From motor unit behaviour to functional ability : the role of dopaminergic medication in Parkinson’s disease

This dissertation aimed to evaluate the influence of dopaminergic medication on the affected upper limb in people with Parkinson’s disease (PD), spanning outcomes from functional performance, such as hand dexterity, to underlying neuromuscular mechanisms at the motor unit (MU) level. Sixteen people with PD and 13 controls participated in this study. Participants with PD completed two identical sessions in a randomized order: one in the ON and one in the OFF medication phase. The control group completed only one session. The specific objectives of this dissertation were: 1) to compare the treated (ON) and untreated (OFF) phases in people with PD to examine effects of dopaminergic medication on hand dexterity, motor symptom severity, strength and twitch contractile properties including peak tension, time to peak tension, contraction duration and half relaxation time of biceps brachii muscle 2) to determine whether dopaminergic medication optimizes motor neuronal properties and force control during an isometric contraction in the more affected upper limb of people with PD, 3) to investigate motoneuron excitability in people with PD during ON and OFF medication phases and to compare them with age-matched controls. Study #1 addressed objective 1 by demonstrating that dopaminergic treatment improved motor symptoms, hand dexterity, and strength, without altering electrically induced contractile properties. Improved hand dexterity during treatment appears to be linked to strength gains independent of peripheral muscle changes. Study #2 addressed objective 2 by showing that dopaminergic medication enhances force control in people with PD by improving force steadiness and reducing MU discharge variability, particularly during the least steady period of a contraction. Also, findings suggest that medication may also reduce unwanted muscle coactivation and increase MU recruitment threshold. Finally, Study #3 addressed objective 3 by revealing that motoneuron excitability altered in people with PD compared with controls, independent of dopaminergic medication. These changes, likely driven by increased calcium influx, reduced synaptic inhibition, and heightened cortical facilitation and are not substantially normalized by dopamine therapy. Overall, this dissertation provides new insight into the effects of dopaminergic medication on MU characteristics in people with PD, highlighting its importance for improving force control and functional outcomes.