Eur J Appl Physiol (1996) 74:119-132 © Springer-Verlag 1996 J.R. Potvin • R.W. Norman • S.M. McGill Mechanically corrected EMG for the continuous estimation of erector spinae muscle loading during repetitive lifting Accepted: 12 October 1995 Abstract Few studies have been carried out on the changes in biomechanical loading on low-back tissues during prolonged lifting. The purpose of this paper was to develop a model for continuously estimating erector spinae muscle loads during repetitive lifting and lower- ing tasks. The model was based on spine kinematics and bilateral lumbar and thoracic erector spinae elec- tromyogram (EMG) signals and was developed with the data from eight male subjects. Each subject per- formed a series of isometric contractions to develop extensor moments about the low back. Maximum vol- untary contractions (MVCs) were used to normalize all recorded EMG and moment time-histories. Ramp con- tractions were used to determine the non-linear rela- tionship between extensor moments and EMG ampli- tudes. In addition, the most appropriate low-pass filter cut-off frequencies were calculated for matching the rectified EMG signals with the moment patterns. The mean low-pass cut-off frequency was 2.7 (0.4) Hz. The accuracy of the non-linear EMG-based estimates of isometric extensor moment were tested with data from a series of six rapid contractions by 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 of 2 h, a model was used to calculate cha- nges in muscle length based on monitored spine kin- ematics. EMG signals were first processed according to the parameters determined from the isometric contrac- tions and then 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 J.R. Potvin (12N) School of Human Biology, Universityof Guelph, Guelph, Ontario, Canada, N1G 2W1 R.W. Norman • S.M. McGill Department of Kinesiology,Universityof Waterloo, Waterloo, Ontario, Canada eccentric and concentric phases of lifting respectively, when compared to load estimates based on the mech- anically corrected EMG. To date, the model has been used to analyze over 5300 lifts. Key words Lumbar spine • Electromyography" Muscle force' Isometric contractions. Dynamic lifting Introduction Manual lifting has been a significant contributor to the alarming incidence of low-back injuries in today's society. Numerous attempts have been made to esti- mate the loading on spinal tissues during lifting in an effort to delineate factors responsible for these injuries. Most have used biomechanical models for addressing this issue and have tended to concentrate on estimating compression forces on intervertebral joints during brief static efforts (Morris et al. 1961; Chaffin 1969; Schultz and Andersson 1981) or non- fatiguing dynamic lifting (Leskinen et al. 1983; Freivalds et al. 1984; McGill and Norman 1985). How- ever, it appears that low-back injuries also result from prolonged and/or repetitive lifting of relatively light loads. There is no dispute that spinal tissues can become progressively damaged or weakened when loaded repeatedly. These chronic effects have been observed with tendons (Goldstein et al. 1987), inter- vertebral discs (Brown et al. 1953; Adams and Hutton 1983), vertebral bodies (Hardy et al. 1958; Liu et al. 1983; Hansson et al. 1987) and have been suggested for lumbar ligaments (Edwards 1988; Frymoyer and Gor- don 1989). The erector spinae muscles have been shown to be the dominant contributors to the loading of lumbar spine tissues during symmetrical lifts (McGill and Norman 1986; Potvin et al. 1991; Marras and Sommerich 1994b). However, current methods for esti- mating these forces are time consuming, limited to