Neck, trunk and limb muscle responses during postural perturbations in humans.

This study examined the EMG onsets of leg, trunk, and neck muscles in 10 standing human subjects in response to support surface anterior and posterior translations, and to plantar and dorsiflexion rotations. The objective of the study was to test the hypothesis that the responses radiating upward from distal leg muscles represent part of a large ascending synergy encompassing axial muscles along the entire length of the body. If these responses are not ascending, then the muscles of the neck, and possibly the trunk, can be independently activated by vestibular, proprioceptive or visual inputs. We analysed the timing of postural muscle responses within and between body segments in order to determine whether they maintained a consistent temporal relationship under translational and rotational platform movement paradigms. Our results did not strongly support an ascending pattern of activation in all directions of platform perturbation. Temporal differences between activation patterns to platform perturbations in the forward or backward directions were revealed. In response to posterior platform translations we observed an ascending pattern of muscle responses along the extensor surface of the body. In addition, responses elicited in the neck flexor and abdominal muscles occurred as early as those of the stretched ankle muscles. This pattern of upward radiation from stretched ankle muscles was not as clear for anterior platform displacements, where early neck flexor muscle responses were observed during the ascending sequence on the flexor surface of the body. Platform rotations caused fewer responses in the neck and upper trunk muscles than translations, and all muscles responses occurred simultaneously rather than sequentially. Probable differences in the stimulation of vestibular and neck proprioceptive inputs and the mechanical demands of the rotation and translation paradigms are discussed.