Surface EMG electrodes do not accurately record 3 from lumbar multifidus muscles

10 Objective. This study investigated whether electromyographic signals recorded from the skin surface overlying the multifidus 11 muscles could be used to quantify their activity. 12 Design. Comparison of electromyography signals recorded from electrodes on the back surface and from wire electrodes within 13 four different slips of multifidus muscles of three human subjects performing isometric tasks that loaded the trunk from three 14 different directions. 15 Background. It has been suggested that suitably placed surface electrodes can be used to record activity in the deep multifidus 16 muscles. 17 Methods. We tested whether there was a stronger correlation and more consistent regression relationship between signals from 18 electrodes overlying multifidus and longissimus muscles respectively than between signals from within multifidus and from the skin 19 surface electrodes over multifidus. 20 Results. The findings provided consistent evidence that the surface electrodes placed over multifidus muscles were more sensitive 21 to the adjacent longissimus muscles than to the underlying multifidus muscles. The R for surface versus intra-muscular comparisons 22 was 0.64, while the average R for surface-multifidus versus surface-longissimus comparisons was 0.80. Also, the magnitude of the 23 regression coefficients was less variable between different tasks for the longissimus versus surface multifidus comparisons. 24 Conclusions. Accurate measurement of multifidus muscle activity requires intra-muscular electrodes.