Postural control in humans: a study using transcutaneous spinal cord stimulation.

The aim of the study was to investigate the spinal mechanisms involved in regulating postural balance in humans. Participants stood in a normal stance, with their spinal postural networks either non-invasively activated or not stimulated by electrical stimulation. Postural sway, muscle activity, joint kinematics and respiratory movements were monitored. Half of the participants depended on external sensory cues for controlling balance (field dependent), whereas the other half did not (field independent). Stimulation was performed at the T11-T12 or L1-L2 vertebral level using intensities below the motor threshold. When participants stood without stimulation, differences in body segment movement in the frontal plane were observed between subgroups, in addition to similarities in segment movement in the sagittal plane. Field-dependent participants demonstrated greater mediolateral sway and pelvic motion and relied more on hip involvement in frontal centre of pressure dynamics than field-independent participants. Stimulation at T11-T12 induced changes in muscle activity and body segment coordination in both groups. It led to a reduction in mediolateral sway and enhanced postural stability, but only in field-dependent individuals. Stimulation at L1-L2 altered muscle activity and joint kinematics in both subgroups without having an effect on postural stability. Stimulation did not affect respiratory movements or posture-respiratory coupling. In the human spinal cord, the interneuronal networks involved in postural regulation are located at the T11-T12 level and operate under supraspinal control. Interneuronal networks at the L1-L2 level also modulate muscle activity and body segment coordination; however, their specific role in regulating upright posture remains unclear.