Affiliations: Departments of Physical Therapy and Neuroscience, Karolinska Institute, Stockholm, Sweden
Note: [*] Reprint address: H. Hirschfeld, Ph.D., Motor Control and Physical Therapy, Research Laboratory, Huddinge Hospital R43, 141 86 Huddinge, Sweden. E-mail: helga.hirschfeld@ki.hs.sll.se.
Abstract: This study investigated whether sudden rotation of the support surface (platform) triggers motor responses similar to reactions to sudden free fall in infants at very early age (2 to 5 weeks). Ten infants in prone position were exposed to sudden head-down rotation (mimicking the falling phase) and head-up rotation of the platform (mimicking landing phase) of 4° or 6° amplitude and 35°/s velocity while EMGs and kinematics were recorded from the neck, trunk, and right arm. One infant, reassessed at 13 weeks, and one adult were tested for complementary developmental information. Sudden downward acceleration of the platform, induced either during head-down rotation or during the deceleration phase of head-up rotation, indeed mimicked falling and evoked in infants two-peaked EMG responses in the neck, trunk, and arm muscles, lasting in the latter over several hundred milliseconds. The activation pattern showed similarities to the adult and 13-week-old control subjects. The results suggested that the first burst may be ascribed to cutaneous pressure changes at the body and to vestibular signals triggering a startle-like response, whereas the second burst of the pattern in the arm is likely a candidate for an early substrate of the landing response normally seen during later stages of motor development. Head control appeared to be related more to its position with respect to the orientation of the trunk rather than to space in the infants and in the adult and might be due to the experimental paradigm, in which the surface accelerated away from the body and not, as during normal falling, when the body accelerates toward the support surface.
