FLEXION RELAXATION DURING LIFTING - IMPLICATIONS FOR TORQUE PRODUCTION BY MUSCLE-ACTIVITY AND TISSUE STRAIN AT THE LUMBOSACRAL JOINT

During the full flexion phase of the back lift movement the lumbar par t of the erector spinae muscle exhibits a reduced activity level (flex ion relaxation). This study addresses the question how the required ex tension torque in the lumbo-sacral joint (L5/S1 joint) is balanced dur ing the period in which apparently the lumbar erector spinae ceases to take its share. Six subjects participated in the experiment in which they performed seven lifting tasks. The load, the range of movement, a nd the phase in which the load was handled (lifting or lowering) were varied. A dynamic linked segment model was applied to determine the mo mentary torques acting at the L5/S1 joint, while the EMGs of the lumba r and thoracic part of the erector spinae muscle were measured. Furthe rmore, the lengths between markers on the lumbar and thoracic part of the trunk were determined to reveal changes in length during the movem ent. The dynamic EMGs were normalized to trunk angle-dependent maximal levels. The L5/S1 joint torques were analysed and combined with the n ormalized EMG data and the kinematics of the trunk, which are assumed to indicate the elongation of passive tissues. Although in the normali zation procedure the change of the length-force relationship of the er ector spinae was taken into account, the dynamic lumbar EMG activity d ecreased to a low-activity level (the phenomenon of flexion relaxation ). This coincided with a 25% increase in lumbar length suggesting that passive tissue strain provided part of the required extension torque. In the tasks where a barbell was handled a significant increase in EM G level of the thoracic part of the erector spinae occurred just befor e the flexion relaxation at the lumbar level, Apparently, the extensor function of the lumbar part is then taken over by the thoracic part o f the erector spinae muscle. This suggests that an intricate coordinat ing mechanism is operative that apportions the load to be balanced ove r active - (lumbar and thoracic part of the erector spinae) and passiv e structures (post vertebral ligaments).