Reverse the effects of age or injury. If three doctors have told you you'll never be able to straighten your left knee again, no trainer is going to help you back to the top of the triathlon podium. What a trainer can do, though, is help you find ways to work around your physical limitations so that you can reach the highest level of fitness possible.

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Do more than he or she has been trained to do. Generally speaking, staff trainers at a gym will be able to help you use gym equipment and teach you a standard program for your age and fitness level. If you want something more complicated, you will need to look into private trainers, who have more certifications in specific fitness areas. And you'll have to pay according to the level of expertise you need.

If you think hiring a trainer is a good choice for your needs, be sure to take your time before signing a contract. Interview several trainers and choose the one whose background is the best match for your goals.