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The effect of acute muscle tendon vibration on motor unit activity in the contralateral, more-affected limb in Parkinson’s disease Larocque, Kaylee Agnes


Acute tendon vibration is commonly used to activate the Ia afferents though excitation of the muscle spindles; increasing excitation of the alpha motoneurons of the vibrated muscle, enhancing muscle contraction and inducing reciprocal inhibition of the antagonist muscle. Previous reports suggest that the excitatory and inhibitory effects of vibration stimulation on muscle activity can be transferred to the contralateral side of the body and that this occurs via excitation of the polysynaptic crossed-extensor reflex loop. The effect of acute tendon vibration on motor unit (MU) activity in the contralateral moreaffected side in Parkinson’s disease (PD) has not been studied. Six males and four females with mild to moderate PD severity maintained isometric elbow flexion at 5% maximum (MVC). Vibration was applied to the biceps tendon of the less-affected side and used to quantify the effect of polysynaptic Ia afferent feedback on MU properties in the contralateral biceps muscle during the 5 sec ramp, 5 sec plateau, and 5 sec deramp contraction. Twenty-nine MUs in the biceps brachii were followed. MU recruitment thresholds, derecruitment thresholds, MU discharge rates (MUDR), MU discharge rate variability (MUDRV) and force steadiness, quantified as standard deviation around a mean force, were compared across conditions (no-vibration, biceps-vibration), phases of the contraction (ramp, plateau, deramp) and between sexes. Recruitment thresholds were higher than derecruitment thresholds. MUDR was higher in females during the plateau and higher in males during the deramp phase. MUDR was also higher during the deramp phase of the no-vibration compared to the biceps-vibration condition. MUDRV was higher in males during the plateau phase of both conditions and during the deramp phase of the biceps-vibration condition. Males were steadier and had a higher PD rating score compared with females. Results suggest that vibration moderately influences MU activity in the contralateral limb in PD and that males and females with PD respond differently to vibration stimulation. These sex differences are likely due to muscle fibre characteristics or level of severity of PD between males and females.

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