In everyday activity we rely on elementary mechanics in more ways than most people guess. Stiffs and planks are easier to manhandle than drunks firstly because rigid bodies present certain advantages of leverage, eg we can raise a plank by one end or swing it by its middle; slack structures mainly require vertical lifting without mechanical advantage. Rigid objects permit us to store work, economising, say by resting raised mass on the structure beneath, whereas a body slumping in every direction collapses wherever it is unsupported, with all the lifting to do again; and flaccid mass passively consumes energy with every change in shape. Himalayan porters who carry visitors or VIPs in litters understand this; they talk loudly to keep their burdens awake – a body slumped in sleep dissipates the forces it absorbs, making it harder to carry than a passenger who reacts dynamically to bumps and jerks.

A static burden’s rigidity is irrelevant – mass is mass. It is as easy to stand with a jouncy flaccid bag straddling your neck as with a firm body of equal mass, but hump it any distance and the flabby energy dissipation saps your stamina.

Frame rucksack design takes account of this.