Orofacial pain and jaw muscle activity: a new model.

Two major theories proposed to explain the effect of pain on muscle activity are the Vicious Cycle Theory and the Pain Adaptation Model. Comprehensive reviews demonstrate conflicting or limited evidence in support of a critical aspect of the Vicious Cycle Theory, namely that pain leads to increased muscle activity. The Pain Adaptation Model proposes that changes in muscle activity limit movement and thereby protect the sensorimotor system from further injury. This model is generally considered the most appropriate explanation of the effect of pain on muscle function. Although there is much literature consistent with the model, there are a number of lines of evidence that appear inconsistent with it. Possible reasons for the lack of consistency between studies include the functional complexity of the sensorimotor system (eg, the possibility of different pain effects at different sites within functionally heterogeneous muscles), and the multidimensional nature of pain. The latter consists of sensory-discriminative, cognitive-evaluative, and motivational-affective components, where factors such as pain location, intensity, and characteristics and other supraspinal/suprabulbar influences may modify the effects of pain on motor activity. The variety of changes in electromyographic (EMG) activity features during pain suggests that pain and motor function are not hardwired. The authors propose that the existing Pain Adaptation Model is a subset of a broader model that could be called the Integrated Pain Adaptation Model. Given the recent view of pain as a homeostatic emotion requiring a behavioral response, this new model states that pain results in a new, optimized recruitment strategy of motor units that represents the individual's integrated motor response to the sensory-discriminative, motivational-affective, and cognitive-evaluative components of pain. This recruitment strategy aims to minimize pain and maintain homeostasis.