Optimal bipolar system positioning to provide information about the trapezius activity associated with scapular retraction during shoulder exercises for resistance training

Objective. Recently, the use of EMG biofeedback to make subjects aware of the stabilizer activation associated with scapular retraction during exercise has been of interest, and challenges related to EMG detection have been addressed. Whether there is an optimal bipolar positioning that discriminates the stabilizer activation with retraction from a neutral scapular position during resistance exercises is an open issue that we address here by simultaneously mapping different positions using high-density surface electromyography (HD-sEMG). Approach. Sixteen resistance-trained males performed five pulling exercises with and without scapular retraction, namely barbell rows, dumbbell rows, pull-downs at a lat machine, seated rows, and TRX (total resistance exercises) system rows. HD-sEMG was acquired in a monopolar mode from the medial and lower trapezius (8 x 4 electrodes and inter-electrode distance (ied): 10 mm) and different bipolar systems were simulated in terms of positioning, interelectrode distance, and orientation with respect to the spine: longitudinal with three ieds (20 mm, 30 mm, and 40 mm), one transversal, and two diagonals (ied: 20 mm), totalling six EMG sets. To identify the optimal electrode pair that was able to distinguish between the presence or absence of scapular retraction, we computed: (i) the root mean square (RMS) map for each condition and the difference between them, obtaining a differential RMS map per subject; and (ii) the intersection of cumulative maps, by summing the differential (binary) maps from all subjects. Main results. For the lower trapezius, the results revealed that the diagonal direction (45 degrees; ied: 20 mm) obtained the greater occurrence of intersecting segments within and between exercises than the other electrode configurations, showing low variability for the optimal positioning across exercises. Electrode configuration varied within and between the pulling exercises for the medial trapezius. Significance. This study allows us to identify an optimal bipolar positioning (consistent across subjects and exercises) for lower trapezius activity assessment, representing a guideline for electrode positioning when EMG biofeedback is adopted for selective activation of the lower trapezius during pulling exercises.