When you carry a pack, your centre of gravity rises. The heavier the pack, the higher it rises. The higher your centre of gravity, the less stable you are, so when carrying a heavier pack, it is worth seeking out extra support, and stability than you might if you were carrying a lightweight rucksack.
When walking, the main thing keeping you upright is the structure of the boot. A boot with a more rigid sole, and stronger materials in the upper will give you more support meaning you are doing less work, so you should fatigue less. Important when walking multiple days in a row.
If you are prone to rolling your ankle, then this is even more important, as rough terrain coupled with a higher centre of gravity will make you even less stable.
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