The great need of knowledge for the muscles' forces and joints reaction forces is of great importance in many biomeclical fields and especially in the study of walking which is the most important daily activity. The moment arms of the muscles have been calculated referring to the knowledge and data from other related studies, clue to lack of information and data on the lines of actions of the muscles around hip and ankle joints. Depending on the muscles anatomical bony land marks, distances from the lower limb joints centers to the points of origin and insertion were used to calculate the muscle lines of actions using trigonometric relations. MATLAB program (version 7) has been adopted in this work to predict the muscles' forces and the joints reaction forces during gait cycle. A two-dimensional model was selected and developed, where walking was assumed to be generated by nine muscles, as these are the major important muscles and effective in walking. Also, the segments and joints were supposed to be frictionless in movements. Muscle forces were predicted using the joints moment's equations, but the number of muscle unknowns was greater than the number of equations, so to solve such type of equations optimization techniques must be applied. Muscles forces were predicted using static nonlinear optimization technique, in order to find a suitable cost function that relates physiologically to the muscle, the sum of the muscle stresses cubed were used as an objective function (cost function). These muscle forces represent the minimum forces required to i n i t i a t e and continue the gait cycle. Dynamic equilibrium equations were used to find the jointsreaction forces, where the muscles forces predicted were used to find the joints reaction forces. The results obtained have been compared with the results from previous researches. These findings are in a qualitative agreement with those found in the literature. It was concluded that the muscle force is directly proportion to both the physiological cross sectional area of the muscle and joint moment. Also, the muscle force is inversely proportional to the moment arm of the muscle. Joints reaction forces for the hip, knee and ankle are found to be 2.6, 2.4, and 4.07 times body weight, respectively. The joint reaction forces are directly proportional to the ground reaction forces and inversely proportional to the muscle line of action.