Novel methods for quantifying neurophysiologic properties of the human lumbar paraspinal muscles

Our understanding the neurophysiologic characteristics of the human paraspinal muscles has historically been hindered by the lack of experimental techniques to examine these muscles function in vivo. In this article we describe a paired-pulse transcranial magnetic stimulation (TMS) protocol to quantify intracortical facilitation (ICF) and short-interval intracortical inhibition (SICI) of the lumbar paraspinal muscles, and an electromechanical tapping protocol to measure the amplitude of the short-latency stretch reflex. Test-retest reliability of these protocols was examined across two sessions separated by 30-min in healthy adults. We assessed relative reliability by calculating the intraclass correlation coefficient (ICC), and absolute reliability was assessed via coefficient of variation (CV). ICF and SICI in the lumbar paraspinal muscles exhibited the classical facilitatory and inhibitory responses observed in appendicular skeletal muscles ( approximately 30% facilitation and inhibition, respectively). The motor evoked potential amplitude (MEP), ICF, SICI, and stretch reflex amplitude measurements did not significantly differ between the two testing sessions (p>0.05). The MEP amplitude, ICF and stretch reflex amplitude exhibited the highest relative and absolute reliability (ICC=0.89-0.91, CV=10.6-11.1%); whereas the SICI measure exhibited somewhat lower reliability (ICC=0.75, CV=20.1%). The stretch reflex protocol performed in the first testing session did not influence the TMS outcome measures in the second testing session (p>0.05). These innovative methods may be useful in studying basic physiology, the pathology of low back pain, as well as the mechanisms of action of treatment interventions.