Even in the case of general fitness, training has its own well established rules. These rules refer to different aspects of doing the exercises: dosing (the number of series), repeating, intensity, rhythm, etc; but considering all these aspects is not enough. It is very important for the exercises to be performed correctly.It is not easy at all for a beginner who has read something about fitness or to whom somebody described exactly how to do an exercise, to do it impeccably starting with the first sessions of training. Many times he has the impression that he does a movement correctly, even when he can see himself in a mirror from the gym. But only the experienced 'eye' of an instructor who has worked many programs for beginners is able to correct him effectively.
On the other hand, it is possible for the beginner to have the feeling that his modality of doing the exercises is wrong. If this is not the case, he needs, again, the instructor, to assure him his performance is correct. It is dangerous for the fitness practitioner's health and body state to self-educate from the first sessions. There are many positions, angles, programs of the machines which could be unadvisable in different situations, determined by every person's characteristics. Some examples of these particularities are: deformations of the spine, old accidents, operations, limits of the mobility of the joints, etc.
Another advantage of working under the supervision of an instructor is a good choice of exercises, machines, movements, which helps avoiding any risk of accidents.
The role of the fitness instructor is not limited to working with beginners. Even the advanced might have the tendency to 'cheat' in doing some exercises or to introduce really difficult exercises, necessitating closer supervision. Also, an experienced sportsman might need, from time to time, a look 'from the outside', which could help him perfect his technique.
Another aspect closely related to the fitness trainer's assistance is the conception of the training programs. While for beginners the impossibility of creating their own training programs is obvious, the advanced could lose themselves in the jungle of possible choices, getting not to see the wood for the trees. In the same case of the advanced, the dangers of routine can be avoided much easier with help from the outside.
Maintaining the motivation and the interest for training is also one of the tasks the fitness instructor must face. He needs to show tact, intelligence and even diplomacy when working with the sportsmen or with the ones who practice fitness just for pleasure or for maintaining their health.
There could be some disadvantages of the involvement of the fitness trainer. His presence might be felt as patronizing, stressing, or unpleasant. Some cultural or social incompatibility between the trainer and the practitioner could make their communication, which is so important in this relation, really difficult.
However, the benefits we can have from working with a competent trainer are much bigger than the possible disadvantages and they make the effort of the sportsman more effective.