Carrying boxes upstairs sits at roughly 8 METs in the Compendium of Physical Activities, the reference database exercise physiologists use to score effort. For context, running a 10 minute mile is about 9.8 METs and walking briskly on flat ground is 3.5. So a full day of hauling furniture is not light housework. It is a long, unstructured, badly programmed strength session with no warmup and no rest intervals.
That framing changes what you should do about it. Nobody would attempt six hours of loaded carries with zero preparation and then wonder why their lower back locked up on day three. Yet that is exactly what moving day is for most people.
what actually hurts you
It is rarely the heaviest object. The couch gets four people, planning and a slow pace. The injury usually comes from the twentieth medium box, lifted one handed, at hour five, while your attention is on the doorframe behind you.
Two specific patterns cause most of the damage. The first is the loaded spinal rotation: you pick up a box in front of you and pivot to set it down beside you without moving your feet. That combination of flexion, compression and rotation is the least tolerant position your lumbar spine has. The second is the unplanned catch, where something shifts mid carry and you correct it with a jerk.
Both are fixable by being boring. Move your feet instead of twisting. Set boxes down on a chair or table rather than the floor so you are not repeatedly bending to the ground. If a load feels like it might shift, put it down and repack it.
pack for the lift, not for the shelf
The single biggest improvement you can make is box weight, and the fix is counterintuitive. Use small boxes for heavy things and large boxes for light things. Most people do the opposite, because a big box looks efficient for books.
A standard 18 by 18 inch box filled with hardcover books can easily pass 60 pounds. The same box filled with bedding weighs about 12. Aim for something you can lift, carry across a room and set down at chest height without your form changing. For most people training a few times a week, that lands somewhere in the 25 to 40 pound range per box. Label the heavy ones so nobody grabs one casually.
the two minutes that make the difference
Do not stretch. Warm up. Those are different things, and a static hamstring stretch before a day of lifting does nothing useful and may briefly reduce force output.
What works is five minutes of movement that rehearses the day: ten bodyweight squats, ten hip hinges with your hands on your thighs, a few sets of walking with a moderately heavy object held at your chest, and some shoulder circles. You are raising tissue temperature and reminding your hips how to do the job so your back does not volunteer for it.
The hinge is worth practicing specifically. Push your hips back, keep your shins close to vertical, keep a long spine, and let your hamstrings take the tension. If you can do that with an empty laundry basket, you can do it with a box.
hydration and fuel, without the mythology
Moving days are usually long, warm and food free. People work from 8am to 6pm on a coffee and a slice of pizza, then blame their back for the ache.
You do not need an electrolyte protocol. You need water available in a place you will actually walk past, and real food at intervals. Keep a gallon jug in the kitchen of the new place and a snack bag in the car: nuts, a banana, a sandwich. The failure mode on moving day is not dehydration from sweat loss, it is simply forgetting to drink anything for four hours because your hands are full.
where I disagree with the standard advice
The usual recommendation is to hire movers and skip the whole problem. If the budget exists, that is reasonable. But the common framing that heavy lifting is inherently bad for your back does not hold up. Loading tissue is how tissue gets stronger, and a person who trains regularly is generally better equipped for moving day than someone who has avoided load for a decade.
The real risk factor is not the weight. It is the volume of unfamiliar movement at unfamiliar angles with a deadline attached. A powerlifter who deadlifts 300 pounds can still tweak something carrying an awkward dresser down a narrow staircase, because a dresser is not a barbell and the staircase does not care about your one rep max.
So the useful advice is narrower: reduce awkwardness, not effort. Take doors off hinges. Empty drawers. Use a furniture dolly, which costs about 25 dollars to rent for a day and removes more risk than any brace or belt.
the day after
You will be sore. That soreness peaks 24 to 48 hours later, which is the normal course of delayed onset muscle soreness after unaccustomed eccentric work, and it is not a sign of injury.
Skip the hard training session you had planned. Walk instead, twenty or thirty minutes, because gentle movement tends to feel better than lying still. Come back to your normal program in two days rather than trying to prove something on day one.
If you have pain that is sharp rather than achy, localized to one spot rather than spread across a muscle group, or accompanied by numbness or tingling down a leg, that is a different category of problem and worth having looked at by a clinician rather than waiting out.