Use of a weight lifting belt has been shown to slightly reduce the injury rate on individuals who have previously had a back injury; however, individuals who a re-injured while using a lifting belt are more likely to have a more serious injury than those who did not, according to McGill.
Furthermore, individuals who have not been injured and use a weight lifting belt display several qualities associated with increased risk of back injury than those who do not. Belts give a false sense of strength to the user, causing him to attempt more weight that he would normally be able to handle. The biomechanics, or movement pattern, of individuals who use a belt may be different from pre-belt usage. This allow individuals to drift into poor lifting habits when performing exercises because they do not have to consciously control their back during the lift since the belt handles spinal location. Finally, use of the weight lifting belt reduces the recruitment of back muscles during exercise. This leads to de-training of the muscles and predisposes you to injury when not wearing the belt.
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http://www.livestrong.com/article/405320-lifting-belt-safety/#ixzz2Mrxu4AeZ