Background: Knee osteoarthritis is common following arthroscopic partial meniscectomy and a higher external peak knee adduction moment is believed to be a contributor. The peak knee adduction moment has been shown to increase over 2 years (from 3-months post-arthroscopic partial meniscectomy). The aim of this study was to evaluate mechanisms underpinning the increase in peak knee adduction moment over 2 years observed in people 3-months following arthroscopic partial meniscectomy. Methods: Sixty-six participants with medial arthroscopic partial meniscectomy were assessed at baseline and again 2 years later. Parameters were evaluated at time of peak knee adduction moment as participants walked barefoot at their self-selected normal and fast pace for both time points. Findings: For normal pace walking, an increase in frontal plane ground reaction force-to-knee lever arm accounted for 30 of the increase in peak knee adduction moment (B = 0.806 [95 CI 0.501-1.110], P <0.001). For fast pace walking, an increase in the frontal plane ground reaction force magnitude accounted for 21 of the increase in peak knee adduction moment (B = 2.343 [95 CI 1.219-3.468], P <0.001); with an increase in tibia varus angle accounting for a further 15 (B = 0.310 [95 CI 0.145-0.474], P <0.001). Interpretation: Our data suggest that an increase in lever arm and increase in frontal plane ground reaction force magnitude are contributors to the increased knee adduction moment observed over time in people following arthroscopic partial meniscectomy.