Human locomotion is a complex system generated by redundant actuators and its interaction with environment. Human manages the redundant body with dexterity for adapting to various environments. Analytical studies have revealed that multiple joints and muscles move simultaneously as if the motion is constraint in low-dimensional structures. These low-dimensional structures, called synergy, should reflect the human control strategy; however a methodology that can deal with an effect of synergy on neural control has not been well-established. This study, thus, proposes a composite approach of analytical and constructive study: a system model that integrates skeletal system built on dynamical simulation and synergies obtained from walking experiment is proposed and discussed. The constructed system model realized a stable walk on both level and slope conditions, and synergy similar to that obtained in human experiment could be observed. By manipulating the contribution of synergy and posture controls inherent in the system model, the model approach revealed the factors for forming average posture during locomotion and the importance of synergy tuning for adapting to slope conditions.