Document Type

Thesis

Degree Name

Master of Science (MSc)

Department

Kinesiology

Faculty/School

Faculty of Science

First Advisor

Dr. Jayne M. Kalmar

Advisor Role

Principal Investigator

Abstract

Spinal motor neuron (MN) persistent inward currents (PIC) increase and sustain the MN output. PIC is amplified by descending monoamines and reduced by afferent inhibition. There is strong coupling between plantar cutaneous sensory input and lower limb MNs. Given that monoaminergic drive is altered with postures and motor tasks, our purpose was to assess posture-dependent plantar cutaneous sensory modulation of tibialis anterior (TA) MNs. We hypothesize that: 1) a supine position will suppress PIC, 2) removing the load from the bottom of the heel will inflate PIC, and 3) cutaneous stimulation of the plantar aspect of the heel will reduce PIC. We developed a novel dynamometer that allows for isometric dorsiflexion without any material in contact with the heel. Thirteen participants (6 female) performed isometric dorsiflexion contractions with and without heel pressure in seated upright and supine body positions with the hip, knee, and ankle at 90°. Cutaneous electrical stimulation (5x1ms pulses at 333Hz at randomized intervals of 0.5-0.8s, 2x perceptual threshold) was applied to the heel during half of these contractions. Intramuscular EMG was recorded from the TA, and surface EMG from the TA and SOL muscles. PIC was estimated using the paired motor unit technique, and post stimulus time histograms were used to quantify the amount of TA MN inhibition elicited via cutaneous electrical stimulation. We found no effect of posture (p=0.857), or of electrical stimulation (p=0.473). However, ∆F estimates of PIC were lower with heel pressure (3.87 ± 1.67pps) than without heel pressure (4.70 ± 1.91pps). Lower PIC with heel pressure could not be attributed to differences in TA or antagonist (SOL) muscle activity. We speculate that lower PIC with heel pressure was due to plantar cutaneous afferent inhibition. This study may contribute to our understanding of top-down and bottom-up regulation of spinal MN excitability during different tasks. This may improve our understanding of motor deficits associated with injury, ageing, and neuromuscular pathologies.

Convocation Year

2024

Convocation Season

Spring

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Available for download on Friday, September 04, 2026

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