The pathophysiology of painful temporomandibular disorders is not fully understood, but evidence suggests that muscle pain modulates motor function in characteristic ways. This study tested the hypothesis that activation of nociceptive muscle afferent fibers would be linked to an increased excitability of the human jaw-stretch reflex and whether this process would be sensitive to length and velocity of the stretch. Capsaicin (10 micro g) was injected into the masseter muscle to induce pain in 11 healthy volunteers. Short-latency reflex responses were evoked in the masseter and temporalis muscles by a stretch device with different velocities and displacements before, during, and after the pain. The normalized reflex amplitude increased with an increase in velocity at a given displacement, but remained constant with different displacements at a given velocity. The normalized reflex amplitude was significantly higher during pain, but only at faster stretches in the painful muscle. Increased sensitivity of the fusimotor system during acute muscle pain could be one likely mechanism to explain the findings.