The effect of short-term changes in the body mass on anticipatory postural adjustments.

The purpose of the study was to investigate whether anticipatory postural adjustments (APAs) are modified with short-term changes in the body mass. Nine subjects were asked to catch a 2.2 kg load with their arms extended under conditions of no weight and when additional weights of 10 and 20% of the subject's body weight (BW) were attached to single body locations or when 20 or 40% BW were attached evenly to two locations. Attaching weights was associated with an increase of the whole body mass, but also involved changes in the vertical position of the center of mass (COM). Electromyographic activity of leg and trunk muscles and ground reaction forces were recorded and quantified within the typical time intervals of APAs. APAs were influenced by the magnitude of the weight attached to the body: an increase in the body mass was associated with anticipatory co-activation of trunk and leg muscles. The level of this co-activation increased with an increase in the magnitude of weight added to the body. At the same time, APAs were affected by the changes in the vertical position of COM. These findings suggest that in the case of short-term changes in the body mass, the CNS might prioritize information regarding the magnitude and location of the additional weight added to the body and utilize a strategy of anticipatory co-activation of postural muscles directed at the stabilization of body segments.