Sex differences in the hemoglobin oxygenation state of the resting healthyhuman masseter muscle

Aims: To determine whether sex differences exist in tissue oxygen saturatio n (StO(2)) and the hemoglobin (Hb) oxygenation state of the resting human m asseter muscle. Methods: Near-infrared spectroscopy (NIRS) was used to meas ure StO(2) and Hb oxygenation state in 20 healthy adult volunteers (10 wome n and 10 men). To determine the measurement range and reliability of the NI RS recording probe, the probe was set up on 12 layers of white acrylic resi n plate, each 3 mm thick. Total hemoglobin levels were measured while a red vinyl resin plate, 1 mm thick, was inserted in turn between each of the 12 layers. Distances from the skin surface to the lateral surface (S-L) and t o the medial surface (S-M) of the right masseter at the middle portion of t he masseter were measured on T1-weighted magnetic resonance images (repetit ion time 500 ms, echo time 23 ms). Thickness of the masseter was calculated by subtraction [(S-M) - (S-L)]. For the study of Hb oxygenation state, the probe was positioned at the same position on the skin surface at the mandi bular postural (rest) position. Results: The measurement range of the NIRS probe was from 9 to 21 mm tinder the skin, and the reliability of the probe was judged by intra- and inter-class correlation coefficients. There was n o sex difference in S-L and the thickness of the masseter; the means of S-L and masseter thickness were 9.3 mm and 15.5 mm in men and 9.8 mm and 14.3 mm in women, respectively. Except for StO(2) values, there were significant sex differences in the Hb oxygenation parameters, with the mean values in the men being approximately twice those in the women. Conclusion: These res ults provide evidence that a sex difference in the Hb oxygenation state may exist in the masseter muscle of normal healthy subjects.