The Smith machine was invented by American Jack La Lanne, who rigged up a sliding apparatus in his gym in the 1950s. It was spotted by Rudy Smith, a bodybuilder, who commissioned Paul Martin to improve it. Smith then installed the improved 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. An article in the July 2005 edition of Muscle & Fitness  reported that Rudy Smith was still alive, aged 79, and was the owner of Las Vegas Athletic Clubs.
Behind each runner is a series of slots on which the barbell can be hooked. 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 makes it safer for those who weight train without a spotter, as one only needs to twist his/her wrist in order to lock the barbell in place in the event that the weight becomes too great.
Because it cannot fall forwards, backwards or sideways, a Smith machine is considered safer to use than an ordinary barbell. Since the weight does not need to be stabilized, this can allow unstable lifters to lift more weight. There is a risk of force loss by applying it improperly, however, which can reduce the amount of weight lifted.
As the Smith machine constrains the body to a single plane of motion, it does not develop the stabilization skills, or ability to force the bar into proper form, so lifting ability on it does not translate into free-weight lifting ability, whereas one can always lift on a Smith machine whatever one can free-weight (The exceptions being those exercises which, performed with free-weights have a form that follows an arc. When these exercises are adapted to the Smith machine, they may result in force loss, such as in the case of an arc-formed bench press). Weights that requires stabilization, generally free weights, are thought to be more 'functional', in that they better mimic most physical tasks that the body would normally be asked to accomplish.
A central dogma within weight training is that the Smith machine is inferior to free weights (the paragraph above pretty much sums up the debate). The benefits of free weights over Smith machines may be overstated. Researchers at Drake University , Iowa tested the premise and found that lifters were able to lift greater amounts on free weights than they were on a Smith machine. As reported in the Journal of Strength and Conditioning Research, bench press strength was about 16% higher for the free-weight bench press as compared to the Smith machine bench press. However, squat strength was about 4% greater for the Smith machine as compared to the free-weight squat.
Clearly this evidence would seem to indicate that Smith Machines might actually provide a greater stimulus than free weights when bench pressing. The solution is to keep an open mind , do what works for you and don't always take the advice of more experienced gym members as gospel.
Due to the necessity of perfect placement under the bar for proper vertical arc, it is difficult to attain perfect form on the machine, which can result in the force loss, and also result in unusual strains on the body. For those who don't believe some exercises have a perfect vertical lifting line, but rather a slight arc (such as certain bench pressing methods), it is by design impossible to do it this way on the Smith machine.
For these reasons a regular barbell with a power cage may be used for the desired safety.
The Smith machine can be used for any barbell exercise in which the barbell moves vertically up and down, such as the squat or the bench press. It is not suitable for exercises in which the barbell moves in an arc, such as the biceps curl, nor for weightlifting lifts such as the clean and jerk.