Why Does a Load Suddenly Feel Heavier When You Hold It Away From Your Body?
― The Bones in Your Lower Back Carry Many Times the Load's Weight
Carrying a cardboard box hugged tight to your chest is no trouble. Hold it out at arm's length and you struggle within a few steps. The weight hasn't changed by a single gram. What has changed is the force on the bones of your lower back, and that force can be several times the weight of the load itself.
It's moving day. You hug a box full of books tight against your body. It's heavy, but you can carry it across the room.
Now think about pulling the same box out from the back of a shelf. With your arms stretched out in front of you, you grunt the moment you lift it.
You might assume your arms are the problem. But the next morning, it's usually your lower back that aches, not your arms.
There are two main reasons
The back muscles pull on the spine at a point only a few centimetres from the hip joint. The load, meanwhile, sits tens of centimetres away. A short arm has to work against a long arm, so the muscles must produce many times the force of the load.
The muscles run lengthwise along the spine. The harder they pull, the more the spine is squeezed from above and below, like a vice. The full force the muscles produce is added on top of the weight of the load.
So holding a load away from your body is hard not because your arms are weak. It's because the lever pivoting on your hip joint suddenly turns into an unfavourable shape. Let's take it step by step.
Your lower back is a terribly lopsided lever
Look from the side at someone bending forward to hold a load. If you treat the bones of the lower back as the pivot, the body is a lever. Take a look at Figure 1. The load hangs about 40 centimetres from the pivot, and the weight of the upper body itself acts about 25 centimetres out. Both try to tip the body forward.
What holds it back are the muscles running lengthwise along the back. But the point where these muscles pull on the bone is only about 5 centimetres from the pivot. Think of a seesaw: the other person sits at the far end, while you sit right beside the pivot. To balance, you need a force larger by the ratio of the arm lengths.
Your spine is squeezed from above and below
Here is where the body's machinery has it rough. The back muscles are attached lengthwise along the spine, so when they pull hard, that force turns directly into a force compressing the spine lengthwise. The large force produced to support the load has nowhere to go and builds up between the bones.
Between the bones sit soft discs called intervertebral discs. As the calculation later shows, holding a 20-kilogram load at arm's length puts a force of roughly six times your body weight on these discs. Hold the same load close to your body and you can cut that by about 30 per cent.
There's no magic in bending your knees itself. Using your knees lets you keep your back upright while bringing yourself close to the load, so the distance between the load and your lower back gets shorter. What matters is the distance, not the shape of your posture. Conversely, if you bend your knees but still hold the load away from your body, the force on your lower back barely drops.
When people carry something heavy, they lean their upper body back without thinking. This brings the centre of gravity closer to directly above the pivot, shortening the arm. Without doing any sums, the body picks the posture that makes the lever work in its favour.
Summary
Move a load away from your body and the lever arm pivoting on your lower back suddenly gets much longer. The muscle supporting it keeps an arm of just a few centimetres, so it has no choice but to produce many times the force. And all of that force turns into force squeezing the spine. A change of a few tens of centimetres in distance changes the force inside your body by hundreds of kilograms.
It isn't the load that sets the weight you feel.
It's those few tens of centimetres between the load and your lower back.
How the discs between your vertebrae change over the course of a day is covered in Why Is Your Height Up to a Centimetre Different Between Morning and Night?, and the story of how bone itself is constantly rebuilt is in Why Is Bone Rebuilt Every Few Years?
- Get a 2-litre bottle of water. Hold it in both hands, pressed tight against your chest, and stand for 30 seconds.
- Next, hold the same bottle with your arms stretched straight out in front and stand for 30 seconds. You should tire at a very different speed.
- Finally, keeping your arms out, lean your upper body back just a little. If it gets easier, that's proof the arm has become shorter.
If you have any concerns about your lower back, do only steps 1 and 3. Even a light bottle is enough to show that distance, not weight, is what matters.
For those who want more ― terms, formulas and links to textbooksEach part is labelled with its level, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics / Physics"
- High school+Advanced high-school material, or textbook sidebar content
- UniversityNot taught in high school; university specialist content (biomechanics)
- ResearchNot yet settled even at university level; what researchers are investigating right now
Middle schoolTerms: this phenomenon has names
- Moment of force (torque): the size of a force's turning effect. It is the force multiplied by the distance from the pivot to where the force acts. It is the effectiveness of a lever itself.
- Lever arm: the distance from the pivot to the point where the force acts. For the lower back, the load's arm is tens of centimetres and the back muscle's arm is a few centimetres.
- Intervertebral disc: a soft, disc-shaped tissue between the vertebrae. It takes the force from above and below and bears it as pressure inside.
Middle schoolHigh schoolChecking with formulas: how much force is on the spine?
