Since the 1990's the quality of fitness instructors has improved drastically. Most health clubs today require the fitness instructor to be certified. If the club is big enough, you should be able to switch instructors without a problem. But let's look at a few things to look for in a quality instructor:
A) Your teacher should be certified at the national level. Anything lower is probably a course than anyone could do in a weekend. It is a good idea to see exactly what they are certified for. A certified pool fitness instructor isn't going to know everything that a weight lifting instructor would.
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B) A good instructor will usually do a quick one-on-one for some basic questions. If he asks if you have any illnesses or physical problems then you can be assured that he is going to check out okay.
C) You want an instructor that is able to give clear concise instructions. You don't want to end up with a teacher who is using a lot of jargon. Usually, the instructors who use a lot of jargon will be trying to make you think they know more than they do.
D) A good reason to have an instructor is that they will motivate you. If they are seemingly on autopilot, you want to drop them at the drop of a hat.
E) A good instructor knows the importance of a warm-up, cool-down, and relaxation. All of these should be incorporated in all of your classes. If not you could be setting yourself up for injury.
There are many factors when deciding on a fitness instructor, so it's important to do your homework.