First, I'll talk about one of the reasons this works, and it's actually proven:
Isometric exercise or isometrics are a type of strength training in which the joint angle and muscle length do not change during contraction (compared to dynamic/isotonic movements). Isometrics are done in static positions, rather than being dynamic through a range of motion. The joint and muscle are either worked against an immovable force or are held in a static position while opposed by resistance. The same applies to when you flex your muscles and hold them for a period of time, just as many bodybuilders incorporate this type of isometric exercise in the form of flexing, into their regular routine.
Basically, without all the excess verbiage, Isometric exercise is a form of exercise involving the static contraction of a muscle without any visible movement in the angle of the joint.
There are various types of isometric exercises that doesn't involve flexing in front of a mirror, but regardless, it seems to have some benefit when it comes to strength (from a certain, limited angle) and sculpting the body. However, I would only use this as a supplemental exercise and not a primary form of training. You need to exercise using a full range of motion, as well, and I believe that it is far superior to isometric training, hands down.
Okay, here's why I keep mentioning the mirror:
Of course, you don't need a mirror to flex your little heart out, but the second reason, even though it is unproven by science, is that this allows your mind to become more involved. Just imagine yourself being more toned and ripped up, while looking in the mirror. Concentrate on the muscles you really want to target, flex, hold, and relax. Now that you've got your mind involved, you have an extra boost and a sense of motivation for future workouts. Plus, I really believe that the mind has more to do with your health, than many people give it credit for.
If you'd like to read about a good example of mind over body, involving physical fitness, visit a recent blog post I wrote (Mystery of Muscle Memory) @ http://health-fitness-guru.blogspot.com/2010/11/mystery-of-muscle-memory.html
Hey, if you get caught flexing your muscles in the mirror too often, just tell 'em: "Hey, I'm not vain or anything like that, I'm just performing some isometric exercises to help sculpt my freakin' body, damn it!" Ha-ha!
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