Stability limits in extreme postures: effects of load positioning, foot placement, and strength.

Although injuries related to postural stability are prevalent, ergonomic job analyses traditionally have not addressed stability issues. In this research functional stability limits are quantified for persons standing in extreme postures under various external load and foot positioning conditions. Participants were asked to lean and displace their center of gravity (COG) as far as possible in eight directions to the sides and front of the body. Stability measures based on these COG displacements were calculated. All controlled variables significantly affected the stability measures. When standing unladen, participants extended their COG to within 99% of their theoretical maximum. Movement was much more restricted when handling a load (89%), especially when holding it with one hand on the shoulder (84%). On average, increased separation of the feet in a particular direction resulted in larger COG displacements in that direction. The results are discussed relative to their effects on balance and stability modeling.