OBJECTIVE: The purpose of this study was to clarify the relationship between changes in masseter muscle oxygenation measured by near-infrared spectroscopy (NIRS) and changes in the electromyographic (EMG) power spectrum during experimental chewing of gum with harder texture, to improve the understanding of the use of NIRS in assessing masseter muscle fatigue. MATERIAL AND METHODS: Ten female volunteers with normal occlusion were examined. Mean age (standard deviation) was 28.4 (3.8) years. Mean fracture stress of gum was 12.5 × 10(4) N/m(2). Subjects were instructed to chew gum for 60 s (75 strokes) on the voluntary chewing side at a pace of 1.25 strokes/s. Simultaneous recordings of NIRS and EMG signals from masseter muscle were performed during gum chewing. RESULTS: Oxygen saturation levels decreased from the start of chewing, then stabilized with a break point between the two phases. The normalized EMG amplitude increased and the mean frequency of the EMG power spectrum decreased during gum chewing. The timing of break point appearance was related to the timing of a significant decrease in median frequency, but no clear relationships were found between break point appearance and increased EMG amplitude. CONCLUSIONS: These results suggest that the break point of the oxygen saturation curve, as obtained from NIRS measurements, could be used as an indicator of masseter muscle fatigue as assessed by a shift in the EMG power spectrum to lower frequencies.
OBJECTIVE: The purpose of this study was to clarify the relationship between changes in masseter muscle oxygenation measured by near-infrared spectroscopy (NIRS) and changes in the electromyographic (EMG) power spectrum during experimental chewing of gum with harder texture, to improve the understanding of the use of NIRS in assessing masseter muscle fatigue. MATERIAL AND METHODS: Ten female volunteers with normal occlusion were examined. Mean age (standard deviation) was 28.4 (3.8) years. Mean fracture stress of gum was 12.5 × 10(4) N/m(2). Subjects were instructed to chew gum for 60 s (75 strokes) on the voluntary chewing side at a pace of 1.25 strokes/s. Simultaneous recordings of NIRS and EMG signals from masseter muscle were performed during gum chewing. RESULTS:Oxygen saturation levels decreased from the start of chewing, then stabilized with a break point between the two phases. The normalized EMG amplitude increased and the mean frequency of the EMG power spectrum decreased during gum chewing. The timing of break point appearance was related to the timing of a significant decrease in median frequency, but no clear relationships were found between break point appearance and increased EMG amplitude. CONCLUSIONS: These results suggest that the break point of the oxygen saturation curve, as obtained from NIRS measurements, could be used as an indicator of masseter muscle fatigue as assessed by a shift in the EMG power spectrum to lower frequencies.