Establishing between-session reliability of TMS-conditioned soleus H-reflexes

Abstract

Transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) can be used to evaluate descending corticomotor influences on spinal reflex excitability through modulation of the Hoffman reflex (H-reflex). The purpose of this study was to characterize between-session reliability of cortical, spinal, and cortical-conditioned spinal excitability measures collected from the soleus muscle. Thirteen able-bodied young adult participants were tested over four sessions. Intraclass correlation coefficients were calculated to quantify between-session reliability of active motor threshold (AMT), unconditioned H-reflexes (expressed as a percentage of M_max), and conditioned H-reflexes using short-latency facilitation (SLF) and long-latency facilitation (LLF). Pearson correlation coefficients were calculated to assess associations between H-reflex facilitation and unconditioned H-reflex amplitude. Between-session reliability for SLF (ICC=0.71) was higher than for LLF (ICC=0.45), was excellent for AMT (ICC=0.95), and was moderate for unconditioned H-reflexes (ICC=0.63). Our results suggest moderate-to-good reliability of SLF and LLF to evaluate cortical influences on spinal reflex excitability across multiple testing sessions in able-bodied individuals.

Keywords: Corticospinal excitability; Facilitation; H-reflex; Reliability; TMS; TMS-conditioning.

Fig. 1

Example of a H-reflex recruitment curve from a single participant. Red dashed lines mark the stimulation intensity used to elicit unconditioned H-reflexes.

Fig. 2

Raw EMG traces of illustrating TMS-conditioning of the H-reflex from a single participant. Top: average of 20 unconditioned H-reflex responses recorded from the soleus muscle during electrical stimulation of the tibial nerve. Middle: TMS-conditioning of the H-reflex during short-latency facilitation (SLF, ISI: −1.5ms, gray line). Bottom: TMS-conditioning of the H-reflex during long-latency facilitation (LLF, ISI: +10ms, gray line) Red dashed line represents onset of peripheral nerve stimulation, and black solid lines represent TMS delivery over the cortical representation of the soleus. In line with previous research [10, 20], larger TMS-conditioned H-reflexes were observed for LLF compared to SLF (statistical results not reported).