A detailed biomechanical model of the low-back musculature that predicts muscleforce distribution in response to external loading is presented. The paper shows that the class of loading tasks that involve an erect posture and an arbitrary load placed on the upper limbs can be described as a loading plane whose axes are the flexion and lateral bending moments. Under these conditions, the individual muscle forces are described as a three-dimensional surface defined by the loading plane and termed the muscle activity surface (MAS). The MAS and the loading plane intersect along the switching curve which separates the load combinations that activate the muscle from those that do not. The paper suggests the existence of a recruitment order of low back muscles in response to external loads and presents a comprehensive framework for examining earlier studies that used EMG measurements to validate physiological and mechanical predictions.

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