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

<p>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.</p> <p>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.</p> <p>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).