Introduction: More than 150,000 people each year undergo anterior cruciate ligament (ACL) reconstruction (ACLR) and associated rehabilitation in hopes of restoring knee joint stability and returning to sport. However, 25% of these individuals will go on to sustain a second ACL injury. The incidence rate of a second ACL injury is as high as 15 times that of someone who has never had an ACL tear. Postoperative rehabilitation is instrumental in the return to pre-injury strength, gait and functional performance. However, rehabilitation does not improve biomechanics, likely due to its reliance on an internal focus of attention. Both internal and external feedback methods are currently used to help patients return to sport, however implementation of the most effective feedback technique may be successful in lowering the rate of secondary injuries.Objective: To quantify differences in biomechanics and functional performance following a single session of external focus of attention (ExFOCUS) versus internal focus of attention (InFOCUS) feedback in individuals after ACL-R compared to controls.Methods: Ten adults were recruited to participate in this study (healthy n=3; ACL-R n=7). All participants completed two testing sessions separated by a minimum of 1 week. InFOCUS feedback cues were given during the first session while and ExFOCUS cues were given during the second. This order was chosen due to the potential of ExFOCUS to change biomechanics long-term. Biomechanics were quantified during a both a single-leg step down and jump-landing tasks using 3D motion capture Participants were outfitted with 36 retroreflective markers that were tracked via a 10 camera (200Hz) motion-capture system (MX-T40S; Vicon, Oxford, UK). A static recording was captured to generate a kinematic model in Visual3D (C-Motion, Inc. Germantown, MD, USA). Joint rotations were calculated in Visual3D using a Cardan rotation sequence and expressed relative to each participant’s static trial. Three-dimensional ground reaction force data were collected synchronously with the kinematic data from two Bertec (Bertec, Columbus, OH, USA) non-conductive force platforms (1000Hz). Kinetic data were smoothed using a 4th order zero lag low pass Butterworth filter with a cutoff frequency of 12 Hz and processed using a standard inverse dynamics approach. Joint moments were normalized to participant body mass and height (Nm/kg*m) and presented as external moments. All biomechanical data were time normalized to 100% of the stance phase (initial contact to toe-off), with initial contact and toe-off representing the instants when the vertical ground reaction force (vGRF) first exceeded or fell below 10N, respectively. Independent variables for analysis were group (ACL-R, control), limb (involved, uninvolved or matched in contralateral in the control group), and condition (ExFOCUS, InFOCUS). All data were assessed for normality prior to analysis. For all aims, change scores (pre – post) were calculated for all biomechanical and functional performance variables. Next, a series of 2x2x2 repeated measures ANOVAs were conducted to identify group x condition x limb differences in knee biomechanics and functional performance. Alpha was set a priori at P<0.05 for all analyses. Post hoc testing was performed using one-way ANOVAs and t-tests in the event of significant interactions. Statistical analysis was conducted using IBM SPSS (v26, IBM Corporation, Armonk, NY, USA). Results: During single-leg step downs there was a significant limb by condition interaction for sagittal plane hip rotation (P=0.023). However, neither the limb (P=0.855) nor the condition (P=0.647) main effects were statistically significant for either variable. Changes in hip frontal, knee sagittal or frontal plane rotations, and hip and knee sagittal and frontal plane moments were not statistically different between groups, limbs, or conditions during the single-leg step downs.Drop vertical jump did not show significant changes in hip nor knee sagittal and frontal plane angles between groups, limbs, or conditions. However, there was a significant limb by group interaction for frontal plane knee moment (P=0.027) and peak vGRF (P=0.044) during drop vertical jumps. Though, neither the limb (P=0.142) nor the group (P=0.792) was statistically significant for either variable. There was a significant main effect of condition for hip frontal plane torque (P=0.025). Specifically, participants demonstrated a greater increase in external hip abduction moment from pre- to post-testing in the InFOCUS compared to the ExFOCUS session. There was a significant main effect of condition for vGRF (P=0.041), with the differences from baseline being greater during the ExFOCUS than the InFOCUS session. Finally, the triple hop for distance test demonstrated a significant group main effect (P=0.016) such that the control group demonstrated greater changes in triple hop distance from baseline compared to the ACL-R group regardless of limb or condition.Conclusions: This preliminary investigation suggests that a single session of InFOCUS or ExFOCUS training is not sufficient to alter lower extremity biomechanics or functional performance in patients after ACL-R or healthy adults. To effectively reduce the risk of ACL injury, this intervention may need to last longer than a single session. Strategies to reduce injury risk among patients after ACL-R are necessary; therefore, future studies should have participants preform multiple sessions of each condition so that it can be observed whether or not changes occur.