This study investigated the impact of changes in weight-bearing line following open-wedge high tibial osteotomy (HTO) on muscle activation within the lower limb kinetic chain in patients with medial knee osteoarthritis (KOA). A cohort study was conducted with 21 patients diagnosed with KOA who underwent HTO. Three-dimensional gait analysis and surface electromyography were performed preoperatively and at 6 and 12 months postoperatively. Seven lower limb muscles were assessed for co-contraction index, integrated electromyography, root mean square, duration, and SMA during the stance phase. Intragroup and intergroup comparisons were made using one-way and repeated-measures analysis of variance. Preoperatively, the co-contraction indices of the vastus medialis–biceps femoris and vastus medialis–gastrocnemius were significantly higher compared with other muscle pairs (p < 0.05), but these indices decreased significantly at 6 and 12 months postoperatively (p < 0.05). The integrated electromyography and root mean square values for six muscles, excluding the vastus lateralis, significantly increased at 6 months postsurgery (p < 0.05). At 12 months, the root mean square value of the vastus lateralis decreased significantly compared with preoperative levels (p < 0.05). The gluteus medius and vastus medialis showed significant increases in integrated electromyography, root mean square, and duration of muscle activation at 12 months postoperatively, along with earlier muscle activation timing (p < 0.05). The vastus medialis demonstrated improved activation compared with preoperative levels. HTO reduces co-contraction indices, enhances muscle activation, and improves lower limb muscle coordination within 1 year postoperatively.