Background Neuromuscular alterations are increasingly reported in individuals with knee osteoarthritis (KOA) during level walking. We aimed to determine which neuromuscular alterations are consistent in KOA individuals and how these may be influenced by osteoarthritis severity, varus alignment and/or joint laxity. Methods Electronic databases were searched up to July 2012. Cross-sectional observational studies comparing lower-limb neuromuscular activity in individuals with KOA, healthy controls or with different KOA cohorts were included. Two reviewers assessed methodological quality. Effect sizes were used to quantify the magnitude of observed differences. Where studies were homogenous, effect sizes were pooled using a fixed-effects model. Findings Fourteen studies examining neuromuscular alterations in indices of co-contraction, muscle amplitude and muscle activity duration were included. Data pooling revealed that moderate KOA individuals exhibit increased co-contraction of lateral knee muscles (ES 0.64 [0.3 to 0.97]) and moderately increased rectus femoris (ES 0.73 [0.23 to 1.22]), vastus lateralis (ES 0.77 [0.27 to 1.27]) and biceps femoris (ES 1.18 [0.67 to 1.7]) mean amplitude. Non-pooled data indicated prolonged activity of these muscles. Increased medial knee neuromuscular activity was prevalent for those exhibiting varus alignment and medial knee joint laxity. Interpretation Individuals with KOA exhibited increased co-contraction, amplitude and duration of lateral knee muscles regardless of disease severity, limb alignment or medial joint laxity. Individuals with severe disease, varus alignment and medial joint laxity demonstrate up-regulation of medial knee muscles. Future research investigating the efficacy of neuromuscular rehabilitation programs should consider the effect of simultaneous up-regulation of medial and lateral knee muscles on disease progression.