Repetitive Work

Submaximal Normalizing Methods to Evaluate Load Sharing Changes in Repetitive Upper Extremity Work

The relationship between EMG and muscle force changes with muscle fatigue, making interpretation of load sharing between muscles over time challenging. The purpose of this investigation was to evaluate the efficacy of normalizing EMG data to repeated, static, submaximal exertions to mitigate the fatigue artifact in EMG amplitude. Participants completed simulated repetitive work tasks, in 60-second work cycles, until exhaustion and surface EMG was recorded from 11 muscles. Every 12 minutes, participants completed a series of 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 Model, (iii) Cubic Model, (iv) Points Forward, and (v) Points Forward/Backward. EMG data were normalized to each novel methods and results were compared to the Standard Method. The significant differences between the novel methods and the Standard Method were dependent on the muscle and the number of time points in the analysis. Correlation analysis showed that the predicted cubic model points correlated better to the actual data points than the linear predicted values. This novel method to create “fatigue debiased” ratios may better reflect the changing muscular loads during repetitive work. This method was evaluated with a novel data set examining the effects of repetitive shoulder exertions, in multiple axes, on load sharing in the shoulder over time. The normalizing method was effective at distinguishing between the effects of fatigue artifact on EMG amplitude and load sharing between muscles over time.
Listed In: Biomechanics


Fatigue and Recovery in the Shoulder Complex While Performing Simulated Repetitive Work

The shoulder complex affords multiple opportunities for kinematic and muscular variability during repetitive work, which could change physical exposure and risk at work. The purpose of this study was to examine kinematic and muscular adaptations during continued performance of submaximal, repetitive work following a fatiguing protocol. Participants (n=12) completed a sequence of three protocols: (1) 20 pre-fatigue work cycles, (2) anterior deltoid fatigue protocol, (3) 60 post-fatigue work cycles. Each work cycle was 60 seconds and consisted of 4 tasks. Reaction forces and moments were recorded with a 6DOF force sensor (MC3-500, AMTI, Watertown, MA, USA) during the work tasks. The fatigue protocol consisted of static and dynamic efforts targeting the anterior deltoid. Fatigue was quantified through changes in strength, RPE and EMG frequency and amplitude. Activity of 14 muscles of the upper extremity and torso were measured with surface electrodes and kinematics were tracked with a passive motion capture system, 30 reflective markers and a scapular tracker. Immediately following the fatigue protocol, there were significant signs of muscle fatigue and reduced physical capacity. These changes were accompanied by significant muscular and kinematic adaptations in the work tasks during the post-fatigue work cycles (p<.05). Although these adaptations allowed for recovery in some muscles, fatigue persisted and developed in other muscles by the end of the post-fatigue work cycles, despite subjective ratings of perceived exertions returning to pre-fatigue levels. If people are unable to perceive negative behavioral changes during repetitive work, they may be at greater risk of developing workplace injuries.
Listed In: Biomechanics