Submaximal normalizing methods to evaluate load sharing changes in the shoulder during repetitive work

The EMG-force relationship changes with muscle fatigue, making interpretation of load sharing between muscles challenging. The purpose of this investigation was to evaluate the efficacy of normalizing EMG to repeated, static, submaximal exertions to mitigate fatigue artifacts in EMG amplitude (increased amplitude without muscle force change). Participants completed repetitive work tasks, in 60-second cycles, until exhaustion. Surface EMG was recorded from 11 shoulder muscles (anterior, middle and posterior deltoids, infraspinatus, upper, middle and lower trapezius, latissimus dorsi, serratus anterior, sternal and clavicular heads of pectoralis major). Every 12 min, participants completed 4 submaximal reference exertions. Reference exertion EMG data were used in 6 normalizing methods including 1 standard (normalized to initial reference exertion) and 5 novel methods: (i) Fatigue Only, (ii) Linear, (iii) Cubic, (iv) Points Forward, and (v) Points Forward/Backward. Data normalized with each novel method were compared to the Standard Method using mixed effects modelling. Significant differences depended on the muscle and the number of time points included (p < .05). The cubic model correlated better to the actual data points than linear predicted values. The novel cubic normalizing method created muscle activity ratios that appear to mitigate the fatigue effects and better reflect muscular loads during fatiguing work.