Effects of touch support on postural stabilization during voluntary movements

Effetti di un leggero supporto sulla postura durante movimenti volontari

Voluntary movements can disturb upright posture due to the generation of reactive torques and changes in body geometry. In a standing person, arm movements are commonly associated with the generation of anticipatory postural adjustments (APAs) in leg and trunk muscles, counteracting the disturbing effects of the upcoming perturbation. The magnitude of these APAs have been shown to depend on the stability conditions (e.g. the size and the slipperiness of the base of support). In very stable conditions APAs are decreased. Touch of the hand to a stationary surface at non-mechanically supportive force levels (<1 N) greatly attenuates postural sway during quiet stance. The stabilizing effects of such a touch may however be viewed as fictitious when a person makes a fast voluntary arm movement. In case a standing person, touching a stable surface, performs a fast movement, no changes in APAs would therefore be expected. On the other hand, if central mechanisms responsible for postural sway and for APAs are similarly affected by a light touch, APAs may be expected to decrease, as during very stable standing. Our study investigated the modulation of APAs during a light touch with the index finger and during ‘mechanically stable’ conditions (a hand grasp) to discern between these two possibilities.