Where you hold it, not what it weighs
- 10 kgHeld closethe same 10 kgHeld at arm's length
Two people lift the same 10-kilo box. One holds it against the chest. The other holds it out at arm's length, the way you carry something you do not want to touch your clothes. Same box. The second person's back is doing several times the work, and will not tell them until later.
- Load, 20 cm in frontThe spine, the pivotMuscles, 5 cm behindpull 4 times the load
A simple lever rule. Picture the lower back as a lever, like a seesaw. The muscles that hold you upright pull about 5 cm behind the spine. A load held 20 cm in front pulls four times as hard against them, because 20 is four times 5. At 60 cm in front, twelve times. A simple rule, but the shape is right.
- Against the chestAt arm's length
Same box, same person. Which lift pulls harder on the lower back?
The same. Ten kilos is ten kilos.
Not yet. Ten kilos is ten kilos for the arms. For the back the question is where the ten kilos are, and the lever rule says distance multiplies.Against the chest.
Not yet. Close to the body is the easier lift. The box sits near the pivot, so the back does the least work it can for that weight.At arm's length.
Right. The box is further from the pivot, so the muscles behind the spine pull harder to balance it. The box has not changed; its distance has. - Waist height, close to the body25 kgWaist height, at arm's length15 kgShoulder height, close10 kgShoulder height, at arm's length5 kg
Guideline figures for a man, occasional lift. HSE. Britain's safety body prints guideline figures for a lift done now and then, in good conditions. For a man at waist height: about 25 kilos close to the body, 15 at arm's length. At shoulder height: 10 close, 5 far. A woman's figures are lower. They are a filter, not a safe limit: they say when to look harder.
Move the control to hold the box further from the spine. The box is 10 kilos. The rule: pull on the back is the box times its distance in front, divided by the muscles' 5 cm behind.
The box10 kgPull on the lower back40 kg20 cm from the spineA 10-kilo box held against the chest, 20 cm in front of the spine. 20 divided by 5 is 4, so the back pulls with about 40 kilos of force to hold it.
The box10 kgPull on the lower back60 kg30 cm from the spineA 10-kilo box held a hand's width out, 30 cm in front of the spine. 30 divided by 5 is 6, so the back pulls with about 60 kilos of force to hold it.
The box10 kgPull on the lower back80 kg40 cm from the spineA 10-kilo box held at bent arms, 40 cm in front of the spine. 40 divided by 5 is 8, so the back pulls with about 80 kilos of force to hold it.
The box10 kgPull on the lower back120 kg60 cm from the spineA 10-kilo box held at arm's length, 60 cm in front of the spine. 60 divided by 5 is 12, so the back pulls with about 120 kilos of force to hold it.
The box is 10 kilos. At arm's length, 60 cm out, what is the pull on the back?
Still 10 kilos. The box has not got heavier.
Not yet. The box is still 10 kilos for the arms. The lever rule is about the back, and for the back the distance multiplies the load.About 120 kilos, twelve times the box.
Right. Sixty divided by five is twelve. A 10-kilo box held out in front asks the lower back for the pull of a 120-kilo load, every time.About 60 kilos, six times the box.
Not yet. Sixty is the distance in centimetres, not the pull. Divide by the 5 cm behind the spine: twelve times, 120 kilos.Move the control.
- 15 kgThe box40 cmIn front of the spine
A 15-kilo box is held 40 cm in front of the spine. Using the lever rule, how many kilos of pull does the lower back give?
kgForty divided by five, then times fifteen.
Right. Eight times fifteen is 120. The same pull as the 10-kilo box at arm's length: a lighter box held further out can load the back exactly as hard. A 12-kilo box needs lifting. Tap the one place where the guideline figure for a man covers it.
Tap the part of the picture that answers it.
Waist height, close to the body25 kgWaist height, at arm's length15 kgShoulder height, close10 kgShoulder height, at arm's length5 kgWaist height, close to the body
Right. Twenty-five kilos is the figure here, and 12 sits well under it. Close, and at waist height, is the lift the body is built for.Waist height, at arm's length
Not yet. Fifteen is the figure, and 12 is under it, but the guideline treats arm's length as a lift to look harder at, and the lever rule says why: the pull on the back is nearly three times the close lift.Shoulder height, close
Not yet. Ten kilos is the figure at shoulder height, and 12 is over it. Lifting above the chest takes the load away from the strongest part of the body.Shoulder height, at arm's length
Not yet. Five kilos. Twelve is more than twice the figure. High and far is the worst place a load can be, and the figures say so.- How heavyHow far outHow oftenTwistingThe floor
What decides the strain of a lift. Weight and distance are not the whole lift. How often matters: a box an hour is not a box a minute. Twisting while holding it matters. So do the carry, the grip, and whether the floor is wet. The guideline figures assume all of these are good, which is why they are a filter, not a permission.
- 20 kg at 20 cmBefore15 kg at 40 cmAfter
A warehouse cuts every box from 20 kilos to 15, and moves the shelves so boxes are now lifted at arm's length, 40 cm out, instead of 20 cm close. What happened to the pull on workers' backs?
It went up, from about 80 kilos of pull to about 120. The lighter box is held twice as far out.
Right. Twenty times four is 80. Fifteen times eight is 120. The warehouse made the boxes lighter and the lifts harder, and the second change is the one the backs will feel.It stayed the same. Less weight and more distance cancel.
Not yet. They would cancel if the weight had halved. It fell by a quarter and the distance doubled, so the back is worse off by half again.It went down. The boxes are a quarter lighter.
Not yet. The boxes are lighter for the arms. For the back the distance doubled, and doubling the distance doubles the pull; a quarter off the weight does not undo that.
Lesson complete
A load held far from the body pulls on the back many times harder than the same load held close.