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The muscle twitch profile assessed with motor unit magnetic resonance imaging

Lookup NU author(s): Dr Linda Heskamp, Matt Birkbeck, Professor Roger Whittaker, Dr Ian Schofield, Professor Andrew Blamire

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This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


Abstract

© 2021 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.Localised signal voids in diffusion-weighted (DW) images of skeletal muscle have been postulated to occur as a result of muscle fibre contraction and relaxation. We investigated the contrast mechanism of these signal voids using a combination of modelling and experimental measurements by employing DW and phase contrast (PC) imaging sequences. The DW signal and PC signal were simulated for each time point of a theoretical muscle twitch. The model incorporated compaction (simulating actively contracting muscle fibres) and translation (simulating passively moving surrounding fibres). The model suggested that the DW signal depended on contraction time and compaction whereas the PC signal depended on contraction time, compaction and translation. In a retrospective study, we tested this model with subgroup analyses on 10 healthy participants. Electrical nerve stimulation was used to generate muscle twitches in lower leg muscles; the resulting force was measured using an MR-compatible force transducer. At current levels causing a visible muscle twitch (~13 mA), the width of the first signal drop in the DW signal (mean ± SD: 103 ± 20 ms) was comparable with the force contraction time (93 ± 34 ms; intraclass correlation coefficient [ICC] = 0.717, P =.010). At current levels activating single motor units (~9 mA), the contraction time determined from the DW signal was 75 ± 13 ms and comparable with the PC contraction time (81 ± 15 ms; ICC = 0.925, P =.001). The maximum positive velocity was 0.55 ± 0.26 cm/s and the displacement was 0.20 ± 0.10 mm. Voxel-wise analysis revealed localised DW changes occurring together with more widespread phase changes. In conclusion, local signal attenuations in DW images following muscle fibre activation are primarily caused by compaction. The PC sequence also detects translating muscle tissue being passively pulled. The magnitude of the changes in DW and PC images depends on the twitch's contractile properties and percentage contraction. DW imaging and PC imaging can therefore measure twitch profiles of skeletal muscle fibres.


Publication metadata

Author(s): Heskamp L, Birkbeck MG, Whittaker RG, Schofield IS, Blamire AM

Publication type: Article

Publication status: Published

Journal: NMR in Biomedicine

Year: 2021

Volume: 34

Issue: 3

Online publication date: 06/01/2021

Acceptance date: 09/12/2020

Date deposited: 19/02/2021

ISSN (print): 0952-3480

ISSN (electronic): 1099-1492

Publisher: John Wiley and Sons Ltd

URL: https://doi.org/10.1002/nbm.4466

DOI: 10.1002/nbm.4466


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