Quadruped motion in legged locomotion provides an outlook to explore both static and dynamic gaits.The leg contact of the quadruped with the environment is an impulsive contact with a non-smooth interaction.This impulsive force is resisted by the torques applied at the leg joints of the model, and hence, the realistic estimate of these resisting torques is vital for the stable operation of the robot.In the current study, the dynamic model of a planar two-legged mobile robot is formulated for an impact problem by utilizing the external impact model as proposed by Lee et al.(Modeling and analysis of internal impact for general classes of robotic mechanisms.In: Proceedings.IEEE/RSJ international conference on intelligent robots and systems, vol.3, pp.1955–1962, 2000 [1]).The impulse encountered during contact of the leg foot with the ground is utilized as the basis to compute the resisting forces at the joints in the model.These resisting joint forces due to impact can be used to decide the configuration of mobile robot leg during landing on the ground.From the simulation, it is established that the impact force on the joints is greatest when the orientation of the lower link of the leg is perpendicular to the contact environment. © 2023, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.