The back pain rule, and judge a moving day by the next morningEmerging evidence
Some discomfort during exercise is fine, and it does not mean you are doing damage: the WHO counts "hurt does not necessarily mean harm" among the messages people with back pain need. Aim for pain you would call mild to moderate, no more than about 5 out of 10. It should settle within an hour or so, and your back should be no worse the next morning. If it is worse, do less next time (fewer reps, a smaller range, a lighter load), not nothing.
Leg symptoms are the exception: stop or shrink any movement that sends pain, pins and needles or numbness further down your leg. If leg symptoms move back towards your spine, that is a good sign, even if your back aches a little more for a while.
If pain climbs day after day, or you notice any warning sign, stop and get it checked. The 5 out of 10 figure matches this site's other pain guides and has not been tested in a back pain trial; in trials of long-lasting muscle and joint pain, exercise that allowed some pain did no harm.
For bending in daily life (our suggestion): a dull ache or stiffness as you bend that eases when you straighten up is fine, and the next morning is the test for yesterday's bending. A sharp catch that stops you, or pain that spreads down a leg, means smaller, slower and lighter bends for now, or the knees, a low stool or a half-kneel for floor jobs. A flare after a heavy or awkward lift is common and usually settles; NHS inform says don't "brace or hold yourself still".
For moving house (our suggestion, built on the rules above, the trigger studies and the HSE's advice on varying work and rest): judge a day's lifting by the next morning. A back ache up to about 5 out of 10 while you lift and carry is fine if it settles within an hour or so of stopping and your back is no worse the next morning. If it builds past that, stop for the day or switch to light jobs, such as packing at a table or unpacking at waist height, and make the next day's loads smaller and the spells shorter, with help for anything heavy. A sharp catch that stops you, pain spreading down a leg, numbness around your bottom, or chest pain while lifting aren't pain-rule questions: see the warning signs.
Sources 33, 34, 6, 8