Aim: Coactivation of the knee agonist-antagonist muscles plays an important role in knee joint stability in different planes of motion. Fatigue is common after high intensity exercise and may affect on muscle co-activation to regulate joint stiffness during unexpected postural perturbations. The aim of the study was to investigate muscle co-activation of the knee flexor and extensor muscles in response to sagital (forward-backward) and frontal (medial-lateral) perturbations before and after dynamic fatiguing exercise. Methods: Bipolar surface EMG signals were recorded from 14 healthy men with five pairs of electrodes located on the knee extensor muscles (vastus medialis, rectus femoris, and vastus lateralis) and knee flexor muscles (the medial and lateral heads of the hamstring) of the right leg during sagittal/frontal perturbations. Results: Maximal voluntary contraction force and time to task failure were significantly reduced after fatiguing exercise with respect to baseline (P<0.001). During the postexercise sagittal/frontal perturbations, reflexive activity in knee extensor muscles was significantly lower than baseline (P<0.05). Moreover, a significant reduction in the level of co-activation between the hamstring and quadriceps muscles was observed during sagittal/frontal perturbations (P<0.05), with a greater reduction identified for the frontal perturbation (P<0.05). Conclusion: Fatigue alters muscle co-activation around knee joint, particularly when knee is subjected to the frontal perturbation.

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