Patterns of Forearm Muscle Activity and Task Parameters Change During a Repetitive Sub-Maximum Forceful Wrist Flexion Task

OCCUPATIONAL APPLICATIONS While completing a sub-maximum forceful wrist flexion task for two 1-hour sessions, the temporal patterns of forearm muscle activity of 10 females changed along with corresponding changes in torque, duty-cycle, and torque-time integral. These temporal changes in muscle activity promote variation in muscle activity during repetitive tasks, and as a result may prove to be a protection mechanism for musculoskeletal disorder development. In addition, the changes observed in torque, duty-cycle, or torque-time integral indicate that even during very defined simulated occupational tasks, workers may find ways to interject variation into a task that may also contribute to this protection. TECHNICAL ABSTRACT Background: Jobs requiring repetitive motion are common in many industries, and such jobs can be associated with the development of musculoskeletal disorders. Temporal changes in muscle activity patterns throughout repetitive tasks, if they occur, may promote variation in muscle activity and therefore may prevent the development of musculoskeletal disorders during repetitive work. Purpose: To assess temporal changes in activity patterns of the forearm muscles during a prolonged (2-hour) repetitive wrist flexion task. Methods: Ten females completed a repeated-measures study by performing a task requiring 80° wrist flexions at a rate of 15 flexions per minute overcoming a resistant force set at 25% of their maximum voluntary isometric wrist flexion torque for two 1-hour sessions, with a 5-minute break between the two sessions. Muscle activity of six forearm flexor and extensor muscles was measured using surface electromyography during the first and last 5 minutes of each of the 2-hour sessions. Task performance parameters including torque, cycle time, angle, duty cycle, work, and torque-time integral were also recorded and calculated during the 5-minute periods. Results: Compared to the first 5 minutes of the first hour, mean extensor carpi ulnaris activity was significantly lower during the last 5 minutes of both hours, and the muscle activity percentage of time in gaps was significantly higher for the last 5 minutes of the first hour and both 5-minute periods of the second hour. Torque, duty cycle, and torque-time integral were also significantly lower during the last 5 minutes of the first hour and both 5-minute periods of the second hour compared to the first 5 minutes of the first hour. Conclusions: The observed temporal changes indicate some variation in muscle activity during the repetitive task, which may affect musculoskeletal disorder development. These results provide some preliminary insight into mechanisms that may prevent the development of musculoskeletal disorder development during repetitive tasks.