High-pass filtering surface EMG in an attempt to better represent the signals detected at the intramuscular level

Surface electromyography (EMG) is often used to represent activation profiles of the underlying musculature. The purpose of this study was to assess the potential of high-pass (HP) filtering to improve the matching of surface EMG signals to those signals recorded intramuscularly. EMG was recorded at the skin surface over the infraspinatus and supraspinatus muscles as well as from fine-wire electrodes placed in the infraspinatus, supraspinatus, and teres minor muscles. The surface EMG signals were HP-filtered at 18 cutoff frequencies (0-510 HZ in 30 HZ increments), and the time-histories were correlated with the signals from the wire electrodes. HP filtering did not significantly alter the correlated muscle activation waveform relationship between the surface and wire signals until cutoffs reached 240 HZ. HP filtering of the surface signals did not improve the representation of the muscle fiber-level activation profile, but the results suggest that enough information resides in the high-frequency components of the signal to reproduce the activation time-history profile of the muscle.