This backpack features the SensiFit construction, which combines both rigid and flexible materials to ensure a perfect fita running vest should not bounce or shift uncontrollably. The main compartment is rain-protected. The 15-liter storage capacity is intelligently distributed between the front and back. - Pockets and compartments: - Main compartment at the back, compatible with up to a 1.5-liter hydration bladder - 2 stretch front pockets for up to 500 ml soft flasks - 2 waterproof front pockets - 2 stretch back pockets - Stretch pockets - Waterproof compartment at the back - Key holder - Features: - Quick Link chest strap, easy to adjust even while running; the elastic material adapts to breathing movements - Pole holders - Reflective details for visibility - Safety whistle (a required item for many trail running events) Choose your size based on chest circumference: - 80-85 cm: XXS - 85-91 cm: XS - 91-97 cm: S - 97-103 cm: M - 103-109 cm: L