Regardless of whether the ladder is wood or aluminium if you've got both hands on the stiles at either side and your foot slips for whatever reason, your legs and body will naturally fall between the stiles until you slide down onto a sound rung.
If your foot slips while you've got one hand on a rung and are in the process of moving the other hand up to the next rung up then your body and legs supported by only the one hand may fall sideways outside of the stiles, making recovery more difficult, and if the ladder is unsupported, the shift in weight could pull it over.
Just a thought.
michael adams
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