Background Non-contact anterior cruciate ligament (ACL) injury has been reported to occur during the later stages of a game, when fatigue is most likely present. Few studies have focused on progressive changes that occur throughout fatiguing. Objective The purpose of this study was to evaluate the effects of a sequential fatigue protocol on lower extremity biomechanics while performing a sidestep cutting task (SS). Design Quasi-experimental. Setting Laboratory. Participants 18 female collegiate soccer players (age=19.2+/-0.9 years, mass=61.6+/-5.1 kg, height=1.66+/-0.5 m) free of injury volunteered for participation. Interventions Lower extremity biomechanics was assessed during the SS task using 3-D motion capture. Participants alternated between a fatigue set of; three squats, agility (5-10-5), three vertical jumps, step-ups, and two unanticipated SS trials. The cycle was repeated until fatigue was attained. Main outcome measurements Dependent variables included: hip and knee flexion and abduction angles and moments, measured at initial contact and peak stance (PS). Repeated measure ANOVA’s with three levels were conducted to assess differences between pre-fatigue, 50% and 100% of fatigue. Alpha level was set at 0.05. Results There was a significant decrease in knee flexion between pre (-17+/-5 degrees ), 50% (-16+/-6 degrees ), and 100% of fatigue (-14+/-4 degrees ),p=0.004; and hip flexion at pre (45.4+/-10.9 degrees ), 50% (46.2+/-11.2 degrees ), and 100% of fatigue (40.9+/-11.3 degrees ),p=0.004. A noticeable decreased was also observed for hip abduction at initial contact between pre (-13.8+/-6.6 degrees ), 50% (-9.1+/-6.5 degrees ), and 100% of fatigue (-7.8+/-6.5 degrees ), as well as for hip adduction moment (Pre:0.14+/-0.13 Nm/kgm, 50%:0.08+/-0.13 Nm/kgm, and 100%:0.08+/-0.13 Nm/kg, p=0.007). Further, similar decreases were observed at PS for knee flexion (Pre: 52.9+/-5.6 degrees , 50%:-56.1+/-7.2 degrees , and 100%:-50.5+/-7.1 degrees ,p=0.001), and hip adduction moment (Pre:0.72+/-0.31 Nm/kgm, 50%:0.59+/-0.33 Nm/kgm, and 100%:0.39+/-0.29 Nm/kgm, p=0.003). Conclusion The detrimental effects of fatigue on lower extremity biomechanics are visible at 50% of the participants’ maximum fatigue, and become more marked when attaining 100%, with a decrease in hip and knee sagittal and frontal mechanics. ACL injury prevention programs should emphasise feedback throughout an entire practice, not only during warm-up.