Grippers, sometimes called hand grippers, are primarily used for testing and increasing the strength of the hands; this specific form of grip strength has been called crushing grip, which has been defined as meaning the prime movers are the four fingers, rather than the thumb. There are differences from brand to brand, but the common features of standard grippers are that they use a torsion spring fitted with two handles. The exact dimensions of these elements vary, as well as the materials used to make them; the springs are made from various types of steel, and the handles are generally made from wood, plastic, steel or aluminum. History Grippers from the early part of the 20th century to the middle of the 20th century typically had wooden handles, were quite easy for a strong person to close, and were a regularly advertised product in bodybuilding-weightlifting magazines. Such mass market grippers were largely replaced with inexpensive plastic-handled grippers that are commonly found in sporting goods and exercise equipment stores today. Top early 20th century professional strongman Thomas Inch had two challenge nutcracker grippers in the 1930s that defied nearly all who tried to close them, and besides their status as a specific challenge, the design and application of this gripper was praised as a sound way to assess grip strength. In 1964, Iron Man magazine introduced the “Iron Man Super Heavy Grip Developer,” and unlike mass market grippers, these grippers had steel handles, thick springs and were very difficult to close. The popularity of these grippers was limited, and they were discontinued in 1977. In 1990, IronMind reintroduced the original Iron Man grippers, and then later that year, the company began modifying the design with the stated goal of improving the accuracy, durability and appearance of the gripper, not just making grippers that could tax the strongest men in the world. IronMind also introduced aluminum handles with permanent markings. These two features were later adopted by other manufacturers, including those in low-cost manufacturing centers such as China and India, and have become common on the more difficult grippers, regardless of brand. At the same time, interest in grippers that challenged even the strongest people in the world broadened beyond audiences directly involved with training or physical performance, and have been reported on in such publications as The Sacramento Bee, and BusinessWeek. Strength levels Grippers come in a range of strengths, suitable