Variability of Time- and Frequency-Domain Surface Electromyographic Measures in Non-Fatigued Shoulder Muscles

OCCUPATIONAL APPLICATIONSLocalized Muscle Fatigue (LMF) can be monitored or predicted based on the relative change in the values of surface electromyography (sEMG) measures with respect to the "fresh" or no-fatigue condition. Quantification of LMF based on relative change, though, relies on the assumption that the sEMG measures recorded in a no-fatigue condition can serve as an appropriate reference. Results of this study indicate that sEMG measures in a no-fatigue condition are affected by various work-related factors and provide further guidance on the variability of commonly used time- and frequency-domain sEMG measures to assist the ergonomist in improving the accuracy of LMF assessment. TECHNICAL ABSTRACTBackground: Surface electromyography (sEMG) is widely used to monitor or predict localized muscle fatigue (LMF). sEMG signals are often analyzed in time and frequency domains - Root Mean Square (RMS), Mean Absolute Values (MAV), and Zero Crossings (ZC) are common time-domain measures; Mean Power Frequency (MnPF), Median Power Frequency (MdPF) and Power Frequency Bands (PFB) are the common frequency-domain measures. LMF prediction is based on the relative change in the values of these measures with respect to "fresh" or no-fatigue conditions. To our knowledge, the assumption that the sEMG measures do not change/vary under no-fatigue conditions due to factors other than LMF has not been thoroughly tested.Purpose: The goal of this study was to quantify the variability of sEMG measures in non-fatigued shoulder muscles and the implication of this variability for assessing LMF.Methods: Twelve participants performed 120 occupationally-relevant static tasks under various conditions of joint angles, anatomical planes, force levels, and force directions. sEMG data were recorded from seven shoulder muscles - supraspinatus, infraspinatus, middle deltoid, anterior deltoid, posterior deltoid, biceps, and triceps.Results: For these shoulder muscles, the variability of RMS, MAV, ZC, MnPF, MdPF, and PFB ranged from 7.9 to 11.1%, 7.1 to 10.1%, 10.2 to 11.0%, 8.6 to 11.4%, 8.7 to 12.2%, and 5.3 to 7.9%, respectively.Conclusions: Both time- and frequency-domain sEMG measures vary due to various work-related factors under no-fatigue conditions. Therefore, it is critical to consider the variability ranges of sEMG measures while monitoring or predicting LMF due to sustained exposure to work-related factors.