The most common material for shoe soles, rubber is lightweight, flexible and has a good level of slip-resistance. Rubber soles also come with the advantage of being water-resistant, allowing for safer use in wet and muddy environments. Rubber has a natural form that boasts higher levels of elasticity, softness and comfort, however it isnât particularly robust and so is better for indoor sports shoes. On the other hand, synthetic rubber is harder and more robust, thus more adequate for outdoor shoes.