The sufficiency of blood flow in human masseter muscle during endurance of biting in the intercuspal position and on a force transducer.

: Blood flow changes caused by endurance of voluntary isometric muscle contraction and the sufficiency of flow during contraction were assessed by measuring 133Xe clearance. Aiming at 50% maximum voluntary contraction (MVC), 9 healthy persons sustained biting in the intercuspal position (ICP group) at an actual medium EMG activity level of 55% MVC, while receiving a visual feedback of the average-integrated EMG activity. Eleven persons sustained unilateral biting (UBF group) on a force transducer at 40% MVC, receiving feedback of the force output. The significantly lower % MVC for the UBF group was due in part to a decrease in the EMG activity during endurance, while force was constant. Blood flow changes over time were significant; however, the changes differed significantly between groups during endurance: the ICP group had a median reduction in blood flow to 0.4 of the initial resting value, and the UBF group had a 0.2-fold median increase. Following endurance, flow changes integrated over 3 minutes were about 43-fold the initial resting values of both groups. This similarity was probably a result of their equivalent effort. The endurance flow accounted for 1% of the total change for the ICP group and 5% for the UBF group. Overall, the proportion of the total flow that took place during endurance decreased logarithmically with greater level of contraction and masseter effort. Therefore, sufficiency of blood flow to maintain muscle fibre homeostasis is less when the rate of metabolic turnover is greater, thus contributing to an earlier onset of masseter fatigue, pain and exhaustion at high contraction levels.