A new method to evaluate staircase phenomenon in skeletal muscle using piezoelectric sensor.

Objective: The staircase phenomenon, which refers to the increases in the force of contraction with repetitive stimulation of the muscle, has been studied for many years, but the method is difficult and not widely used. Our objective was to evaluate the staircase phenomenon in skeletal muscle using a piezoelectric sensor. Methods: Thirty-five subjects without neuromuscular diseases (normal controls), 11 patients with Becker muscular dystrophy (BMD), and 19 patients with myotonic dystrophy type 1 (MyD) were studied. Compound muscle action potential (CMAP) and movement-related potential (MRP) waveforms were recorded using piezoelectric sensors during repetitive stimulation of the median nerve, and the amplitudes and durations were measured. Excitation-contraction (E-C) coupling time (ECCT) was calculated from the difference between onset latencies of CMAP and MRP. Results: In normal controls, MRP amplitude ratio (relative to baseline) increased significantly with increase in stimulation duration and with increase in stimulation frequency. In BMD and MyD, however, MRP amplitude ratio did not change significantly with increase in stimulation duration. Especially, in MyD, there was no change in MRP amplitude ratio with increase in frequency. Conclusion: Staircase phenomenon abnormalities can be evaluated easily using piezoelectric sensors, indicating their potential utility for evaluating E-C coupling impairment in myopathies. Significance: Piezoelectric sensors may be a useful tool to evaluate staircase phenomenon in skeletal muscle.