| Stability during manual material handling is an important issue in the prevention of falls and overexertion injuries. The purpose of this work was to assess stability while handling loads in various positions. Standing and walking with loads, i.e. holding and carrying, were the specific tasks examined. A second purpose was to formulate generalizations regarding stability while holding and carrying loads.; Two holding tasks and one carrying task were explored. To facilitate comparison across experiments, the same five load-holding postures, two load magnitudes and fifteen human subjects were tested. Specific characteristics of load position, namely load height and frontal plane symmetry, were analyzed. The tasks were also performed while unladen. The first experiment quantified functional stability limits while standing on a stable surface. In the next experiment, unexpected support-surface perturbations induced postural sway while standing. The third experiment incorporated gait, investigating stability while carrying loads.; The results showed functional stability ranges averaged 50% to 60% of the total support base, both laterally and in the anteroposterior direction. Anteroposterior perturbations induced an increased sway response with heavier loads and in asymmetric holding postures. While carrying, the body-and-load system's center of mass had increased lateral displacement with asymmetric, higher and heavier loads. However, lateral displacements were not affected similarly on both sides of the body.; Generalizing across experiments, adding an external load, especially asymmetrically, decreased stability compared to unladen tasks. In some instances, however, the additional load improved lateral stability. Holding loads lower and in frontally symmetric postures significantly improved stability compared to asymmetric and higher load positions. In addition to posture and load, dynamics and complexity of the experimental condition also affected stability.; The consistencies found across holding and carrying tasks and the effectiveness of identifying posture and task characteristics demonstrate a potential for developing general material handling guidelines from a postural stability perspective. |
