According to my knowledge, when you are lifting a weight with ur back in flexion the centre of gravity moves up and outward causing an tenfolds increase of weight you are lifting whereas, the COG remains at S2 level if you are keeping your back straight. Along with shift of COG, there is also increase in disc pressure in the posterior aspect, one of vulnerable area for a disc prolapse (postero lateral). The flexed position also slightly increases the intrabdominal more than when compared with erect one.
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