Objective：To investigate the relationship between clinical assessment of pain and muscle activity during walking in patients with severe knee osteoarthritis (OA)．Methods：Eleven patients diagnosed as having severe knee OA (13 knees) were evaluated with electromyography and numerical rating scale (NRS) of pain during walking. Electromyography was recorded from the vastus medialis (VM), semitendinosus (ST), vastus lateralis (VL), and biceps femoris (BF) muscles. Co-contraction indexes (CCIs) for the extensor and flexor muscles of the knee joint were calculated as follows：using the VM and ST data for CCI at the medial aspect of the knee and the VL and BF data for CCI at the lateral aspect. Spearman's rank correlation coefficients between the average individual muscle activities or CCI and pain were calculated.Results：During the terminal stance of walking, NRS significantly correlated with the activation patterns of BF (r ＝－0.61, p＜0.05), the CCI of VL-BF (r ＝－0.582, p＜0.05), and the CCI of VM-ST (r ＝－0.596, p＜0.05)．Conclusion：This study suggests that pain severity is reflected in increasing CCI at the medial and lateral aspects during the terminal stance phase in severe knee OA.