Older adults (OA, over 65 years) typically use compensatory gait strategies for safe stair descent. Adequate lower limb stiffness, including leg and individual joint (i.e., hip, knee, and ankle), may be necessary to prevent falling, particularly when negotiating a slick surface or when the individual is distracted. Yet, it is unknown if OA change lower limb stiffness for safe stair descent during these challenging situations. Thus, 13 young adults (YA, between 18-25 years) and 12 OA had leg and lower limb joint stiffness quantified during a stair descent (18.5 cm rise). Each participant performed the stair descent on normal and slick surfaces, and with and without cognitive distraction. Leg stiffness (quantified as change in leg length when ground reaction force is applied) and hip, knee and ankle joint stiffness (calculated as change in joint angle when a joint moment is applied) were submitted to 3-way mixed model ANOVA. 3-way interaction for ankle stiffness (p = 0.036) was observed. Without a distraction, the YA exhibited a stiffer ankle on the normal compared to slick surface (p = 0.007), but YA did not change ankle stiffness when distracted (p = 0.125). Neither distraction, nor surface impacted OA ankle stiffness (p \u3e 0.05). YA exhibited a stiffer hip than OA (p = 0.009), and all participants increased hip stiffness on the slick surface (p = .001). Leg and knee stiffness did not differ by age, or change with surface or distraction (p \u3e 0.05). YA may possess the neuromuscular function to increase hip and ankle stiffness during stair descent. Future work is needed to determine whether OA changes in lower limb stiffness are an attempt to prevent accidental fall or from compromised muscular function
