A bladder fits. I found it felt like it pulled the shirt if I put more than about 1.5 litres in though. I suspect the larger the person/shirt, the more you could get away with putting in the bladder.It can’t move as it’s already tight fitting but the fabric being stretchy allows for it without being too tight. Wonder if a bladder would fit in the back maybe, never explored that option, would look weird though having the hunch back look however
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