yes that certainly was common practice with longbowmen, due to the sheer size of their bows ( over six foot without the string) and the arrows (sometimes larger than 3 foot) it made it difficult to reach around and pull an arrow out of the quiver. this hampered the firing rate.Added (2011-08-31, 7:10 PM)
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This wasnt the case with crossbowmen though so keep the quiver animation! I would suggest though that they carried their quivers on their hips as opposed to on their backs. They also reloaded by using various methods, in the animation however It would look like them bending over. Ill explain via a picture its much easier!
Added (2011-08-31, 7:12 PM)
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http://www.google.com/imgres?q=crossbowman+reloading&um=1&hl=en&safe=off&rlz=1C1RNNZ_enFR386FR391&biw=1680&bih=959&tbm=isch&tbnid=or7vcPEgFsOrLM:&imgrefurl=http://www.theotherside.co.uk/tm-heritage/visit/visit-azincourt-battle.htm&docid=uESrkPixXyW1QM&w=92&h=146&ei=505eTtj7Oeei4gS-9NhF&zoom=1&iact=hc&vpx=889&vpy=484&dur=2668&hovh=116&hovw=73&tx=70&ty=30&page=2&tbnh=116&tbnw=73&start=42&ndsp=45&ved=1t:429,r:31,s:42
http://www.sthubertsrangers.org/crossbows.htm
sorry about the huge web address couldnt be bothered to shorten it!!