Purpose: Muscle fatigue affects proprioception, and it causes problems in spinal stability. The purpose of this study was to examine the effect on the accuracy of reproducing the lumbar angles before lumbar exercise and after fatiguing isokinetic lumbar exercise. Methods: Thirty healthy adults participated in this study. Before induction of fatigue by exercise, the proprioception was measured by Biodex. Lumbar positions were passively maintained on stimulation position ($25^{circ}$ flexion and $25^{circ}$ extension), and back to the starting position. Subjects actively repositioned the remembered stimulation position, and error degrees between the stimulation position and reposition were measured. Using an isokinetic device at $120^{circ}$/sec of velocity of angle lumbar flexion/extension exercise resulted in muscle fatigue. The post-fatigue proprioceptive position sense was used in the same way as in pre-fatigue measurement. Results: Means of position sense of pre-fatigue were $2.19{pm}1.97$ on flexion angle, and $5.04{pm}2.84$ on extension angle. After exercise induced fatigue, means of position sense were $2.37{pm}1.83$ on flexion angle, and $4.93{pm}2.57$ on extension angle. Results of this study showed significant differences of lumbar proprioceptive position sense between pre- and post-fatigue. Conclusion: Lumbar proprioception sense in active repositioning in flexion and extension was affected in the presence of muscle fatigue. Therefore, it should be noted that therapeutic exercise for patients with abnormal proprioceptive sense or elderly people must be performed with care because muscle fatigue can cause secondary damage.