Postural Muscle Tone in the Body Axis of Healthy Humans

1 Neurological Sciences Institute and 2 Department of Neurology; Oregon Health and Science University, Portland, Oregon; and 3 School of Physiotherapy, Curtin University of Technology, Perth, Australia Submitted 17 April 2006; accepted in final form 28 June 2006 Across the entire human body, postural tone might play its most critical role in the body's axis because the axis joins the four limbs and head into a single functioning unit during complex motor tasks as well as in static postures. Although postural tone is commonly viewed as low-level, tonic motor activity, we hypothesized that postural tone is both tonically and dynamically regulated in the human axis even during quiet stance. Our results describe the vertical distribution of postural muscle tone in the neck, trunk, and hips of standing human adults. Each subject stood blindfolded on a platform that axially rotated the neck, trunk, or pelvis at 1°/s and ±10° relative to the neutral position (i.e., facing forward). The measured resistance to axial rotation was highest in the trunk and lowest in the neck and was characterized by several nonlinear features including short-range stiffness and hysteresis. In half of the subjects, axial muscle activity was relatively constant during axial rotation, and in the other half, muscle activity was modulated by lengthening and shortening reactions, i.e., decreasing activity in lengthening muscles and increasing activity in shortening muscles, respectively. Axial resistance to rotation was reduced in subjects whose muscle activity was modulated. The results indicate that axial tone is modulated sensitively and dynamically, this control originates, at least in part, from tonic lengthening and shortening reactions, and a similar type of control appears to exist for postural tone in the proximal muscles of the arm. Address for reprint requests and other correspondence: P. J. Cordo, Neurological Sciences Institute, Oregon Health and Sciences University, 505 NW 185th Ave., Beaverton, OR 97006 (E-mail: cordop{at}ohsu.edu )