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Surface electromyography (EMG) has been used to estimate deep trunk muscle activity. However, it remains unknown whether surface EMG provides an accurate estimation of this activity. The purposes of this study were to compare surface and intramuscular EMG activity measurements and investigate the efficacy of surface EMG measurement for the transversus abdominis (TrA) and the multifidus (MF) muscles. Eight healthy men participated in the study. TrA and MF activities were simultaneously measured by both intramuscular and surface EMG during isometric trunk exercises. Spearman correlation coefficients for the relationship between the two activity measurements for the right TrA, left TrA, right MF, and left MF were 0.55, 0.36, 0.67, and 0.79, respectively. For the TrA, Bland–Altman plots revealed that mean differences between measurements obtained by intramuscular EMG and surface EMG were not close to zero, with a systematic bias toward higher surface EMG values. In conclusion, surface and intramuscular EMG activity measurements were strongly correlated for MF muscles, but poorly correlated for TrA muscles.

Trunk muscles may not be able to function appropriately when individuals lift an object that is much heavier than expected.