Masticatory Muscle Fatigue Does Not Modulate the Jaw-Stretch Reflex

The short-latency jaw-stretch reflex can be used to study the consequences of trigeminal pain on the control of jaw movements. Although this reflex circuit principally involves a monosynaptic arc, experimental pain is known to enhance the amplitude of the reflex response. However, the contribution of muscle fatigue to this modulation is unknown. Objectives: To examine the effects of experimentally evoked muscle fatigue, without and with muscle pain, on the short-latency jaw-stretch reflex, using a randomized crossover design. Methods: Fifteen men (24.0 ± 4.1 years) and thirteen women (24.8 ± 2.3 years) participated in this study. Reflexes were evoked in both the masseter and temporalis muscles, using a 1-mm downward displacement, a ramp time of 10 ms, and a pre-set clenching level of 15% of the maximum voluntary EMG contraction level. The study was performed in two blocks, both containing three experimental conditions (before, directly after, and 15 min after provocation). Provocation consisted of a fatiguing chewing test, followed by an intramuscular injection of either isotonic saline (IS; non-painful) or hypertonic saline (HS; painful). Results: ANOVA for repeated measures indicated no significant effect of the experimental condition ‘fatigue + IS' on onset latency and normalized peak-to-peak reflex amplitude. For each muscle, the ‘fatigue + HS' condition yielded significantly higher normalized reflex amplitudes than the other conditions (F > 3.90; P < 0.02). Several muscles displayed gender differences regarding onset latency and normalized reflex amplitude, which may, at least in part, be explained by anatomical differences in motor units between men and women. Conclusion: Experimentally evoked, mild-to-moderate muscle fatigue does not modulate the human jaw-stretch reflex. (Supported by IOT.)