Inline skating is a multi-disciplinary sport that encompasses a variety of activities performed on inline skates. Inline skates generally feature two to five polyurethane wheels mounted in a single line on the underside of the boot by a metal or plastic frame. The sport also includes skateboarding, which is performed on regular boards with trucks under each foot and ankle straps attached to the board behind the rider's feet. Inline skating is often considered a form of physical activity or exercise because it forces the user to move their legs quickly in order to stay aloft. The first inline skating rinks were opened in the United States in the early 1990s. They differed from traditional roller skating rinks in that they were designed for multiple users at once, and they usually included features such as seating, lighting, and landscaping. These new rinks were inspired by classic American sports facilities such as baseball diamonds and football fields, and they attracted participants from around the world who were looking for alternative forms of entertainment. Since then, more inline skating venues have been built across the United States, and many other countries have followed suit by constructing their own. Inline skating is popular among adults as well as children. There are many clubs and organizations that cater to inline skaters of all abilities and sizes. Inline skating is also used as a means of transportation in certain cities where there is a lack of public transport.