Anyone who goes to a gym sees people standing in front of a mirror as they lift weights. But exactly what is that person looking at?
A mirror can be a workout's best friend, but only for those who know how to use it. There's nothing to be gained by squinching the eyes shut at the heaviest part of the lift, or staring at it to admire the facial reflection of effort. The mirror is not your audience. It is your judge.
Use a mirror to check for tilting, especially when using dumbbells. While it's easy to see whether you tilt to one side while lifting, other off-center stances are more subtle. For example, do you tilt your head slightly sideways during a particular lift, or during a certain part of it? Is one shoulder more forward than the other while you are performing a particular exercise? Look for slight tilts of your torso and hips. These can cause your muscles to develop unevenly, or impact your movement patterns. It's especially important to train yourself not to tilt if you are involved in a sport; off-center movement patterns will not only slow you down, they will make you awkward.
Turn sideways and look while you do a biceps curl. Is your body straight or are you leaning backwards? If you lean back, that means your biceps are getting help from your lats; and most of the weight is being supported by your lower spine---a real short cut to back strain and pain. Use a lighter weight, and lift only with your biceps, keeping your back straight.
Watch yourself in the mirror as you walk across the gym. Does one side of your body lead more than the other? This is a setup for injury, especially if you're a runner. Use the mirror to retrain your walk so that both feet step forward the same amount, and one leg isn't doing more work than the other. How about your feet; do both feet point forward as you walk, or do your toes point out? A "duck walk" makes your knees more vulnerable and puts stress on the inner side ligament of the knee, the medial collateral, which is the most frequently injured knee ligament. It's a good practice to start and end each workout by watching yourself walk across the gym.
If you have a treadmill at home, place it in front of a large mirror at least five feet high. Watch yourself as you run and make sure you are lifting each knee an equal amount. If one knee rises higher than the other, it directly affects your hip flexor and thigh, as well as the balance of your core. Changing your walking and running posture will take constant concentration, but training for a more balanced stance will keep those nagging aches and pains at bay.
Nearly everyone has a "favored" side, a dominant hand and a dominant leg. But sometimes the favored side can get so dominant, the other side actually weakens. You will be able to see if this is the case if you look for it in a mirror. Use your eye to check how you move, even when racking your weights. The mirror will not only help you recognize problems, it can help you retrain and correct them. Use it to judge how you move, and let it help gauge how close you are getting to the way you want to move---balanced and athletic.