Predicting Isometric Force Using The Force-velocity Relationship From Isokinetic Testing After Acl Reconstruction

Evaluating the force-velocity relationship (FVR) through isokinetic testing at multiple velocities provides a holistic approach to characterizing muscle function after ACL reconstruction (ACLR). FVR can predict isometric force production, but the best velocities for prediction are unknown. PURPOSE: To determine which method for calculating FVR best predicts measured isometric force and relates to subjective knee function. METHODS: 41 patients (21 M,20F; 28 ± 1y) 4 months postop completed 6 reps of isokinetic knee extension at 60, 120, and 240°/s and 1 of isometric knee extension for 5 s at 30, 60, and 90° knee flexion. Subjective knee function was evaluated by IKDC and KOOS. Peak isokinetic torque and angular velocity across speeds were converted to force and linear velocity. FVR was determined as the slope and predicted isometric force as the intercept of the line calculated from isokinetic peak torque at 60-120°/s, 120-240°/s, 60-240°/s, and line of best fit for 60, 120, 240°/s. Mean absolute error (MAE) determined the difference in the predicted and measured isometric force. Correlations were used to determine the relationship between FVR, predicted isometric force, and subjective knee function. RESULTS: 3-point slope from isokinetic testing best predicted isometric force at 30°, 60-120°/s best predicted force at 60°, and 60-240°/s best predicted force at 90° (Table 1). For each method, the MAE between predicted and measured force was smallest at 90°. Significant small-medium correlations were present between FVR and predicted isometric force and KOOS sport (p < .05). CONCLUSION: Prediction of isometric force using the FVR from isokinetic testing provided variable MAE when compared to measured isometric force. Using the FVR to characterize muscle function is associated with perceived function in patients post-ACLR. Table 1. Measured isometric force (N), predicted force (F) (N), and mean absolute error (MAE) (N) between measured and predicted forces.