Postural stabilization by gripping a stick with different force levels.

Hand contact with a stationary surface reduces postural sway in healthy individuals even when the level of force applied is mechanically insufficient. To make this phenomenon more applicable to a real-life situation, where a stationary support is not available, a mobile stick was used to measure and control grip force. The effect of a supra-postural task of stick gripping on stability was tested in 18 healthy individuals during quiet standing, standing in semi-tandem, and with eyes closed. Subjects stood either holding no haptic stick, or gripping with one of six force levels ranging from 1 to 9N and a self-selected force in the same range. The path length and velocity of the center of pressure (COP) were measured and compared within and between experimental conditions. Gripping the stick reduced the COP path length and velocity by up to 23% and 25%, respectively, and postural stability was increased at all force levels, including self-selected. The results confirmed the stabilizing effects of gripping an external portable object regardless of the amount of force applied. This knowledge may be useful for counseling people on prevention of stability loss in real life situations where balance is challenged.