A METABOLISM STUDY OF HUMAN MASSETER MUSCLE BY P-31 MAGNETIC-RESONANCE SPECTROSCOPY DURING LONG PERIODS OF EXERCISE AND RECOVERY

The metabolism of the human masseter muscle was investigated using pho sphorus (P-31) magnetic resonance spectroscopy (MRS) during long perio ds of exercise and recovery. Eleven subjects aged 19 to 28 yr were exa mined by P-31 MRS during four consecutive periods of 13 min each: rest , exercise, recovery 1 and 2. For each subject, a biting force equal t o 20% of maximum voluntary biting force was applied and controlled dur ing the exercise period to produce maximum fatigue. P-31 MR spectra we re localized from a 24 cm(3) volume of interest using an image selecte d in vivo spectroscopy (ISIS) sequence and a 6 cm diameter surface coi l placed on the left masseter. Compared to the resting level, the phos phocreatine (PCr) content decreased by 26% during exercise, while the inorganic phosphate (Pi) concentration increased by 65%. During the tw o recovery periods, the Pi content remained decreased compared with th e resting level by 36% and 30%, respectively. The Pi/PCr ratio was inc reased from 0.30+/-0.04 at rest to 0.63+/-0.13 during exercise while t he adenosine triphosphate (ATP)/Pi ratio was decreased. The pH decreas ed from 7.02+/-0.03 to 6.93+/-0.04 during exercise and returned to con trol level (7.09+/-0.08) only during the second recovery period. These results suggest that the masseter muscle is characterized by high ATP turnover and, therefore, high oxidative phosphorylative activity in a greement with its constitution of predominantly fatigue resistant type I fibers.