The purpose of this study was to quantify changes in postural stability with fatigue of the frontal and sagittal movers of the lower extremities. There were four test sessions, with a randomized order assigned according to the muscles tested and the plane of motion. Subjects were 20 healthy men (age: 22.6+/-2.4 years, height: 173.7+/-3.6 cm, weight: 63.3+/-7.9 kg). During each session, one set of muscle groups was fatigued using isokinetic contractions: ankle plantar/dorsi flexors, ankle evertor/invertors, hip flexor/extensors or hip abductor/adductors. The Biodex Stability System was used to assess anterior/posterior and medial/lateral stability before and after muscle fatigue. Repeated measures ANOVAs revealed that fatigue was associated with a significant increase in all stability indices. Fatigue of the hip movers, whether in the frontal or sagittal planes, led to greater increments in stability indices than fatigue of the ankle musculature. Fatigue of the frontal movers resulted in greater increases in the medial/lateral stability index compared to fatigue of the sagittal movers. In conclusion, fatigue of proximal lower extremity muscles affects postural stability and fatigue of the frontal movers is associated with postural instability in the frontal plane.