<p>Osteoarthritis of the medial knee is strongly associated with an increased external knee adduction moment (KAM). Although trunk-lean gait has been shown to effectively reduce KAM, the underlying mechanism remains unclear. This study aims to investigate the biomechanical mechanisms behind this reduction. An experiment was conducted comparing two gait strategies: normal gait and trunk-lean gait, with fourteen healthy participants. Key variables were analyzed in the frontal plane. The trunk-lean gait significantly reduced KAM peaks compared to natural gait (<i>p</i> &lt; 0.01). The major factor contributing to this reduction was the decrease in the moment arm (<i>R</i>² &gt; 0.91), primarily driven by the medial shift of the knee joint center (<i>R</i>² = 0.60–0.77) rather than the changes in ground reaction force. The fundamental cause of these changes was the rotation of the stance limb in the opposite direction of the trunk lean, leading to the medial shift of the knee joint.</p>