Masseter motor unit recruitment is altered in experimental jaw muscle pain

Some management strategies for chronic orofacial pain are influenced by models (e.g., Vicious Cycle Theory, Pain Adaptation Model) proposing either excitation or inhibition within a painful muscle. The aim of this study was to determine if experimental painful stimulation of the masseter muscle resulted in only increases or only decreases in masseter activity. Recordings of single-motor-unit (SMU, basic functional unit of muscle) activity were made from the right masseters of 10 asymptomatic participants during biting trials at the same force level and direction under infusion into the masseter of isotonic saline (no-pain condition), and in another block of biting trials on the same day, with 5% hypertonic saline (pain condition). Of the 36 SMUs studied, 2 SMUs exhibited a significant (p < 0.05) increase, 5 a significant decrease, and 14 no significant change in firing rate during pain. Five units were present only during the no-pain block and 10 units during the pain block only. The findings suggest that, rather than only excitation or only inhibition within a painful muscle, a re-organization of activity occurs, with increases and decreases occurring within the painful muscle. This suggests the need to re-assess management strategies based on models that propose uniform effects of pain on motor activity.