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

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.