Biomechanical analysis of materials handling manipulators in short distance transfers of moderate mass objects: joint strength, spine forces and muscular antagonism

Although often suggested as a control measure to alleviate musculoskeletal stresses, the use of mechanical assistance devices (i.e. manipulators) in l oad transfers has not been extensively studied. Without data describing the biomechanical effects of such devices, justification for decisions regardi ng implementation of such tools is difficult. An experimental study of two types of mechanical manipulators (articulated arm and overhead hoist) was c onducted to determine whether biomechanical stresses, and hence injury risk , would be alleviated. Short distance transfers of loads with moderate mass were performed both manually and with manipulator assistance under a varie ty of task conditions. Using analysis and output from new dynamic torso mod els, strength demands at the shoulders and low back, lumbar spine forces, a nd lumbar muscle antagonism were determined. Strength requirements decrease d significantly at both the shoulders and low back when using either manipu lator in comparison with similar transfers performed manually. Peak spine c ompression and anterior-posterior (a-p) shear forces were reduced by about 40% on average, and these reductions were shown to be primarily caused by d ecreases in hand forces and resultant spinal moments. Two metrics of muscul ar antagonism were defined, and analysis showed that torso muscle antagonis m was largest overall when using the hoist. The results overall suggest tha t hoist-assisted transfers, although better in reducing spine compression f orces, may impose relatively higher demands on coordination and/or stabilit y at extreme heights or with torso twisting motions. The relatively higher strength requirements and spine compression associated with the articulated arm may be a result of the high inertia of the system. Potential benefits of practice and training are discussed, and conclusions regarding implement ation of mechanical manipulators are given.