Stabilization of Posture by Precision Touch
We have carried out extensive studies demonstrating that a light touch of the hand at mechanically non-supportive force levels attenuates body sway and enhances postural control under a wide variety of circumstances in normal subjects and in patients with bilateral vestibular loss.
Subjects were tested under six randomly presented experimental conditions involving vision (V) of the test chamber or darkness (D) with either no touch (N), the arms passively by the sides, or precision touch (T), contact of the right index finger at less than 1N (@100 grams) of force, or force contact (F), as much force as desired. Touch forces were measured with full bridge, temperature compensating, semiconductor strain gauges. We calculated the mean sway amplitude (MSA) of the center of foot pressure.
Subjects typically applied about 40 grams of force in the touch conditions (DT, VT). Mechanically, 40 grams of force applied at the fingertip can attenuate sway at most by 4% (Holden, Ventura, Lackner, 1987, 1994), but touch contact (DT) reduced sway by 50-60% relative to the no-contact control condition (DN). Forty grams at the fingertip corresponds to the maximal dynamic sensitivity range of the finger's cutaneous receptors (Westling & Johansson, 1987). This force level thus allows optimal resolution of contact force vectors that provide information about body sway. Clapp and Wing (1999) repeated this study with subjects in a feet side by side stance and replicated our results. Thus, precision touch greatly attenuates body sway and is even more effective than vision by itself. When vision is allowed, precision touch further attenuates sway.
Other findings from our Precision Touch experiments include:
- Precision touch decreases leg muscle EMG activity. Less energy is required to maintain balance when precision touch information about sway is present because sway is detected and corrected earlier.
- Precision touch contact stabilizes congenitally blind subjects. This means postural destabilization arising from aberrant leg muscle or neck muscle activity can be suppressed by precision touch.
- Precision touch stabilizes individuals with bilateral vestibular loss. Light touch of the index finger with a stationary surface allows labyrinthine defective subjects to balance as accurately as normal subjects.
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