The primary objective of this study is to determine the lower extremity strength requirements when healthy young and older'adult participants take a step to recover from a posterior postural disturbance. The secondary objective is to determine the effects of time period (double stance, single stance, landing) on the strength requirements throughout the step response. The performance of using a single step for balance recovery in response to posterior waist pulls was measured in 12 young adult (YA:​ mean age 23.3, SD = 4.2 yrs) and 12 young-old adult (YOA:​ mean age 69.7 yrs, SD - 5.0 yrs) healthy female participants. The effects of age, disturbance magnitude, time period, and their interactions on the primary outcome variables quantifying the center of pressure and center of mass movement were examined. Foot-support surface reactions and body segment kinematics were measured and analyzed using an inverse dynamics model to determine the joint kinematics, the net joint torques, and power time histories in the ankle, knee, and hip joints within three step periods:​ double stance, swing, and landing. The effects of age, time period, and their interactions on the primary outcome variables of peak angle excursion, peak joint torque, and peak power at each joint were examined. The older adult's response had shorter center of reaction path lengths and allowed their center of reaction to come closer to limits of their base of support We found substantial age group differences in the temporal, kinematic, and kinetic single step response used to regain balance to large posterior waist pulls. The landing period may represent the most challenging period of the response in terms of joint kinematics and joint kinetics, especially for those with limited strength capacity and joint ranges of motion.