People associated with athletics commonly state that injuries are more common following a fatigued condition. Fatigue and its impact on the biomechanical factors associated with injury are not fully understood. Fatiguing of joint dynamic stabilizers may yield a greater load on the viscoelastic passive stabilizers of the joint thus creating a deformation in the passive stabilizers. This deformation may extend beyond the elastic region of the load/deformation curve into the plastic region creating a deformation that may be permanent or eventually a rupturing of soft tissue. The purpose of this study was to investigate fatigue and its effect on joint force production at the ankle, knee and hip during an impact from a countermovement vertical jumping. Methods. Twenty unskilled untrained females with no history of lower extremity orthopedic concerns performed a vertical jump in a non-fatigued and fatigued condition. The non-fatigued and fatigued conditions were randomly performed on different days separated by a week. A Wingate Anaerobic Power Test was used as the fatiguing mechanism for the fatigued condition with the vertical jump occurring immediately following the Wingate test. Lower extremity kinematics was determined by a Peak 5 motion analysis system and ground reaction forces were collected by an AMTI force platform. The greater trochanter of the hip was tracked to determine the displacement in vertical jump height. Results. There were no significant differences between the anterior/posterior shear forces, compression forces, and flexion/extension moments at the ankle, knee and hip. There was a significant difference in vertical jump height with a decrease of 6.65 cm occurring during the fatigued condition. Ground reaction forces significantly decreased during the impact of the fatigued vertical jump. Discussion. The kinetic changes in the joint occurred because of a decrease in jump height. This was attributed to the lower extremity muscles being fatigued and as a result, the lower extremity muscles were unable to produce enough force to displace the mass of the subject into the air for the same distance. This would produce a decrease in the ground reaction force during the fatigued condition and a decrease in the amount of force that is needs to be attenuated at each joint. Conclusion. This study provides information to the body of knowledge of anaerobic fatigue and its effect on lower extremity biomechanics associated with sagittal plane movement such as vertical jumping. Additional research is needed to determine if this same effect occurs in frontal plane movements.Keyword(s): athletics/sports, exercise/fitness