Objective: An association exists between repetitive movements and the development or perpetuation of neck-shoulder muscle pain. The mechanisms underlying this association remain unclear. This observational study investigated the effect of upper trapezius muscle pain on the distribution of upper trapezius activity during repetitive lifting. It was hypothesized that nociception would change the distribution of activity resulting in activation of muscle regions which would not normally be active during the task.
Materials and Methods: Healthy men repeatedly lifted a box with a cycle time of 3 seconds for 50 cycles, at baseline, following injection of isotonic and hypertonic saline into the upper trapezius muscle and 15 minutes after the last injection. High-density surface electromyography (EMG) was recorded from the upper trapezius using a grid of 64 electrodes. The EMG amplitude was computed for each location to form a map of the EMG amplitude distribution.
Results: During the painful condition, the overall EMG amplitude was lower compared with all other conditions (P<0.05) and in addition, the center of upper trapezius activity was shifted toward the caudal region of the muscle (P<0.01), a region not normally active during the task. The described alterations of muscle activity likely play an important role in the perpetuation of pain during repetitive activity.
Discussion: Novel mapping of the spatial distribution of upper trapezius muscle activity showed that nociception induced a redistribution of activity during repetitive lifting. This knowledge provides new insights into the mechanisms underlying the perpetuation of pain with repetitive activity.
*School of Sport, Exercise and Rehabilitation Sciences, Centre of Precision Rehabilitation for Spinal Pain (CPR Spine), College of Life and Environmental Sciences, University of Birmingham, Birmingham, UK
†Rehabilitation Research Laboratory, Department of Business Economics, Health and Social Care, University of Applied Sciences and Arts of Southern Switzerland, SUPSI, Manno, Switzerland
‡Department of Health Science and Technology, Center for Neuroplasticity and Pain (CNAP), Center for Sensory-Motor Interaction (SMI), Aalborg University, Aalborg, Denmark
D.F., C.C., R.L.: contributed to the conception and design of the study. C.C. and R.L.: collected the data. C.C., D.F., and M.B.: analyzed the data. D.F. and M.B.: wrote the first draft of the paper. All authors contributed to the interpretation of findings, revising the manuscript for important intellectual content, and approved the final version to be published. All authors had full access to all of the data (including statistical reports and tables) in the study and can take responsibility for the integrity of the data and the accuracy of the data analysis.
The authors declare no conflict of interest.
Reprints: Deborah Falla, PhD, School of Sport, Exercise and Rehabilitation Sciences, Centre of Precision Rehabilitation for Spinal Pain (CPR Spine), College of Life and Environmental Sciences, University of Birmingham, Birmingham B15 2 TT, UK (e-mail: email@example.com).
Received June 3, 2016
Received in revised form January 17, 2017
Accepted May 1, 2017