Short term effects of contralateral tendon vibration on motor unit discharge rate variability and force steadiness in people with Parkinson's disease

Abstract

Background: Vibration of one limb affects motor performance of the contralateral limb, and this may have clinical implications for people with lateralized motor impairments through vibration-induced increase in cortical activation, descending neural drive, or spinal excitability.

Objective: The objective of this study was to evaluate the effects of acute biceps brachii tendon vibration on force steadiness and motor unit activity in the contralateral limb of persons with Parkinson's disease.

Methods: Ten participants with mild to moderate Parkinson's disease severity performed a ramp, hold and de-ramp isometric elbow flexion at 5% of maximum voluntary contraction with the more-affected arm while vibration was applied to the distal biceps brachii tendon on the contralateral, less-affected arm. Using intramuscular fine wire electrodes, 33 MUs in the biceps brachii were recorded across three conditions (baseline, vibration, and post-vibration). Motor unit recruitment & derecruitment thresholds, discharge rates & variability, and elbow flexion force steadiness were compared between conditions with and without vibration.

Results: Coefficient of variation of force and discharge rate variability decreased 37 and 17%, respectively in post-vibration compared with baseline and vibration conditions. Although the motor unit discharge rates did not differ between conditions the total number of motor units active at rest after de-ramp were fewer in the post-vibration condition.

Conclusion: Contralateral tendon vibration reduces MU discharge rate variability and enhances force control on the more affected side in persons with Parkinson's disease.

Keywords: electromyography; motor neurons; muscle rigidity; recruitment; tremor.

Figure 1

Schematic representation of the experimental setup and protocol. (A) Intramuscular fine wires were inserted into the biceps brachii of the more-affected arm. (B) Vibration was applied to the biceps brachii tendon during the ramp, sustained contraction and de-ramp of the less-affected contralateral arm. (C) The 5% isometric ramp contraction tasks were performed 3x consecutively for three conditions of baseline (without vibration) vibration, and post-vibration (without vibration) with 3–5 min of no vibration rest between contractions. (D) Representative force, detected MUs, and separated MU action potentials of one post-vibration trial. (E) Three MUs (different colors) were extracted from this trial, and were tracked across the three conditions. Mean discharge rates were calculated between vertical dotted lines, from 7.5 s to 17.5 s and the middle 5 s reported to account for transition to and off the plateau. MUs, motor units; MUAPs, discriminated.

Figure 2

CV of force across three conditions for an isometric contraction at 5%: (A) CV of force. There were 3 contractions performed per condition for the 10 participants. (B) CV of force normalized to baseline condition for visual purpose. Δ, female; ◯, male; Horizontal line, average of trials; *, significantly different between baseline and vibration (p < 0.05); CV, coefficient of variation; Vib, vibration; post, post-vibration.

Figure 3

Motor unit discharge rates (MUDR) and motor unit discharge rate variability (CoV_ISI) of 28 MUs tracked across the plateau of the three conditions. The MUDR (A) and CoV_ISI (C) were normalized to the value of the baseline condition for MUDR (B) and CoV_ISI (D) for visual purposes. Δ, female; ◯, male; Horizontal line, average of individual trials; *, significantly different from baseline and vibration (p < 0.05); MUDR, motor unit discharge rate; CoV_ISI, motor unit discharge rate variabilit; Vib, vibration; post, post-vibration; CoV, coefficient of variation; ISI, interspike interval.