The main objective of this paper is to find out the muscle which plays a major role during flexion and extension on models of different age groups and the extent of mechanical deformation which takes place in the muscles of lower limb with the help of BRG.Life Mod software which works in ADAM’s environment. The muscles moment is obtained with the help of inverse and forward dynamics simulation of human model that shows us which muscle is active during flexion and extension and how much moment (torque) that muscle is exerting, which causes muscle deformation. According to Hill’s mechanical model of muscle, it consists of three elements: one contractile element attached in series with an elastic element and both of these attached in parallel to an elastic element. During muscles contraction and expansion the deformation of muscles takes place and during one complete gait cycle it regains its original shape and size due to its viscoelastic nature. The deformation in the muscles is obtained by finding the change in the length of muscles during extended flexion and extension in complete gait cycle on different age group models. After conducting the simulation on models of different age group, it is observed that deformation in the muscles of lower limb is maximum obtained in Rectus femoris which is one of the four quadriceps muscles of the human body, the others being Vastus medialis, Vastus intermedius, and Vastus lateralis. On the basis of our experiment the conclusion is made that Rectus femoris plays a major role in regulating knee flexion and extension.