A long, slim shape allows for great amounts of upper body movement.
The Aeroflex mesh back system is airy and has a useful length adjustment. It’s a bit complicated to work out, but is excellent once you do, with a decent range of adjustment. The face fabric is rugged and hard-wearing, if heavier than some, but the lack of a zip entry to the main compartment necessitates some careful packing. Tall thin pockets on the sides are a decent volume too, being large enough to fit a flask in, although you will have to deal with the compression straps to access them.