Introduction:​ Knee osteoarthritis (OA) is a progressive joint disease that negatively affects physical function and quality of life. The disease is exacerbated by overweight and obesity. Individuals with higher body mass index (BMI) and knee OA patients with a healthy weight both differ from their healthy counterparts in their gait mechanics as they descend stairs.

Purpose:​ To determine the association between overweight or obesity on stair descent mechanics in adults with knee OA.

Methods:​ This is a cross-sectional study of 28 adults with obesity and knee OA. Pearson’s correlation coefficients (r), coefficients of determination (r²), and significance levels (p) were computed to determine the association between BMI and lower extremity joint kinematics and kinetics during stair descent.

Results:​ A higher BMI was associated with increases in vertical ground reaction forces (p=0.001;​r=0.58), vertical ground reaction force impulse (p=0.02;​r=0.45), vertical ground reaction force impulse to impact peak (p = 0.01;​r=0.49), peak ankle plantar flexion moment (p<0.001;​r=0.66), peak ankle plantar flexion moment during the shock absorption phase, defined as the first 20% of stance (p= 0.003;​r=0.53), peak knee extension moment (p=0.04;​r=0.40).

Conclusions:​ Adults with a higher obesity level exert greater vertical ground reaction forces during stair descent. Stair descent mechanics were influenced by obesity through an increase in ankle plantar flexion moment to control a heavier leg and stabilize a larger body mass.