Haemodynamic changes induced by submaximal isometric contraction in painful and non-painful human masseter using near-infra-red spectroscopy

Although mechanisms underlying chronic muscle pain are poorly understood, one prevalent theory is that it is due, in part, to localized hypoxia. The purpose of this study was to evaluate this theory using non-invasive near-infra-red spectroscopy that monitors relative changes in intramuscular haemoglobin (Hb) concentration and oxygen saturation levels. Data were collected for the human masseter muscle during and following three isometric 30-s trials at 50% maximum voluntary contraction. Ten females, with a history of chronic muscle pain in the jaw, and eight matched healthy females without muscle pain (controls) participated. Results showed that, upon initiation of masseter muscle contraction, there was a rapid reduction in the intramuscular Hb concentration concomitant with a reduction in oxygen saturation levels. After cessation of the contraction, the Hb concentration increased rapidly and then fell toward the baseline. Significant differences in the recovery profile for oxygen saturation were found between the first trial and the following two trials for both the muscle pain- and control group. Looking at the first trial only, and adjusting for covariates of height, weight and bite-force in the analysis, revealed a marginally significant postcontraction difference between the two groups with a lower level of oxygen saturation during recovery in the group with chronic muscle pain. Significant group differences were found in Hb concentrations without any significant trial effect. It is likely that the well-known changes in intramuscular blood flow that occur during and after contraction in human muscles are reflected in these altered relative Hb concentrations. The group with chronic muscle pain showed a clearly reduced magnitude of the Hb concentration change in the postcontraction recovery period. The results support the concept that patients with chronic muscle pain have a slower intramuscular reperfusion during the recovery phase after sustained isometric contractions.