Maintaining constant voluntary force in generalized myotonia despite muscle membrane disturbances: insights from a...

At medium and high force levels, patients with generalized myotonia (GM) cannot produce normal force because of a peculiar transient paresis. The authors have previously demonstrated that, during transient paresis, there is a disturbed propagation of muscle fiber action potentials along the sarcolemma, resulting in conduction block and paresis. At low force levels, however, these patients can produce a constant force. It is as yet unknown how patients with GM, despite these muscle membrane abnormalities, are able to produce constant force during isometric voluntary low-force contractions. Using high-density surface EMG (SEMG), the authors tested the hypothesis that in patients with GM, muscle membrane function is already disturbed at low force levels despite constant force and that abnormal motor unit recruitment acts as a compensatory mechanism to obtain normal force stability. High-density SEMG was measured on the biceps brachii at 20% of the maximal voluntary contraction (MVC) in seven patients with GM previously shown to have transient paresis at higher force levels. High-density SEMG provides spatial and temporal information that is used to analyze propagation disturbances and recruitment strategies. In addition, needle EMG was performed in two patients simultaneously. The SEMG abnormalities included disturbed propagation of motor unit action potentials (MUAPs) over the muscle fiber membrane and abnormal motor unit recruitment patterns, pointing to central adaptation mechanisms. Prolonged periods of low SEMG amplitudes were sometimes present despite constant force, indicating that mechanisms other than motor unit recruitment also contribute to force preservation. During the periods of low SEMG, the density of myotonic discharges, recorded simultaneously with needle EMG, was not increased. Patients with GM can show, despite muscle membrane dysfunction, normal force stability. Abnormal motor unit recruitment as a compensatory mechanism occurs in patients with GM but cannot fully explain constant force.