Early sub-maximal quadriceps motor unit firing characteristics and six-month knee extensor strength after anterior cruciate ligament reconstruction: a prospective longitudinal study

Rob Buhmann; Abram Reeves; Daniel Mellifont; Max Stuelcken; Mark Sayers; David N Borg; Steven Lawrie

doi:10.1080/07853890.2026.2740295

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Early sub-maximal quadriceps motor unit firing characteristics and six-month knee extensor strength after anterior cruciate ligament reconstruction: a prospective longitudinal study

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Early sub-maximal quadriceps motor unit firing characteristics and six-month knee extensor strength after anterior cruciate ligament reconstruction: a prospective longitudinal study

Rob Buhmann, Abram Reeves, Daniel Mellifont, Max Stuelcken, Mark Sayers, David N Borg and Steven Lawrie

Annals of Medicine, Vol.58(1), pp.1-13

2026

DOI: https://doi.org/10.1080/07853890.2026.2740295

PMID: 42804596

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Abstract

Anterior cruciate ligament reconstruction

quadriceps

motor units

high-density surface electromyography

knee extensor strength

Objectives To determine whether quadriceps motor unit firing rates measured pre-operatively and during the first three months after anterior cruciate ligament (ACL) reconstruction are associated with isometric knee extensor strength at six months, and to explore the relationship between motor unit action potential (MUAP) amplitude and firing rate. Methods Twenty-seven participants (15 female) underwent surface electromyography of vastus medialis and vastus lateralis during sub-maximal isometric knee extension from pre-operation to three months post-reconstruction. Six-month knee extensor MVC force was modelled using Bayesian regression and interpreted using a region of practical equivalence (ROPE; ±15 N). A secondary hierarchical analysis examined MUAP amplitude and firing rate. Results Motor unit firing rate was not consistently associated with six-month knee extensor strength across time points and limbs. In the ACL reconstructed limb at two weeks, a 1-SD greater mean firing rate was associated with 45.8 N greater six-month strength (95% credible interval [CrI] − 18.5 to 114.6 N). Six-month strength was greater in the contralateral limb (β = 198.4 N, 95% CrI 171.6 to 225.1 N), in males (β = 184.3 N, 95% CrI 32.0 to 325.1 N), and with greater body mass (β = 119.9 N per 1-SD, 95% CrI 33.4 to 207.2 N). Greater relative MUAP amplitude was associated with lower firing rate (−3.7 Hz per 1-SD, 95% CrI −3.8 to −3.6 Hz). Conclusions Sub-maximal quadriceps motor unit firing rate was not consistently associated with later knee extensor strength, although a stronger positive association was suggested in the reconstructed limb at two weeks. Firing rate was strongly related to relative MUAP amplitude, and its predictive value for later strength appears limited.

Details

Title: Early sub-maximal quadriceps motor unit firing characteristics and six-month knee extensor strength after anterior cruciate ligament reconstruction: a prospective longitudinal study
Authors: Rob Buhmann (Corresponding Author) - University of the Sunshine Coast
Abram Reeves - University of the Sunshine Coast
Daniel Mellifont - University of the Sunshine Coast
Max Stuelcken - University of the Sunshine Coast
Mark Sayers - University of the Sunshine Coast
David N Borg - University of the Sunshine Coast
Steven Lawrie - Sunshine Coast Orthopaedic Group (Australia, Birtinya)
Publication details: Annals of Medicine, Vol.58(1), pp.1-13
Publisher: Taylor & Francis
Date published: 2026
DOI: 10.1080/07853890.2026.2740295
ISSN: 1365-2060
PMID: 42804596
Copyright note: © 2026 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent.
Data Availability: The data and analysis code supporting the findings of this study are openly available in Zenodo at https://doi.org/10.5281/zenodo.22255496.
Grant note: This study received in-kind support from Delsys Inc., which provided equipment for the study.
Organisation Unit: Healthy Ageing Research Cluster; School of Health - Sports & Exercise Science
Language: English
Record Identifier: 991264992802621
Output Type: Journal article

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