Evaluation of control strategies for overhead work using electromyography of rotator cuff muscles
Evaluation of control strategies for overhead work using electromyography of rotator cuff muscles Hossein Motabar Work-related musculoskeletal disorders (MSDs) of shoulder have a significant impact on overall health and economics in the industrialized nations. Rotator cuff is the most frequently injured region in the shoulder complex. Overhead work is considered as one of the most predominant causes of rotator cuff injuries. The workers in construction, automotive, and aerospace industries are
... ace industries are routinely exposed to overhead exertions. Elimination of overhead work is nearly impossible due to various task and technology constraints. Therefore, appropriate intervention strategies based on the engineering and administrative controls opt to be used to prevent the hazardous impact of overhead exertions. The objective of this study was to design administrative controls based on the principles of ergonomics and biomechanics and test their influence on the fatigue response of the rotator cuff muscles. As a first step towards developing such controls, a preliminary study was completed to understand baseline behavior of the rotator cuff muscles in terms of their strength, endurance, and fatigue response and subsequently two specific aims on control strategies were completed. The first specific aim evaluated the effect of four different task rotation sequences on the activation and fatigue response of rotator cuff muscles. In the sequences, three different exertions (Hard (H), Medium (M), and Low (L)) were performed in the following orders: increasing (LLMMHH), decreasing (HHMMLL), upward parabolic (HMLLMH), and downward parabolic (LMHHML). Trends in the median frequency of surface electromyography (EMG) data, maximum strength, and ratings of perceived exertion were used as the fatigue indicators. Despite the similar levels of muscle activation, the effect of task rotation sequence on muscle fatigue development was significant. The median frequency-based fatigue indicator reduced by 54% for supraspinatus, 53% for infraspinatus, and 39% for teres minor when high fatigue sequence (decreasing) was compared with low fatigue sequence (upward parabolic). In the second specific aim, a moderate gripping exertion was added to the task rotation sequences to evaluate the effectiveness of a refresher exertion on the active recovery and the fatigue response of the rotator cuff muscles. On average, presence of the griping task reduced the percent drop in the median frequency by 36% for supraspinatus, 56% for infraspinatus, and 58% for teres minor when compared with the task rotation without gripping exertions (aim 1 study). In conclusion, the results of this study indicate that performing tasks with a constant workload in different orders can alter rotator cuff muscles' fatigue development. A shorter duty cycle for the intense tasks, warm up exertions prior to the intense tasks, and low intensity/refresher exertions between the work cycles seem to be the key factors in designing control strategies based on task rotations.