Page 142 - 《运动与健康科学》(英文)2024年第2期

TaggedAPTARAEndTaggedAPTARAFigure Available online at www.sciencedirect.com

TaggedAPTARAEnd TaggedAPTARAFigure TaggedAPTARAEnd

Journal of Sport and Health Science 13 (2024) 264276

Original article

TaggedAPTARAH1Lower limb suspension induces threshold-specific alterations of motor units

properties that are reversed by active recoveryTaggedAPTARAEnd
a
a
a,
a
b
TaggedAPTARAPGiacomo Valli *, Fabio Sarto , Andrea Casolo , Alessandro Del Vecchio , Martino V. Franchi ,
a a
Marco V. Narici , Giuseppe De Vito TaggedAPTARAEnd
a
TaggedAPTARAP Department of Biomedical Sciences, University of Padova, Padova 35131, Italy
b
Department Artificial Intelligence in Biomedical Engineering, Friedrich-Alexander University, Erlangen-N€urnberg 91052, Germany
TaggedAPTARAEnd Received 4 November 2022; revised 17 March 2023; accepted 16 May 2023
Available online 17 June 2023
2095-2546/Ó 2024 Published by Elsevier B.V. on behalf of Shanghai University of Sport. This is an open access article under the CC BY-NC-ND license.
(http://creativecommons.org/licenses/by-nc-nd/4.0/)
TaggedAPTARAPAbstract
Purpose: This study aimed to non-invasively test the hypothesis that (a) short-term lower limb unloading would induce changes in the neural
control of force production (based on motor units (MUs) properties) in the vastus lateralis muscle and (b) possible changes are reversed by
active recovery (AR).
Methods: Ten young males underwent 10 days of unilateral lower limb suspension (ULLS) followed by 21 days of AR. During ULLS, partici-
pants walked exclusively on crutches with the dominant leg suspended in a slightly flexed position (15˚20˚) and with the contralateral foot
raised by an elevated shoe. The AR was based on resistance exercise (leg press and leg extension) and executed at 70% of each participant’s 1
repetition maximum, 3 times/week. Maximal voluntary isometric contraction (MVC) of knee extensors and MUs properties of the vastus lateralis
muscle were measured at baseline, after ULLS, and after AR. MUs were identified using high-density electromyography during trapezoidal
isometric contractions at 10%, 25%, and 50% of the current MVC, and individual MUs were tracked across the 3 data collection points.
Results: We identified 1428 unique MUs, and 270 of them (18.9%) were accurately tracked. After ULLS, MVC decreased by 29.77%, MUs abso-
lute recruitment/derecruitment thresholds were reduced at all contraction intensities (with changes between the 2 variables strongly correlated),
while discharge rate was reduced at 10% and 25% but not at 50% MVC. Impaired MVC and MUs properties fully recovered to baseline levels
after AR. Similar changes were observed in the pool of total as well as tracked MUs.
Conclusion: Our novel results demonstrate, non-invasively, that 10 days of ULLS affected neural control predominantly by altering the discharge
rate of lower-threshold but not of higher-threshold MUs, suggesting a preferential impact of disuse on motoneurons with a lower depolarization
threshold. However, after 21 days of AR, the impaired MUs properties were fully restored to baseline levels, highlighting the plasticity of the
components involved in neural control.

TaggedAPTARAPKeywords: Disuse; High-density EMG; Muscle disuse; Neural impairment; Neuromuscular degenerationTaggedAPTARAEnd

muscle, reduction in muscle force is considered a primary
TaggedAPTARAH11. IntroductionTaggedAPTARAEnd
3
consequence of disuse. Currently, several mechanisms under-
TaggedAPTARAPSkeletal muscle disuse is a condition that can be experi-
lying disuse-induced muscle force reduction have been
enced at any age and for a broad variety of reasons, including 3
proposed, such as muscle atrophy and altered contractile
injury, hospitalization (e.g., during disease and after surgery), 4
properties of muscle fibers (i.e., excitationcontraction
and even during space flight. In order to investigate the effects
coupling) based on observations following limb suspension
of disuse on neuromuscular health, and to identify potential
and bed-rest. However, these mechanisms are considered
countermeasures, different experimental models have been insufficient to fully explain the loss of muscle force,
proposed 1,2 (e.g., limb suspension, limb immobilization, bed-rest). 3,4
suggesting that other key processes might be involved. TaggedAPTARAEnd
Regardless of the implemented model or the investigated
TaggedAPTARAPDisuse has been also defined as a reduction in the number of
1
action potentials delivered by motoneurons; in light of this, it
TaggedAPTARAEndTaggedAPTARAEnd Peer review under responsibility of Shanghai University of Sport. is surprising how little attention has been given to the role of
TaggedAPTARAEnd* Corresponding author.
E-mail address: giacomo.valli@phd.unipd.it (G. Valli). neural signaling to the muscle in disuse conditions. Indeed, the
https://doi.org/10.1016/j.jshs.2023.06.004
Cite this article: Valli G, Sarto F, Casolo A, et al. Lower limb suspension induces threshold-specific alterations of motor units properties that are reversed by
active recovery. J Sport Health Sci 2024;13:26476.