Haemodynamic changes in human masseter and temporalis muscles induced by different levels of isometric contraction

This study evaluated the influence of low contraction forces on intramuscul ar haemodynamics in human masseter and temporalis using near-infrared tissu e spectroscopy. This method allowed the intramuscular haemoglobin (Hb) to b e assessed dynamically before, during and after a 5, 15, 25 and 100% maximu m voluntary contraction (MVC). Twenty volunteers, 10 males and 10 females, without pain or dysfunction in the masticatory system were included in this study. Data were recorded for 30 s before, 30 s during and 5 min after the four sustained contraction tasks. The results showed that all four levels of voluntary contraction produced a clear haemodynamic response (during and after contraction) in both muscles. For analytical purposes, the maximum H b achieved after 100% MVC was set equal to 1.00. In the masseter the mean p eak Hb during the 5, 15, 25 and 100% MVC was 0.49, 0.92, 1.30 and 1.73 whil e after the contractions it was 0.50, 0.65, 0.78 and 1.00, respectively. In the temporalis the peak Hb during the contractions was 0.23, 0.36, 0.48 an d 0.66 and after the contractions 0.32, 0.45, 0.56 and 1.00, respectively. Repeated-measures analysis of variance revealed a significant main effect f or the different contraction levels both in the masseter (during contractio n, p=0.001; after contraction, p < 0.001) and the temporalis (during contra ction, p=0.002; after contraction, p < 0.001). These data suggest that low levels of contraction induce a clear haemodynamic response, even at 5% effo rt. When compared, the masseter and anterior temporalis showed clearly diff erent patterns for the Hb signal during the contraction (p < 0.001) as well as after it (p = 0.007). Specifically, the Hb during the contractions in t he masseter appeared more stable than in the temporalis, which showed a str ong return to baseline. Obviously the contracting masseter had a stronger a nd more sustained venous occlusion than the contracting temporalis. It is s peculated that variation in architecture between the two muscles contribute s to these differences in blood flow. (C) 1999 Published by Elsevier Scienc e Ltd. All rights reserved.