Postural adjustments in sitting humans following external perturbations: muscle activity and kinematics

Abstract

There are several controversies concerning the organization and induction of postural adjustments in standing humans. Some investigators suggest the responses are triggered by somatosensory inputs (especially from the ankle in standing subjects), while others emphasize the vestibular input induced by head acceleration. We examined postural responses in sitting subjects in order to describe the muscle activation pattern during various perturbations and to test whether somatosensory or vestibular stimulation elicited the responses. The kinematics and EMG patterns in response to perturbations caused by movements of the support surface were studied in adults. The postural muscle activation following a backward sway was mainly the same, whether it was elicited by a forward translation or a legs-up rotation. This is remarkable, since, except for pelvis rotation, the movements of all body segments including the head differed in the two conditions. Furthermore, a second experiment showed that the direction of the initial head movement could be reversed with retainment of the same postural muscle activation pattern. The results suggest that somatosensory signals derived from the backward rotation of the pelvis, and not vestibular information from the head, trigger postural responses during sitting. There was a slight but consistent difference in the muscle activation pattern, whether the backward sway was elicited by a forward translation or legs-up rotation. The difference seemed to reflect the sensory information from head and other body parts (except the pelvis). This finding allowed us to speculate in a central pattern generator for postural adjustments containing two levels. At the first level, a simple format of the muscle activation would be generated; at the second level, the centrally generated pattern could be shaped and timed by interaction from the entire somatosensory, vestibular, and visual input.

Cervical electromyographic activity during low-speed rear impact

“…A central response, involving the central nervous system, as opposed to a stretch reflex, was suggested in a study monitoring the activity of lumbar extensors and neck extensors and flexors [4]. Later, Szabo and Welcher presented a study that supported the idea of a central response, and concluded that muscle activities in different parts of the body occurred at approximately the same time and were not dependent on acceleration or movement of that area of the body [23].…”

Age-related Changes and Sex Differences in Postural Control Adaptability in Children during Periodic Floor Oscillation with Eyes Closed

“…Age-related changes and sex differences in postural control adaptability Forssberg and Hirschfeld (1994) distinguished two functional levels involved in the development of postural control to externally triggered disturbances. The first level is the selection of basic postural synergies, and the second level is the regulation of basic synergies based on multisensory information.…”

The Neuronal Group Selection Theory: a framework to explain variation in normal motor development

“…the epigenetically determined boundaries of direction speci-ficity. Direction specificity denotes the mechanism primarily to activate the muscles on the dorsal side of the body when the body sways forwards, and primarily to activate the ventral muscles when the body sways backwards 67 . Indeed, already before infants can sit independently, at 4 to 5 months, the postural activity of neck and trunk muscles is direction specific 44,64 .…”