In weightlifting, form and technique are everything. You can think you're doing everything right, but you wouldn't know for sure unless you've been trained by a professional, actually watched yourself lift, or ideally, both. Sure, you can ogle yourself in the mirror, but a better way to learn is to record a video of yourself with your phone's camera. Then you can review it later (ideally with a trainer) and see what you should fix.
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We've stressed the importance of form before. If you're unsure about your form, a video is the perfect way for a personal trainer to critique it, but it also helps you become aware of your own quirks and tendencies. For instance, you may learn that you tend to favour your right side in a bench press and do weird things with your shoulder to move the weight. Then you can work together with a professional to address your quirks head-on. (Additionally, people sometimes post their videos on a public forum for advice, but I generally advise against this because of the potentially conflicting information you can get from both qualified and unqualified responders.)
Don't worry, it's normal to feel self-conscious about recording yourself in the gym, but remember, this is for your own good! As for how to set up your phone to record, you can always ask someone for a couple minutes to help you take video, or Macguyver together a makeshift tripod, using a bench to hold the phone and small weights to sandwich it in place.
If you need ideas for exercises to record, these are just a few of the exercises that tend to be problematic for a majority of people:
- Overhead presses
The list of exercises can actually be quite extensive, depending on who we're talking about, so this is merely a starting point. I highly recommend getting videos from different angles -- front, back, and side -- for the best assessment. Then you can look them over, or better yet, send one or all of them to a trainer for review.