An EMG technique for measuring spinal loading during asymmetric lifting

Objective. To compare two methods of calibrating the erector spinae electro myographic signal against moment generation in order to predict extensor mo ments during asymmetric lifting tasks, and to compare the predicted moments with those obtained using a linked-segment model. Methods. Eight men lifted loads of 6.7 and 15.7 kg at two speeds, in varyin g amounts of trunk rotation. For each lift, the following were recorded at 60 Hz; the rectified and averaged surface electromyographic signal. bilater ally at T10 and L3, lumbar curvature using the 3-Space Isotrak. movement of body segments using a 4-camera Vicon system, and ground reaction forces us ing a Kistler force-plate. Electromyographic (EMG) and Isotrak data were us ed to calculate lumbosacral extensor moments using the electromyographic mo del, whereas movement analysis data and ground reaction forces were used to estimate net moments using the linked-segment model. For the electromyogra phic technique, predictions of extensor moment were based on two different sets of EMG-extensor moment calibrations: one performed in pure sagittal fl exion and the other in flexion combined with 45 degrees of trunk rotation. Results. Extensor moments predicted by the electromyographic technique incr eased significantly with load and speed of lifting but were not influenced by the method of calibration. These moments were 7-40% greater than the net moments obtained with the linked-segment model, the difference increasing with load and speed. Conclusions. The calibration method does not influence extensor moments pre dicted by the electromyographic technique in asymmetric lifting, suggesting that simple, sagittal-plane calibrations are adequate for this purpose. Di fferences in predicted moments between the electromyographic technique and linked-segment model may be partly due to different anthropometric assumpti ons and different amounts of smoothing and filtering in the two models, and partly due to antagonistic muscle forces, the effects of which cannot be m easured by linked-segment models.