We're all familiar with the cliche, old habits die hard. (Yes, yes, I know, that's what makes it a cliche. But bear with me.) Nowhere is this more true than at the gym---not only because most of us are slaves to routine, but because we have such a strong psychological attachment to what we've been told about how to get fit, lose weight, build muscle.
In fitness as in all areas of study, we formulate theories about what works; we put those theories into practice; with practice we modify our assumptions about what we thought we knew; and with time our knowledge evolves to include new and better information (often at the disposal of old ideas).
However, many who work to improve their fitness get hung up on the old ways of doing things. They rely on outdated workout routines and techniques to achieve updated goals. The former college athlete who learned powerlifting (for more explosive movement) and mass-building (for improved strength for their sport) ten years ago, who now wants simply to lose weight, maintain some flexibility, and stay lean, lifts weights that very same way he did back then. The young mom who's trying to shed the last of her baby weight sticks to a circuit of machine exercises, machines designed for the isolation of individual muscle groups, because, like him, that's what she knows.
Admittedly, many of these folks don't know that what they're doing isn't ideal. In my experience, however, an equal number do know that there's a better way they could be exercising, yet they don't seek help. And of those who do, many don't want to listen to new suggestions.
The women are afraid of bulking up, even in the face of loads of information out there (which, I might add, has been around for years) telling them it's not gonna happen. You see those guys that are in the gym 6 days a week, doing 12 sets apiece of various chest and bicep exercises and lifting as heavy as they possibly can, that still aren't getting any bigger? Your two strength workouts a week with 5-lb dumbbells probably aren't going to turn you into the Hulk.
And the men still believe the only way to achieve more defined muscles (Note: I come across VERY few guys these days who want to put on size) is to lift as heavily as possible, often at the expense of both form and courtesy for the people around them.
Here's my advice: even if you think you know everything there is to know about working out, schedule a consultation with a trainer at your health club. (These are usually free.) At the least, you'll pick up some tips about how to get better results, and you can go forth with even more confidence than you've already got.