Hydration backpack backpack capacity 20 liter pack volume 1 litre applications trail running trekking walking model 2016 suitable for women men chest straps with chest strap adjustable hip belts with waist strap adjustable back panel pods 1 outside pockets mesh side pockets front pocket chest pocket internal pockets hydration bladder compartment special features padded back signal whistle reflective print compression straps fastener zip properties