Coherence between surface electromyograms is influenced by electrode placement in hand muscles

We used multi-channel surface electromyograms (EMGs) to examine if electrode location influences coherence measures derived from pairs of EMGs recorded from two hand muscles during a pinch task. A linear probe of 16 electrodes was used to estimate the location of the innervation zone in first dorsal interosseous (FDI) and abductor pollicis brevis (APB). Four electrodes were then placed on the skin overlying each muscle and three bipolar electrode configurations were constructed with their center points directly over the innervation zone, and 15 mm distal and proximal to the innervation zone. Ten subjects performed two force-matching tasks for 120 s at 2 N and 3.5 N by pressing a force sensor held between the thumb and index finger. Coherence spectra were calculated from pairs of EMGs recorded from the two muscles. Maximal coherence from 1 to 15 Hz and 16 to 32 Hz was calculated at both force levels from the EMGs with electrodes centered over the innervation zones of FDI and APB. These values were compared to the maximal coherence from all other EMG comparisons across muscles recorded with electrodes that avoided the innervation zones. ANOVA revealed significant main effects only for electrode location, with a 58.1% increase (p = 0.001) in maximal coherence for EMGs detected from pairs of electrodes that avoided the innervation zone (from 0.11 ± 0.02 to 0.18 ± 0.03; mean ± 95% confidence interval). These results indicate that electrode location relative to the innervation zone influences EMG-EMG coherence and should be carefully considered when placing EMG electrodes on hand muscles.

Research highlights▶ We examined if electrode location influences EMG-EMG coherence during a pinch grip task.▶ Multi-channel surface EMGs were used to identify the innervation zone in two hand muscles.▶ EMG-EMG coherence increased by 58.1% if electrodes were placed to avoid the innervation zone.▶ These results suggest EMG electrodes should be carefully placed to avoid the innervation zone.▶ EMG-EMG coherence can be altered independently of rhythmic activity across muscles.