Human performance augmentation exoskeleton is beneficial in human motion assistance and enhancement. Human muscle force is an important indicator to evaluate the assistance of an exoskeleton. This study applied an inverse dynamics method based on muscle synergy to analyze human muscle force of the human-exoskeleton hybrid system. We conducted experiments with three subjects equipped with a hip joint-assisted exoskeleton walking at various speeds. Kinematic data, ground reaction forces and electromyography (EMG) signals were measured, and joint torques and forces of representative lower-limb muscles were calculated. The results indicated that both human hip torque and ankle torque were reduced due to wearing the hip joint-assisted exoskeleton, which implied assistance could be transferred across joints. The results also showed the effects of the exoskeleton on muscle force modulation. The dynamics analysis with muscle force calculation of this study may provide more insights of exoskeleton assistance, and thus be helpful in evaluation of performance augmentation and in design optimization of exoskeleton.