Background: Lower extremity fatigue causes a number of neuromuscular alterations that may increase the risk of knee (re)injury. It has therefore been suggested that fatigue elements be incorporated into injury prevention and rehabilitation programs. Objective: The purpose of this study was to determine the effects of fatigue on knee-muscle activity patterns after anterior cruciate ligament injury during lading task. Methodology: 12 healthy, 12 ACL reconstructed (ACLR) and 12 ACL deficit (ACLD) recreationally active male volunteers were recruited for this case-control study. This study had 2 independent variables: group (ACLR, ACLD and control) and fatigue state (prefatigue and postfatigue). The main outcome measures were knee joint muscle activity onset times and reactive muscle activity of the vastus lateralis(VL), vastus medialis (VM), lateral hamstring (LH), medial hamstring (MH). Fatigue was induced via repetitive sets of double-leg squats and landing. We used 3 × 2 repeated-measures analysis of variance (ANOVA) to detect the main effects of group (ACLR, ACLD and control) and fatigue state (prefatigue, postfatigue) on muscle activity onset times and reactive muscle activity. Where appropriate, post hoc Bonferroni multiple-comparison procedures were used. Results: There was a significant group x fatigue state interaction for VL (F=4.70, P=0.016) and LH (F=3.87, P=0.031) onset times. Further, there was a significant main effect of group for VL (F=3.35, P=0.045), LH (F=3.89, P=0.03), and MH (F=3.63, P=0.037) onset times. Post hoc analysis revealed that VL, LH, and MH muscles turned on earlier in the ACLR compared to the control group post-fatigue (P<0.05). There was a group significant (F=3.39, P=0.041) main effect for VM activity. Specifically, ACLD patients used more VM activity compared to the other groups postfatigue. Conclusion: ACLR patients had earlier muscle activity onset times compared to controls post-fatigue. Thus, rehabilitation needs to focus on the p
