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°), 50% (−16±6°), and 100% of fatigue (−14±4°),p=0.004; and hip flexion at pre (45.4±10.9°), 50% (46.2±11.2°), and 100% of fatigue (40.9±11.3°),p=0.004. A noticeable decreased was also observed for hip abduction at initial contact between pre (−13.8±6.6°), 50% (−9.1±6.5°), and 100% of fatigue (−7.8±6.5°), 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°, 50%:−56.1±7.2°, and 100%:−50.5±7.1°,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.