H-reflex Is Depressed by Conditional Magnetic Brain Stimulation during Lengthening Contraction in Man

Introduction It is well known that the integrated electromyographic (EMG) activity during lengthening (LEN) contractions is lower than that during shortening (SHO) contractions against the same load (Bigland and Lippold 1954). Even if the EMG activity is at a similar level in both contraction types, the excitability in spinal reflex pathway and/or corticospinal tract is lower during LEN contractions than that during SHO contractions (Romano and Schieppati 1987, Nakazawa et al. 1997 and Abbruzzese et al. 1994). However, it still remains unclear as to how peripheral or central nervous mechanisms are implicated in the reduction of the excitability during LEN contractions. In general, the comparison between motor evoked potential (MEP) elicited by transcranial magnetic stimulation (TMS) and Hoffmann (H-) reflex has been used to estimate the contribution of corticospinal neurons to the neural control during a specific motor task. Morita et al. (2000), however, suggested that the contribution of corticospinal neurons to neural control may in some cases be too small to be revealed from a comparison between MEP and H-reflex modulation. Therefore, they proposed that not only comparisons of MEPs and H-reflexes are made, but also that PSTH or H-reflex facilitation experiments should be conducted to disclose task-dependent changes in corticospinal excitability. This H-reflex facilitation experiment based on the following concept; when the intensities of the conditional magnetic brain stimulation are sufficiently low, any changes observed on the test reflex (H-reflex) during a specific voluntary motor task is due to the increased excitability of corticospinal neurones that are voluntarily activated by the subject oneself (Nielsen et al. 1993). Therefore, by using the H-reflex facilitation technique, we attempted to clarify the contribution of corticospinal neurons to the H-reflex modulation depending on the different contraction types.