Assessment of the H-reflex using two synchronized magnetic stimulators in order to increase stimulus durations: A comparison with electrical stimulation.

Magnetic nerve stimulation (MNS) may be a less painful alternative to electrical nerve stimulation (ENS) for Hoffmann reflex (H-reflex) measurement, however standard MNS (sMNS) techniques utilize a short stimulus duration, thereby limiting its use for H-reflex assessment. This limitation may be partly overcome by coupling two magnetic stimulators to increase the pulse duration (coupled MNS: cMNS). The aim of this study was to test this assumption by comparing the H-reflex characteristics evoked by ENS, sMNS and cMNS. Thirteen healthy volunteers were tested with ENS and both MNS in the prone position. Maximal soleus H-reflex (H) and M-wave (M) amplitudes were measured to compute the H/M ratio. H was evoked at rest and during both isometric submaximal (10%MVC) and maximal plantar-flexions (MVC). At rest, MNS techniques underestimated H (ENS: 8.32 ± 2.73 mV; sMNS: 6.85 ± 2.29 mV; cMNS: 7.48 ± 2.23 mV; p < 0.05). In contrast, no difference was observed for H/M (ENS: 0.59 ± 0.17; sMNS: 0.45 ± 0.28; cMNS: 0.47 ± 0.29; p = 0.11). sMNS, cMNS and ENS similarly detected H facilitation during MVC (ENS: +120 ± 248%; sMNS: +228 ± 350%; cMNS: +162 ± 180% of the rest value; p = 0.344). Owing to their shorter stimulus duration, both MNS techniques underestimated the H compared to ENS. However, when the gold standard ENS technique cannot be used, coupled MNS may be recommended since it provides better H-reflex characteristic assessment than standard MNS due to its longer stimulus duration.