Estimates of persistent inward currents in tibialis anterior motor units during standing ramped contraction tasks in humans

Abstract

Persistent inward currents (PICs) play an essential role in setting motor neuron gain and shaping motor unit firing patterns. Estimates of PICs in humans can be made using the paired motor unit analysis technique, which quantifies the difference in discharge rate of a lower-threshold motor unit at the recruitment onset and offset of a higher-threshold motor unit (∆F). Because PICs are highly dependent on the level of neuromodulatory drive, ∆F represents an estimate of level of neuromodulation at the level of the spinal cord. Most of the estimates of ∆F are performed under constrained, isometric, seated conditions. In the present study, we used high-density surface EMG arrays to discriminate motor unit firing patterns during isometric seated conditions with torque or EMG visual feedback and during unconstrained standing anterior-to-posterior movements with RMS EMG visual feedback. We were able to apply the paired motor unit analysis technique to the decomposed motor units in each of the three conditions. We hypothesized that ∆F would be higher during unconstrained standing anterior-to-posterior movements compared to the seated conditions, reflecting an increase in the synaptic input to MNs drive while standing. In agreement with previous work, we found that there was no evidence of a difference in ∆F between the seated and standing postures, although slight differences in the initial and peak discharge rates were observed. Taken together, our results suggest that both the standing and seated postures are likely not sufficiently different--both being "upright" postures--to result in large changes in neuromodulatory drive.

Antagonist tendon vibration dampens estimates of persistent inward currents in motor units of the human lower limb

“…When utilizing decomposition of HDsEMG to identify MUs, each test unit can have tens of usable test unit-reporter unit pairs that can be used to calculate ΔF. To reduce the number of ΔF estimates obtained from each unit, while also reducing the variability across the pool of identified MUs, we chose to take the mean of all ΔF values obtained from each test unit that satisfied our inclusion criteria (Khurram et al ., 2021; Hassan et al ., 2021). This value will henceforth be referred to as simply ΔF.…”

Effects of reciprocal inhibition and whole‐body relaxation on persistent inward currents estimated by two different methods

“…Foley & Kalmar, 2019; Marchand‐Pauvert et al., 2019; Wilson et al., 2015) or high‐density surface EMG electrode arrays (e.g. Hassan et al., 2021; Khurram et al., 2021; Orssatto et al., 2021) to detect single MU firings using decomposition techniques. These approaches exploit the 1‐to‐1 ratio between the firing rate of a motoneuron and MU action potentials in the muscle, providing a unique window into the central nervous system, but require computational procedures that are relatively time‐consuming and complex.…”

Longitudinal changes in intrinsic motoneuron excitability in amyotrophic lateral sclerosis are dependent on disease progression

“…During the MVCs, bipolar electromyography (sEMG) signals were collected from two channels in the middle of the HD‐sEMG electrode grid, and the peak root mean square (RMS) was recorded. Two minutes later, participants performed two ramp‐shaped contractions to 40% of their peak RMS obtained during the MVCs (Khurram et al., 2021), at a rate of RMS rise and decline of 2.67%/s (15‐s up and 15‐s down). Participants received real‐time visual sEMG RMS‐trace feedback during each contraction and were instructed to follow the sEMG RMS path displayed on a 14 inch laptop monitor (Khurram et al., 2021).…”