Magnetic stimulation of the quadriceps: analysis of 2 stimulators used for diagnostic and therapeutic applications

Compromised muscle function can be evaluated in respiratory disease patients by supramaximal magnetic stimulation (isometric twitch) of the quadriceps, a technique that is reproducible and objective. We validated the technique, comparing a device used in our laboratory with another reference electromagnet. We also assessed whether the technique could potentially be used to train the muscle by repetitive stimulation. The Medtronic Magpro (MED) device with a circular coil and the Magstim 200 device (MAG) with a figure-of-eight coil (reference device) were used to stimulate the femoral nerve of 6 volunteers at different percentages of maximal output. MED stimulation was also applied on the quadriceps muscle. We measured voluntary contractions, comparing measurements from the 2 devices and on different days. The stimulation achieved with MED was lower than with MAG, showed greater day-to-day variability, and was not clearly supramaximal. MED quadriceps stimulation was 80.7% of MAG stimulation. In conclusion, supramaximal stimulation of the quadriceps cannot be guaranteed with MED and the circular coil. However, this device generates sufficient contraction when applied to the muscle to be used for repetitive stimulation.