The smith machine is often used by many gym-goers in order to simulate the various compound movements that are generally performed using a standard barbell (the squat, bench press, overhead press, etc.).
Since the weight is stabilized by the apparatus, the prime movers in a particular exercise are localized and stimulated far more directly. The problem is, many will completely substitute compound exercises with free weights in favor of the smith machine. The thinking is that smith machines are the training wheels of compound movements. This is incorrect and here is why.
When using a smith machine, the bar path travels directly up and down in a straight line. Subsequently, the lifter is forced into a fixed and unnatural movement pattern that can potentially put his/her joins at risk for injury. It is difficult to imagine a 400lb barbell traveling directly up and down during a free-weight deadlift, considering the multiple variables involved in the movement.
As mentioned previously, the prime movers for each particular exercise will be the only muscles stimulated. For example, the quads, hamstrings, and glutes are not used alone during a standard squat. Other muscles are vital, such as the spinal erectors, obliques, and abdominals. Using the smith machine for the squat will not allow these muscles to develop. This will cause an overdevelopment of the prime movers in relation to the stabilizers.
When this occurs, the functionality of that individual’s body will severely decrease in the long-term. Running, jumping, and the ability to change directions quickly is needed for the majority of sports. Stabilizer strength is necessary for this. If lacking, the risk for injury will be increased exponentially.
Additionally, if the lifter’s use of the smith machine is to learn compound movements, it is important to understand that the technique learned will not translate over to the corresponding free-weight movements because of the underdeveloped stabilizers and the fixed path of the bar. Once using free weights, the form will need to be readjusted and relearned to adapt to the greater movement potential of the body and bar. The lifter will also find that they must lower the weight significantly because of the added coordination and total-body strength required, essentially placing them back to square one.
It is more efficient to use very light weight in the beginning and perform many repetitions with good form in order to habitualize the nervous system to these good habits. Shortly after, it will be safe to increase the resistance and build not only bigger, but functional, muscle that can be carried into all walks of life.