Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C47
Is the standing configuration a postural set that is aimed for?
I. Di Giulio1, I. D. Loram1
1. Manchester Metropolitan University, Manchester, United Kingdom.
Posture might be controlled via pre-programmed set points and feedback mechanisms associated with maintaining or changing body configuration (1). The processes involved in, and the adjustments associated with, gaining the standing configuration are revealed using perturbations. Lying down on a horizontal board was used to deprive the body of the vertical reference point to investigate the way the upright configuration goal was aimed for. The question asked was whether, after the individual was passively brought upright, the nervous system either (a) maintained the body configuration imposed by the board, or (b) sagged randomly into a position or (c) actively aimed for a different multi-joint point of equilibrium. 14 healthy participants, aged 47±13 years, laid horizontally on a table with feet against a perpendicular support for 5min and were then brought passively to the upright position by rotating the table until the feet on the support were planted onto a force plate. With the participant self supporting and posturally quiet, the vertical table was released from the foot support surface and wheeled away leaving the participant free-standing for 160s. A 10-camera motion analysis system measured body kinematics. The quantities calculated were: sagittal CoG; joint centres, flexion-extension and internal-external rotation angles of the ankle, hip and knee. Quantities were tested for difference between the time when the board was removed and “steady state” standing (160s later) (Krusal-Wallis). At the initiation of standing (i.e. board removed) hip and knee joint centres were in front of the CoG, the ankle was behind. During the 160s after the board was removed, all the quantities approached their final values asymptotically. On average the CoG moved more in front of the ankle and knee joint by 22.4±1.3mm and 16.2±1.4mm respectively (p<0.001 both) and behind the hip by 13.6±3.8mm (p<0.001).The knee and hip flexion were reduced by 3.7±0.5deg and by 3.64±0.07deg (p<0.001 both). The hip internally rotated by 3.2±0.4deg (p<0.001). This experiment showed that the individuals’ nervous system initially tried to maintain the configuration imposed by the board and then actively aimed for a distinct point of equilibrium which was sustained during the trial. This progression was not due to mechanical momentum because participants started at rest. The active aiming for a new standing configuration is confirmed by the fact that knee increasingly extended: this is consistent with joint torque acting against the gravity vector, possibly to increase joint stiffness and improve perceived stability. While small dynamic adjustments of configuration may have occurred continually, the broad multi-joint configuration remained stable and distinct from the onset of standing. This validates our previous findings that a postural set-point typical of standing exists, is aimed for and is defended (2,3).
Where applicable, experiments conform with Society ethical requirements