Trunk Muscle Response to Trunk Asymmetry, Velocity, and Load Level

A significant amount of epidemiologic evidence suggests that asymmetric motions of the trunk during lifting are related to the risk of low back disorders. However, a void exists in the biomechanical literature which describes how the internal forces of the trunk respond to these motions. An experiment was performed to investigate this relationship. Eighteen subjects were asked to produce lifting motions of the back under highly controlled conditions. Trunk asymmetry, angular concentric and eccentric velocity, torque production, and trunk position were controlled while the activity of ten trunk muscles was observed. This article presents preliminary data from this experiment. Saddle-shaped response surfaces described the reactions or “costs” to the muscles to the various controlled variables. These trends are described and the effects of these trends on spine loading and biomechanical modelling are discussed.