PHOSPHORUS MAGNETIC-RESONANCE SPECTROSCOPY OF HUMAN MASSETER MUSCLE

Masseter muscle metabolism is poorly understood. P-31 Magnetic Resonan ce Spectroscopy (MRS) provides an opportunity for non-invasive study o f muscle metabolism during rest, exercise, and recovery. The aim of th is study was to investigate the changes in high-energy phosphates and pH in human masseter muscle associated with exertional pain. Phosphate s and pK were measured with P-31 Magnetic Resonance at 2.0 Tesla. The bite force was simultaneously measured with a force transducer. Contin uous biting at maximum voluntary bite force (MVBF) and two intermitten t biting exercises with different duty cycles were performed to pain i ntolerance. The light intermittent exercise did not produce pain. Brie f MVBF requested at the beginning, during, and end of each exercise sh owed no decay. Qualitatively, changes in phosphates were similar to th ose reported from comparable limb muscle exercises: increased inorgani c phosphate (Pi), decreased phosphocreatine (PCr), and no changes in A TP level. Quantitatively, however, the Pi/PCr ratio did not reach the levels reported in limb muscles during similar exercises. Also, the pH changed very Little. Thus, the lack of fatigue was no surprise, since the level of changes in Pi/PCr and pH, reported to be associated with fatigue in limb muscles, was far less in the masseter. Pain developme nt toward the end of the heavy exercises prevented further depletion o f metabolites. Thus, the lack of fatigue generally postulated for the masseter muscle may not be due to resistance to fatigue of these fiber s, but rather to the presence of pain preventing the fatigue. However, no specific metabolic changes associated with exertional pain were fo und.