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 (Hmax) and M-wave (Mmax) amplitudes were measured to compute the Hmax/Mmax ratio. Hmax was evoked at rest and during both isometric submaximal (10%MVC) and maximal plantar-flexions (MVC). At rest, MNS techniques underestimated Hmax (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 Hmax/Mmax (ENS: 0.59 ± 0.17; sMNS: 0.45 ± 0.28; cMNS: 0.47 ± 0.29; p = 0.11). sMNS, cMNS and ENS similarly detected Hmax 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 Hmax 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.
Keywords: Electromyography; M-wave; Recruitment curve; Recruitment threshold; Spinal excitability.
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