Mechanomyographic determination of post-activation potentiation in myopathies

Objective There is a need to provide an index of muscle contractility in evaluating myopathies especially in the clinical setting. This study was conducted to investigate if the mechanomyogram post-activation potentiation (MMG-PAP) can be used as an index of muscle contractile force potentiation (force-PAP), if it differs between normal and myopathic muscles, and if it can reflect abnormalities in muscle fiber anatomy. Methods The correlation between MMG-PAP and force-PAP was evaluated in 12 normal subjects after maximum voluntary contraction (MVC) of the biceps brachii muscle. The same method was then applied to study MMG-PAP in 16 patients with myopathies, 16 disease and 25 normal controls. Mean fiber diameters and the proportions of type 1 and 2 fibers in biopsied biceps brachii muscle were determined and compared with MMG-PAP values. Results There was a significant positive correlation between force-PAP (197±148%) and MMG-PAP (135±68%) immediately after MVC (P<0.05). The mean MMG-PAP in myopathies (66±53%) was significantly lower than those of the disease (128±34%; P<0.005) and normal controls (120±56%; P<0.005). Patients with non-dystrophic myopathies, including those with myositis, had significantly lower MMG-PAP values (38±20%; P<0.005) than those with muscular dystrophy (148±23%). MMG-PAP did not clearly correlate with either type 2 fiber atrophy or type 2 fiber disproportion based on muscle biopsy analysis of myopathic patients. Conclusions This study shows that MMG-PAP can be used as an index of muscle contractility and that it is significantly lower in non-dystrophic myopathies compared to normal subjects. MMG-PAP does not seem to reflect abnormal muscle fiber anatomy. Significance MMG-PAP may become a valuable non-invasive tool in augmenting routine clinical electrophysiologic studies especially in evaluating muscle contractility in myopathies.