A barbell is a piece of exercise equipment used in weight training, weightlifting and powerlifting. Barbells range in length from 4 feet (1.2 m) to above 8 feet (2.4 m), although bars longer than 2.2 metres (7.2 ft) are used primarily by powerlifters and aren't commonplace. The central portion varies in diameter from 25 millimetres (0.98 in) to even 2 inches (51 mm) (as it's the case with Apollon's Axle), and is often engraved with a knurled crosshatch pattern to help lifters get a good grip. Disc weights (plates) are slid onto the outer portions of the bar to obtain the desired total weight. These weights are often secured with collars to prevent them from sliding off during the exercise (which can result in injuries) or flinging the unevenly-loaded barbell through the air.