Nerve conduction velocity of motor and sensory 1α-fibres of the tibialis nerve:: A magnetic nerve stimulation study

Peripheral motor fibres and sensory I alpha-fibres of the tibialis nerve can be excited by means of electromagnetic pulses. The compound muscle action potential (CMAP) and the H-reflex can be recorded with surface electrodes from the M. triceps surae following magnetic stimulation. We used a Magstim 200 magnetic stimulator and a magnetic coil with 14 cm outer diameter to evoke H-reflexes and CMAPs at different stimulation sites (knee, gluteal fold, vertebral column) along the tibialis nerve of 27 healthy subjects. Onset latencies of the CMAP and peak latencies of the H-reflex were determined and the distance between stimulation sites was measured. From these data the nerve conduction velocity (NCV) for motor fibres and I alpha-fibres were calculated. In the nerve segment between the vertebral column and the knee a motor NCV of 50.0 +/- 5.6 m/s and a sensory NCV of 58.1 +/- 7.2 m/s was calculated. A significantly faster NCV was found for motor fibres (68.0 +/- 13.4 m/s) and for sensory fibres (69.6 +/- 19.3 m/s) in the proximal nerve segment between the vertebral column and the gluteal fold. For the nerve segment including both sections of the nerve we calculated a mean NCV of 57.4 +/- 6.2 m/s for motor fibres and 63.3 +/- 8.0 m/s for sensory lu fibres. We conclude that this method provides an adequate tool for investigating the NCV of proximal motor and I alpha-fibres of the tibialis nerve separately.