Compromised quadriceps and hamstring force control, not maximal strength, is associated with gait biomechanics at 9 months following anterior cruciate ligament reconstruction

To understand the changes in muscle force control ability and its association to gait biomechanics following anterior cruciate ligament reconstruction (ACLR).

Twenty-five participants, 9 months following ACLR, performed isometric quadriceps (20%, 40% and 100% maximum voluntary isometric contraction [MVIC]) and hamstring (20% and 40% MVIC) contractions on an isokinetic dynamometer, tracing a target line. Torque variability using coefficient of variation, and complexity using sample entropy were calculated. Knee moments and knee flexion excursion were calculated from walking gait. Two-way mixed model analyses of variance were used for between-limb comparisons and Pearson's correlations to evaluate the associations between force variables and gait biomechanics.

Quadriceps and hamstring torque variability changed depending on the limb and the effort level. Quadriceps variability was higher in the ACLR limb only at maximum effort (p = 0.039, Cohen's d = 0.437). Hamstring variability showed no differences at 40% but lower torque variability (p = 0.035, Cohen's d = -0.448) at 20%. Muscle force complexity was consistently higher in the ACLR limb compared to the noninjured limb, regardless of effort level. Additionally, as effort increased, muscle force became less complex in both limbs. Quadriceps variability was negatively associated with knee moments (r = -0.410, p = 0.042, better force control = higher knee extension moments). Hamstring complexity was positively associated with knee flexion excursion (r = 0.417, p = 0.038). Maximal strength was not associated with any gait biomechanics.

Quadriceps and hamstring force control ability are altered and associated with gait biomechanics at 9 months post-ACLR. Rehabilitation should include exercises focused on force control to potentially help restore gait biomechanics.

Level IV.