Objective To evaluate the test-retest reliability of electromyography (EMG) normalization in patients with knee osteoarthritis by the standard isometric contraction (SIC) and maximum voluntary isometric contraction (MVIC ) tests. Methods For patients with knee osteoarthritis, the EMG data in gluteus maximus, semitendinosus, rectus femoris, lateralis femoris, tibialis anterior, lateral gastrocnemius and soleus of their affected legs during jogging, SIC tests and MVIC tests were collected using the wireless EMG system. Intra-class correlation coefficients (ICC) with 95% confidence intervals were used to assess the test-retest reliability. Results For patients with knee osteoarthritis, the test-retest reliability of all 7 muscles was good during SIC tests, but the test-retest reliability was high for 2 muscles and moderate for 5 muscles during MVIC tests. The test-retest reliability of all 7 muscles normalized by SIC was higher than that normalized by MVIC, and the test-retest reliability of rectus femoris normalized by SIC was significantly higher than that normalized by MVIC. Conclusions For patients with knee osteoarthritis, SIC is a more reliable EMG normalization method than MVIC, and it has good promotion potential in clinic.