A barbell is a piece of exercise equipment used in weight training, bodybuilding, weightlifting, powerlifting and strongman, consisting of a long bar, usually with weights attached at each end. Barbells range in length from to above , although bars longer than are used primarily by powerlifters and are not commonplace. The central portion of the bar varies in diameter from 25 millimetres (0.98 in) to 50 millimetres (1.96 in) (e.g., Apollon's Axle), and is often engraved with a knurled crosshatch pattern to help lifters maintain a solid grip. Weight plates slide onto the outer portions of the bar to increase or decrease the desired total weight. Collars are used to prevent plates from moving outward unevenly so that the lifter does not experience uneven force. The barbell is the longer version of the dumbbell that is used for free weight training and competitive sports, such as powerlifting, Olympic weight lifting, and CrossFit. Many exercises can be done using the barbell, such as bicep curl, bench press, Olympic weightlifting, overhead press, deadlift, and squat. Olympic barbells are usually an estimated weight of . Many fitness categories use the barbell for different reasons. For example, powerlifters use the barbell to perform compound exercise movements. Use in Olympic weightlifting Men's bar A men's Olympic bar is a metal bar that is long and weighs . The outer ends are in diameter, while the grip section is in diameter, and in length. The bars have grip marks spaced apart to allow intuitive grip width measurement. It is the standard used in competitive weightlifting where men and women compete at the highest level: the Commonwealth Games, Pan-American Games, World Championships, and the Olympics. Bars of this kind must have suitable "whip" (ability to store elastic energy) and sleeves which rotate smoothly, as well as the capacity to withstand dropped lifts from overhead. Women's bar A women's Olympic bar is similar to the men's bar, but is shorter and lighter—, and —with a smaller grip section diameter (). Also in contrast to the men's bar, the women's bar does not sport a center knurl. Bumper plates Weight plates used in Olympic lifting, which are often termed "bumper" plates, are coated in solid rubber to make them safe to drop from above head height. General strength training or powerlifting plates are made from cast iron and are considerably cheaper. Currently, the following colour code is required by