Analyses of muscular activity, energy metabolism, and muscle fiber type composition in a patient with bilateral masseteric hypertrophy

Hyperwork of the masseter muscles due to habitual parafunction is thought t o induce masseteric hypertrophy (so called work hypertrophy). However, the causes underlying this disease are not yet fully understood. Recently, we h ad a patient with bilateral masseteric hypertrophy, and we performed a part ial excision of the masseter muscles. In this patient's case, we examined m uscular activity, energy metabolism, and fiber type composition of the mass eter muscles using electromyograms (EMG), P-31-magnetic resonance spectrosc opy (MRS), and enzyme-histochemistry. The EMG showed no hyperactivity, and the P-31-MRS showed normal energy spectral patterns and PCr contents of the masseter muscles. The fiber type composition, however, in the muscles in t his case was very different from that in muscles with "work hypertrophy" an d also that in normal masseter muscles: 1. Loss of type IIB fibers; 2. Incr eases in type IIA and in type IM & IIC fibers; and 3. Decrease in type I fi bers. The findings suggest that this is not a case of work hypertrophy but a case of compensatory hypertrophy possibly due to a lack of high-tetanus-t ension type IIB fibers.