Double stimuli paradigms should be careful interpreted when applying lumbar magnetic stimulation

Abstract

We investigated the neural structures activated by transcutaneous magnetic stimulation of the spinal cord, by examining the evoked electromyographic responses in the lower limbs. Only in few cases posterior root-muscle reflexes could be elicited by activation of the posterior root afferent fibers. Generally, M-waves were elicited by direct stimulation of the alpha motoneuron axons within the anterior root or spinal nerve. However, the double stimulus paradigm in the interpretation of the muscle responses by their refractory behavior was inefficient. Stimulation evoked strong contraction of the paraspinal muscles by the first stimulus and increased the distance between magnetic coil and the back during the second pulse, i.e. changed the stimulation conditions. To avoid misinterpretation of the data, additional neurophysiological methods to condition the responses should be applied for their identification, such as tendon vibration.