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The Smith machine is a piece of equipment used in weight training. It consists of a barbell that is fixed within steel rails, allowing only vertical or near-vertical movement. A Smith machine often includes a weight rack in the base to help stabilise it. Some Smith machines have the barbell counterbalanced. The machine can be used for a wide variety of exercises such as a squat shown in the image to the right.
The Smith machine was invented by American Jack LaLanne, who rigged up a sliding apparatus in his gym in the 1950s. It was spotted by men's bath house manager Rudy Smith, who commissioned Paul Martin to improve it. Smith then installed the modified model in a gym he was managing at the time, Vic Tanny's gym in Los Angeles. By the end of the 1950s, Rudy Smith was an executive in Tanny's chain of gyms, and the Smith machine was being manufactured and sold more widely.
Use as a self-spotting device
Behind each vertical post (runner) is a series of slots on which the barbell can be hooked. This means that unlike an ordinary barbell, the Smith machine need not be re-racked after a set of repetitions: it can be secured at any point. This supposedly makes it safer for those who lift without a spotter, as one only needs to twist the wrist in order to lock the barbell in place in the event that the weight becomes too great. Most models also incorporate blocks, pegs, or other devices which can be adjusted to automatically stop the barbell at a predetermined minimum height.
Unlike a free-weight barbell, the bar on a Smith machine does not move forward, backward, or sideways. Because lifting on a Smith machine requires less stabilization by the lifter, lifters can usually lift more weight on a Smith machine than on a free-weight barbell.
Schwanbeck et al. reported that free-weights caused 43% more activation of the muscles measured than a Smith machine. Men's Health reported that the straight motion on a Smith machine is an unnatural movement that stresses the knees and lower back. Men's Health also reported that traditional squats produced 50% more muscle activity in quadriceps compared to squats done on a Smith machine.
When Jack LaLanne invented the Smith machine, he used it as part of a complete muscle-toning regime utilizing free weights too. LaLanne never intended it to be used, particularly, to isolate and bulk up one muscle group. The Smith machine works very well because it mitigates the potential for bodily injury since the motion is stabilized and the supporting vertical bars have safety 'notches' that help prevent the horizontal bar holding the weights from dropping uncontrolled to the bottom of the device.