Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCA301

Poster Communications

Exploring the validity of lumbar stimulation for the assessment of lower limb motoneuron excitability in humans

P. Ansdell1, J. Škarabot1, C. Brownstein1, K. Thomas1, G. Howatson1, S. Goodall1, R. Durbaba1

1. Northumbria University, Newcastle upon Tyne, Tyne and Wear, United Kingdom.

  • Figure 1. Assessment of occlusion and facilitation in TA and RF. * indicates significant difference from single-pulse MEP values (p ≤ 0.05).

Mastoid or higher thoracic spinous process electrical stimulation is typically used to assess descending pathway excitability of motoneurons, but is accompanied with high discomfort levels when targeting lower limbs (1). Modelling studies have suggested that electrical stimulation at the lumbar spinous process level (LS) would activate the lateral spinal cord white matter of the L1 to S1 spinal segments (2, 3). Thus, the purpose of this study was to assess whether LS activated the same descending pathways to lower limbs as transcranial magnetic stimulation (TMS). Motor and lumbar evoked potentials (MEPs and LEPs) were recorded in the rectus femoris (RF) and tibialis anterior (TA) of six healthy participants (age: 27 ± 3 years). For LS, cathode and anode electrodes were placed over the L1 and T8 spinous processes, respectively. MEP and LEP amplitudes were matched at 10-15% Mmax at rest, with 10 sets of 18 single and paired stimuli delivered at ISIs ranging from −16 ms (TMS before LS) to 14 ms (LS before TMS). The LEP response was graphically subtracted from the paired response at each ISI. The amplitude of the resultant trace was then expressed as a ratio compared to single pulse MEP amplitude, with values of <1 and >1 indicating occlusion and facilitation, respectively (4). Effects of neural drive on MEPs and LEPs were investigated during 90° joint angle isometric contractions at 10, 25, 50, 75, and 100% of maximum voluntary contraction (MVC). For this part of the study, stimulus intensities were set so that MEP and LEP amplitudes matched at 50% Mmax during a 50% MVC contraction. Five respective stimuli were averaged at each contraction intensity. Stimulation intensity to elicit resting LEPs was 167±63 and 188±95 mA, and TMS intensity was 54±18 and 68±20% for TA and RF, respectively. Significant occlusion was observed for most ISIs ≥ −8 ms (p ≤ 0.035; Figure 1). For TA, significant facilitation was observed at −14 ms (p = 0.039), and for RF at −16 and −14 ms ISIs (p ≤ 0.048). To elicit a response amplitude of 50% Mmax during a 50% MVC contraction, the LS intensity was 198 ± 31 and 146 ± 61 mA and TMS was 42 ± 11 and 56 ± 24% for TA and RF, respectively. LEP and MEP amplitudes increased with contraction intensity in both muscles (p ≤ 0.031), with a plateau in responses at 50 and 75% MVC for RF and TA, respectively. These results suggest that LS was successful in activating the same corticospinal axons as TMS in TA and RF, and is consistent with upper and lower limb data for mastoid stimulation (4, 5). Also, LS intensities are ~70% lower than those reported for eliciting the same amplitude response from mastoid stimulation (5). Thus, LS could be a useful tool for assessing motoneuron pool excitability in the lower limbs at rest and during contraction.

Where applicable, experiments conform with Society ethical requirements