Maintaining mediolateral (ML) balance is very important to prevent falling during walking, especially at very slow speeds. The effect of walking speed on support and propulsion of the center of mass (COM) has been focus of previous studies. However, the influence of speed on ML COM control and the associated coupling with sagittal plane control remains unclear. Simulations of walking at very slow and normal speeds were generated for twelve healthy subjects. Our results show that gluteus medius (GMED) contributions to ML stability decrease, while its contributions to sagittal plane accelerations increase during very slow compared to normal walking. Simultaneously the destabilizing influence of gravity increases in ML direction at a very slow walking speed. This emphasizes the need for a tight balance between gravity and gluteus medius action to ensure ML stability. When walking speed increases, GMED has a unique role in controlling ML acceleration and therefore stabilizing ML COM excursion. Contributions of other muscles decrease in all directions during very slow speed. Increased contributions of these muscles are therefore required to provide for both stability and propulsion when walking speed increases.