The aim of the study was to analyze the ML knee displacement and its influence on barbell tilt and rotation displacement from the back squat. Based on the weight distribution (WtD) test, 13 were in the equal WtD group and 14 were in the unequal WtD group. All subjects performed 75% of 1RM barbell back squat with reflective markers on selected locations for motion analysis. Medial-lateral (ML) knee displacement, barbell tilt and rotation displacements were considered. The results showed a statistically significant difference between groups (p<0.01), but no significant difference in ML knee displacement of the left and right sides (p=0.63). There were no correlations between ML knee displacement asymmetry and barbell tilt (p=0.12) and rotational displacement (p=0.07), indicating frontal knee action may not explain unwanted barbell movements in back squat. INTRODUCTION: Unlike clinical analysis of gait, bilateral resistance exercises, such as the back squat, were assumed to be symmetrical for years (Donnelly, Berg, & Fiske, 2006; Fry, Smith, & Schilling, 2003; McLaughlin, Dillman, & Lardner, 1977). The exercise is used to increase muscular strength in the lower extremity and trunk, and it requires balance and stability to perform it symmetrically between the left and right sides of the body without compromising unwanted bar movement. The back squat is predominantly a sagittal plane exercise with about 30 degrees (°) of ankle dorsiflexion, near/over 90° of knee flexion, depending on the depth of the squat, and 85 to 110° of hip flexion (Donnelly et al., 2006; Fry et al., 2003; McLaughlin et al., 1977). Along with the kinematics of the lower extremity, other studies addressed joint torques, mechanical powers, and superficial muscle activities (Escamilla, Fleisig, Lowry, Barrentine, & Andrews, 2001; Zink, Perry, Robertson, Roach, & Signorile, 2006). However, practitioners are also interested in other components of the squat kinematics such as frontal plane motion of the knee. After observing how strength and conditioning and weightlifting coaches check their athletes' technique, it appears that many of them use a frontal and/or diagonal position to check lifting technique, including the motion of the knee. From scientific literature, a study reported knee varus and valgus motions activate knee extensor muscles differently (Sogabe, 2009), but no further information on the threshold of the knee varus and valgus range of motion was discussed. Based on the practitioners' viewpoint as well as recent scientific study, it is speculated that carefully observing frontal plane knee kinematics may have an important role on detecting other portions of unwanted movement from the barbell back squat. The current literature, however, has not established a normal range of frontal plane knee displacement and the consequences of that motion on the back squat. Practitioners would then be better prepared to observe and comment on knee motion during the back squat. Therefore, the purpose of the study was to analyze the medial-lateral (ML) knee displacement characteristics from populations who display equal weight distribution (WtD) and unequal WtD to determine if a difference exists. Further analysis was done by investigating the relationship between the level of knee displacement asymmetry and barbell tilt and rotation during the barbell back squat. The study hypothesized; a) similar ML knee displacement from left and right sides are present from equal WtD group, but not from unequal WtD group, b) unequal WtD group who displays unequal ML knee displacement between left and right sides would be related to higher barbell tilt and rotation during the back squat. The significance of the study