The Smith machine is a weight machine used for weight training. It consists of a barbell that is fixed within steel rails allowing only vertical or near-vertical movement. Some Smith machines have the barbell counterbalanced. The machine can be used for a wide variety of exercises though it is most commonly used to perform "Smith machine squats". 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 is intended to make 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. This does not fully eliminate the inherent danger in having a heavy weight on one's back, however, and may instill a false sense of confidence. In 2001, a Smith machine user became quadriplegic when the apparatus crushed his spine. Bench pressing while alone is dangerous in general, but even more so in a Smith machine; if one gets trapped under the bar, it cannot be rolled or tipped sideways off one's chest or neck, which has led to at least one fatality. Advantages and disadvantages Use of the Smith machine is frowned upon by many strength training devotees as it forces the user to adopt an unnatural straight-up-and-down "bar path" which can put shear stress on the knees or back (if squatting) or shoulders (if pressing). The constrained movement of the bar also reduces the role that stabilizing muscles play versus an exercise using free weights. This may allow heavier weights to be lifted, at the expense of engaging less muscle mass overall. It is also hard to keep an accurate training log because manufacturers usually don’t indicate bar weights. As with other exercise machines, the Smith machine is often preferred by casual or inexperienced strength trainees who do not know how to safely perform free weight exercises involving heavy weights. Many gyms cater to casual trainees and thus may provide Smith machines instead of power racks, which are a piece of equipment