The purpose of this study was to determine the effects of knee muscle fatigue and blood lactic acid elevation on static balance. Sixteen healthy volunteers (aged, 22.75 +/- 2.21 years) with no previous lower extremity injury, neurological deficits or vestibular impairments participated in this study. Fatigue was induced by an isokinetic dynamometric protocol and static balance was assessed using a Kinesthetic Ability Trainer (KAT) diagnostic and rehabilitation device during unilateral stance with eyes open. Subjects participated in a fatigue protocol in which continuous concentric movements at 60 degrees/s was performed until the torque output for both knee flexion and extension dropped below 25% of the calculated peak torque for 3 consecutive movements. Before and immediately after the fatigue protocol blood lactic acid measurements and single-leg static balance measurements were performed. The effect of muscle fatigue on static balance was assessed by the difference in pre- and post-test KAT scores. An increase in lactic acid levels was detected in all patients, which biochemically confirmed the occurrence of the fatigue. KAT scores increased significantly following the fatigue protocol. There was no correlation between lactic acid accumulation (change between pre- and post fatigue levels) and KAT score differences. It can be concluded that knee muscle fatigue has an adverse effect on balance, however, this impairment was not correlated with the degree of lactic acid accumulation.