An important aspect of maintaining balance in sitting and standing is integrating and weighting the sensory information from different sources (touch, proprioception, vestibular and visual) needed for estimating the position of the body as a whole. This integration is reflected in the presence and extent of postural sway.
Carpenter et al's research (2010) suggests that postural sway may be used by the CNS as an exploratory mechanism to ensure that continuous dynamic sensory inputs are provided.
"Humans and other species are unable to stand perfectly still; their bodies continuously sway during stance even during concentrated efforts to avoid such movement. Traditionally, this phenomenon has been viewed as an inability of the central nervous system (CNS) to maintain perfect equilibrium because of its reliance on feedback from sensory signals to control corrective ground-reaction forces. Using a novel method to minimize movements of the body during stance without subject awareness, we have made the unique discovery that ground-reaction forces are generated independent of body sway, as evidenced by observations of increased centre of pressure variability when postural sway is minimized experimentally. Contrary to traditional views, our results suggest that postural sway may be used by the CNS as an exploratory mechanism to ensure that continuous dynamic inputs are provided by multiple sensory systems. This novel paradigm has the potential to significantly shift long-standing views on balance, and questions the theoretical basis behind conventional treatment strategies for balance deficits associated with age and disease."
Observing postural sway as infants learn to balance in standing
Observing infants as they learn to balance in standing clearly illustrates the presence of postural sway as the first learn to balance. Over time the extent of the sway in the lateral and A-P directions decreases to a more "mature" level.
The presence of postural sway is best illustrated by the following short video clips.
Will 9 months: postural sway standing with light hand touch
Will has learned to stand with the trunk erect and the hips in extension, using light touch of the hand on the support surface to aid in balance. In this position the postural sway is clearly seen.
Roam 12 months: postural sway standing with waist high support
In this clip Roan has increased her base of support by leaning against the waist high support surface. Her trunk is tilted forwards and her hips are slightly flexed. Despite the extensive external support afforded by her position, you can see the continuous small range movements of the pelvis along with the larger movements associated with adapting her weight distribution as she reaches for toys.
Carpenter MG, Murnaghan CD, Inglis JT. Shifting the balance: evidence of an exploratory role for postural sway. Neuroscience. 2010 Nov 24;171(1):196-204. Abstract
Murnaghan CD, Carpenter MG, Chua R, Inglis JT. Keeping still doesn't "make sense": examining a role for movement variability by stabilizing the arm during a postural control task. J Neurophysiol. 2017 Feb 1;117(2):846-852 Abstract