Kinematic analysis of footstep strategies in asymmetrical lifting and lowering tasks

Abstract

Two footstep strategies associated with expert workers and two associated with novice workers were studied for lifting and lowering a 12 kg box. Kinematic variables, L5/S1 resultant moments and body postures of eight healthy male subjects were used to analyse the effects of the footstep strategies. A tridimensional dynamic rigid body model was used to estimate L5/S1 resultant moment, using inertial forces to estimate external forces; these procedures were validated with force platforms. Results showed that expert strategies either reduced the path of the subject's centre of gravity or reduced the asymmetry of posture at deposit. The footstep strategies showed no consistent effect on L5/S1 resultant moment. The use of different footstep strategies might be aimed at different goals, which may depend on the context. On the one hand, the reduction in the path of the subject's centre of gravity might represent a mean to reduce the effort required and avoid fatigue, which might be appropriate in some circumstances; on the other hand, the reduction of asymmetry of posture at deposit might be more appropriate in other circumstances. Footstep strategies will deserve further attention since this study showed that they can play an important role in reducing the risks of injury. Relevance to industry Manual materials handling is a major known source of industrial injury. Biomechanical studies of handling strategies really used by workers should help understand the loadings encountered in real handling tasks, which may lead to the identification of safer handling strategies.