OBJECTIVE:To determine the immediate effects of a varus unloader knee brace on lower-limb electromyographic activity in individuals with lateral knee osteoarthritis and valgus malalignment after anterior cruciate ligament reconstruction. METHODS:Electromyographic data were recorded in 19 individuals with lateral knee osteoarthritis and valgus malalignment after anterior cruciate ligament reconstruction during walking under three conditions: (i) no brace, (ii) unadjusted brace (no varus adjustment), and adjusted brace (varus adjustment). Variables of interest were statistically analyzed using repeated measures analysis of variance. RESULTS:There were no significant differences in muscle co-contraction between the three test conditions. The adjusted brace resulted in delayed offset of gluteus maximus (mean difference [95% CI]: 72ms [24-119]), and earlier onset of gluteus medius (59ms [21-97]) compared to no brace. The adjusted brace delayed onset of lateral gastrocnemius compared to no brace (53ms [28-78]) and the unadjusted brace (39ms [7-71]) and reduced average activation amplitude of gluteus maximus (-4mV [-6 to -1]) and lateral gastrocnemius (-9mV [-16 to -2]) compared to no brace. CONCLUSIONS:The unloader brace did not produce significant changes in muscle co-contraction in individuals with lateral knee osteoarthritis and valgus malalignment after anterior cruciate ligament reconstruction. Significant changes in gluteal and gastrocnemius muscle activation timing and amplitude were observed, however, it is not clear whether these changes are of clinical importance.