs contributing to balance control are reduced, so that they have less redundancy of sensory information. Thus, when both somatosensory and visual inputs are made in congruent with postural sway, the older adult shows significantly increased sway compared with the young adult, and many older adults lose balance completely. This characteristic is also similar to that seen in young children. Muscle (ie, ankle dorsiflexor) weakness may also be a factor in balance dysfunction in the older adult. Given the many similarities in functional capabilities of the different systems contributing to balance control in the child and the older adult when compared with the young adult, do these results support the strict vertical hierarchy hypothesis [hat as children mature, higher nervous system tenters take over function from more primitive reflex systems, and that as adults age and higher centers deteriorate, lower-level systems begin to show functions that reemerge? A though there are limited data to show that there is some emergence of spinal reflexes in the older adult, all other similarities in function between the different musculoskeletal and nervous subsystems can be explained by developmental changes in functional status of each system independently. There is no need to invoke the existence of a strict vertical hierarchy. For example, the similarities in use of antagonist muscles along with agonists in posture control in the two age groups (children versus older adults) simply imply that each may use the agonist-antagonist coactivation to stiffen the ankle joint and thus limit the degrees of freedom needed for postural control. This is a typical strategy found in any motor skill when function is not optimal; it is not an indication of a “lower level” of the vertical hierarchy reemerging in dominance. The systems model can be used to evaluate changes in the different systems contributing to balance control across the life span by asking questions such as: When the function of one system contributing to balance control is unavailable, what other systems can compensate? Are there specific environmental conditions that threaten balance control when specific systems are impaired, and can these conditions be avoided? and Can balance strategies be modified to improve balance function when a specific system is no longer functioningat optimal levels? Thus, this model has great flexibility and great potential in contributing not only to our understanding of balance changes across the life span, but to therapeutic interventions in the child or the older adult with balance dysfunction. However, our understanding of the clinical implications of many of the experimental findings has only recently been explored. As a result, effective approaches to assessment and treatment of some types of postural problems identified through systems research are still limited.
[Woollacott MH, Sbumway-Cook A. Changes in posture control across the life span–a systems approach. Phys Ther. 1990; 70: 799407.1
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