The effect of noxious stimulation of the right masseter muscle on single motor unit activity at two sites in the masseter muscle during standardized jaw closing tasks

Abstract

Background and aims: The Pain Adaptation Model and the Vicious Cycle Theory are older theories of the effects of pain on motor activity. While some data sets are consistent with these theories, other data sets are not, and a number of new models (the Integrated Pain Adaptation Model; Theory of Motor Adaptation to Pain) have been proposed. Two of these models propose a reorganization of motor activity in pain. The aim of the present study was to determine whether experimental masseter muscle pain resulted in a change in muscle activity at two different sites within the masseter muscle during the performance of isometric jaw-closing tasks in asymptomatic participants. Methods: Single motor unit (SMU) activity was recorded with bipolar fine wire electrodes at 2 sites within the right masseter muscle during the performance of standardized isometric biting on an intraoral force transducer in 17 healthy participants. Participants performed standardized biting trials during infusion of 5% hypertonic saline into the right masseter (pain), and during isotonic saline infusion (control). Recruitment patterns, force thresholds and firing rates were compared between pain and control trials. Results: A total of 50 SMUs were discriminated from the 2 sites. Changes in recruitment patterns of SMUs at one site within the masseter muscle during pain in comparison with control was observed during the tasks and some of these changes were different to the changes occurring at the other site. There were no changes in thresholds of SMU firing nor in SMU firing rates between pain and control trials. Inter-individual variability in the motor response to pain was observed at the two different sites of the masseter. Discussion and Conclusions: In general, the data are not consistent with the earlier models of pain-motor interactions, but provide support for more recent models proposing a re-organization of motor activity in the presence of pain.