MECHANICALLY CORRECTED EMG FOR THE CONTINUOUS ESTIMATION OF ERECTOR SPINAE MUSCLE LOADING DURING REPETITIVE LIFTING

Few studies have been carried out on the changes in biomechanical load ing on low-back tissues during prolonged lifting. The purpose of this paper was to develop a model for continuously estimating erector spina e muscle loads during repetitive lifting and lowering tasks. The model was based on spine kinematics and bilateral lumbar and thoracic erect or spinae electromyogram (EMG) signals and was developed with the data from eight male subjects. Each subject performed a series of isometri c contractions to develop extensor moments about the low back. Maximum voluntary contractions (MVCs) were used to normalize all recorded EMG and moment time-histories. Ramp contractions were used to determine t he non-linear relationship between extensor moments and EMG amplitudes . In addition, the most appropriate low-pass filter cut-off frequencie s were calculated for matching the rectified EMG signals with the mome nt patterns. The mean low-pass cut-off frequency was 2.7 (0.4) Hz. The accuracy of the non-linear EMC-based estimates of isometric extensor moment were tested with data from a series of six rapid contractions b y each subject. The mean error over the duration of these contractions was 9.2 (2.6)% MVC. During prolonged lifting sessions of 20 min and o f 2 h, a model was used to calculate changes in muscle length based on monitored spine kinematics. EMG signals were first processed accordin g to the parameters determined from the isometric contractions and the n further processed to account for the effects of instantaneous muscle length and velocity. Simple EMG estimates were found to underestimate peak loading by 9.1 (4.0) and 25.7 (11.6)% MVC for eccentric and conc entric phases of lifting respectively, when compared to load estimates based on the mechanically corrected EMG. To date, the model has been used to analyze over 5300 lifts.