Intramuscular haemodynamic responses to different durations of sustained extension in normal human masseter

Abstract

Ten healthy non-smoking males (mean age 24.3±0.8 years) with no history of chronic muscle pain or migraine participated in this study. Intramuscular total haemoglobin (Hb), an indicator of blood volume in the illuminated area, was measured with a non-invasive, near-infrared spectroscopic device. Each participant was told to maintain maximal mouth opening to extend the masseter muscle for 30, 60 or 120 s in random order. Data were continuously recorded from the right masseter 1 min before, at set times during and for 5 min after sustained muscle extension in each trial. Each trial was separated by a 10-min interval. Heart rate (HR) and blood pressure (BP) were also recorded. The mean normalized Hb decreased during muscle extension and rebound hyperaemia was observed after it in each trial (P=0.0001). Hb returned to baseline within 60 s. The magnitude of the decremental change during extension and of the incremental change in the rebound hyperaemia was not significantly different among the three trials (P=0.9071); neither were mean normalized HR and BP. These data suggest that sustained extension of the masseter produces a reduction in total intramuscular Hb during extension and a secondary increase in Hb following a return to the resting muscle's normal length.

Introduction

Stretching, shortening (with or without contraction) and isometric contraction are biomechanical factors that might influence haemodynamic measurements in jaw muscles. With the exception of a recent study by Kim et al. (1999), the effect that these factors might have on muscle blood volume has not been evaluated systematically. Such information would be unimportant if all research on blood flow was on resting (neither stretched nor shortened) muscles. Of course this is not the case and several recent studies on muscle blood flow involve stretching and/or contraction of the muscle studied. For example, Larsson et al. (1994) measured the microcirculation in the upper portion of the trapezius in 25 patients with pain persisting after a soft-tissue injury of the neck. They reported that the ability to increase muscle blood flow during arm elevation was impaired on the painful side in the patients with unilateral pain. Delcanho et al. (1996) demonstrated less perfusion of the masseter muscle immediately after maximal isometric contraction in patients with chronically painful masticatory muscles than in asymptomatic individuals. Tsukiyama et al. (1999) looked at the combined effects of cold pressor stimuli and muscle contraction on perfusion in the masseter. In all of these studies, some degree of stretching, shortening or contraction occurred.

In the one study in which contraction was evaluated systematically, Kim et al. (1999) describe the influence that various degrees of isometric muscle activation can have on intramuscular haemodynamics (specifically blood-volume recordings) in the masseter and temporalis. What is missing now is research on the influence that various amounts of stretching might have on jaw muscle haemodynamics (e.g. blood volume). Intramuscular blood flow has been measured during stretching of other muscles in several animal studies, but not in humans. Those animal studies revealed that intramuscular blood flow is impeded by stretching (Gimlette and Nasrallah, 1969, Wisnes and Kirkebø, 1976, Supinski et al., 1986). Gimlette and Nasrallah (1969) measured blood flow in a skeletal muscle (dog gastrocnemius) during and after stretching, using both ¹³³Xenon clearance and venous occlusion plethysmography, and observed a decrease of flow during stretching with both methods. However, although by plethysmography they found a small increase in blood flow after the stretching, the decreased rate of ¹³³Xenon clearance did not return to baseline. These studies are helpful, but in vivo human studies on specific muscle groups are needed.

We now elected to assess changes in intramuscular blood volume in the human masseter under conditions of maximum stretch. To determine whether any observed haemodynamic changes were dependent on the conditions of the stretch itself or on its duration, we also used varying periods of muscle extension. The null hypothesis was that there is no difference in the blood volume of the masseter before, during and after maximum extension of different durations.