Is voluntary motor control of mediolateral rhythmic sway ruled by modular organization? Answering this question has potential implications in diagnosis and rehabilitation of neurologically impairments. Superficial EMG and computerized dynamic posturography has been used in this study to investigate modular control of six healthy subjects. Postural movements have been performed at three different frequencies to also test the influence of speed on the composition of synergies and activations. Results showed that two synergies account for more than 75% of EMG variance and are shared by all subjects across all frequency conditions. These evidences, together with a functional interpretation of computed muscle synergies, support the existence of consistent modular control across healthy subjects during mediolateral voluntary movements.