Let's work out the force on the lower-back bones when bending forward with a 20-kilogram load. We assume a person weighing 60 kilograms.
| In symbols | F × a ＝ W1 × b ＋ W2 × c |
| In words | Muscle force × muscle arm ＝ load weight × arm to the load ＋ upper-body weight × arm to the upper body's centre of gravity |
| Where it comes from | It is the balance of moments of force about the hip joint. When the body is held still without tipping forward, the tipping effect and the righting effect are equal. The force on the spine is then found from P ＝ F ＋ W1 ＋ W2. |
| Load weight W1 | 20 kg's worth; as a force, about 200 N (N is the unit of force) |
| Upper-body weight W2 | 30 kg's worth, about half of body weight; as a force, about 300 N |
| Arm to the load b | 0.40 m (bent forward with arms out) |
| Arm to the upper body's centre of gravity c | 0.25 m |
| Back muscle arm a | 0.05 m (only a few centimetres) |
| Tipping effect of the load | 200 × 0.40 ＝ 80 |
| Tipping effect of the upper body | 300 × 0.25 ＝ 75 |
| Total tipping effect | 80 ＋ 75 ＝ 155 |
| Muscle force F needed to balance | 155 ÷ 0.05 ＝ 3100 |
| Total weight pressing from above | 200 ＋ 300 ＝ 500 |
| Force compressing the spine P | 3100 ＋ 500 ＝ 3600 |
| Converted to weight | 3600 ÷ 9.8 ≒ 367 |
| Multiple of body weight | 367 ÷ 60 ≒ 6.1 |
The force between the vertebrae comes to roughly 367 kilograms' worth, about six times body weight. Doing the same calculation with the load pulled in to 0.15 m from the body gives the following.
| Tipping effect of the load | 200 × 0.15 ＝ 30 |
| Total tipping effect | 30 ＋ 75 ＝ 105 |
| Muscle force needed | 105 ÷ 0.05 ＝ 2100 |
| Force compressing the spine | 2100 ＋ 500 ＝ 2600 |
| Converted to weight | 2600 ÷ 9.8 ≒ 265 |
Pulling the load in by just 25 centimetres cut the force on the spine by 100 kilograms' worth. It isn't arm strength or grit; it's distance doing the work. Note that this is a very rough estimate that treats the body as a single rod. In a real body, the abdominal muscles and the pressure inside the belly also help to bear the load.
High schoolHigh school+Why does the body use such a bad lever?
High schoolIn physics, this is treated as the balance of moments of force about a pivot. This lower-back lever is what is called a third-class lever, in which the effort and the load lie on the same side of the pivot, and in terms of force it always loses out. In exchange, a small contraction of the muscle moves the hand a long way, and fast.
High school+Most of the arms and legs in the human body are made of this third-class lever. The body sells force in order to buy speed and range of motion. The lower back isn't an exception; the whole body makes the same trade.
UniversityMeasuring disc pressure, and musculoskeletal models
In biomechanics, this force has been measured directly as pressure inside the disc. Since the 1960s, researchers have pushed fine needles into discs to measure pressure, showing that pressure is far higher when bending forward than when standing. Today the usual approach is to apply inverse dynamics to a musculoskeletal model containing many muscles, and estimate the compressive and shear forces on the lumbar spine. Treating the muscles as a single one, as in the calculation above, can over- or underestimate the force, so the problem is handled as a muscle-force distribution problem that works out how several muscles share the load.
📖 Derivation of the formulas, and further reading: Moment of force (Japanese Wikipedia) ／ Intervertebral disc (Japanese Wikipedia)
ResearchWhat is still not well understood
- Size of force and pain don't match. A larger force on the lumbar spine doesn't always mean more pain, and many people with abnormalities on imaging feel no pain at all. Mechanics alone leaves part of the picture unexplained.
- The effect of pressure inside the belly isn't settled. Bracing the abdomen raises the pressure inside it and is thought to reduce the load on the spine, but studies give different values for how much.
- The effect of "correct lifting technique" is still under debate. Some reports say teaching people to bend their knees hasn't clearly reduced lower-back problems, and researchers are asking whether posture training alone is enough.
In other words, this article too is "an explanation within what is known today." The lever balance is sound, but whether it translates directly into bodily complaints is not yet known.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: how levers work | Pivot, effort and load, and the ratio of arm lengths |
| High school | Basic Physics: moments of force and balance | ⓪ and ② of "Checking with formulas" |
| High school+ | Physics: equilibrium of rigid bodies | Why a third-class lever loses out on force |
| University | Biomechanics, orthopaedics | Measuring disc pressure and musculoskeletal models |
| Research | Epidemiology of back pain, ergonomics | The mismatch between force and pain |
| ― | Links to daily life | Holding a load close to your body cuts the force on your lower back by about 30 per cent |
- Ministry of Health, Labour and Welfare (厚生労働省), "Guidelines for Preventing Low Back Pain in the Workplace" (職場における腰痛予防対策指針) (guidelines that include handling heavy loads close to the body)
- Nachemson, A. "The load on lumbar disks in different positions of the body", Clinical Orthopaedics and Related Research, 1966 (direct measurement of disc pressure in different postures)
- Japanese Orthopaedic Association (日本整形外科学会) and Japanese Society for the Study of Low Back Pain (日本腰痛学会), eds., "Clinical Practice Guidelines for Low Back Pain" (腰痛診療ガイドライン) (a summary of the relationship between causes of low back pain and imaging findings)
- Moment of force (Japanese Wikipedia)
※This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand how things work. If pain or numbness in your lower back persists, don't rely on your own judgement; consult a medical professional.