Interaction between voluntary and postural motor commands during perturbedlifting

Study Design. An experimental study was conducted to evaluate the effect of an unexpected postural perturbation during a lifting task. Objectives. To investigate electromyographic responses in the erector spina e to a postural perturbation, simulating slipping, during an ongoing volunt ary lifting movement it was hypothesized that specific combinations of volu ntary movement and postural perturbation present a situation in which injur y caused by a rapid switch between conflicting motor commands can occur. Summary of Background Data. Studies of postural perturbations have mainly f ocused on behavior during static tasks such as quiet, upright standing. To date, there are no published studies of the effect of a perturbation during an ongoing voluntary lifting movement. Methods. Subjects standing on a movable platform were exposed to random per turbations while lifting a 20-kg load. Muscle activity was recorded from fl exor and extensor muscles of the trunk and hip. Trunk flexion angle in the sagittal plane was recorded with a video system. Results. Perturbations forward were followed by an increased activity in er ector spinae superimposed on the background activation present during the l ift, indicating that both the voluntary and postural motor programs caused an activation of erector spinae. During backward perturbation however, ther e was a sudden cessation of erector spinae activity followed by an extended period of rapid electromyographic amplitude fluctuations while the trunk w as. flexing, indicating an eccentric contraction of the erector spinae. Conclusions, This erratic behavior with large electromyographic amplitude f luctuations in the erector spinae after a backward slip during lifting may indicate a rapid switch between voluntary and postural motor programs that require conflicting functions of the back muscles. This may cause rapid for ce changes in load-carrying tissue, particularly in those surrounding the s pine, thus increasing the risk of slip-and-fall-related back injuries.