Is Carrying a Bag on One Shoulder Bad for Your Back?
Carrying a bag on one shoulder measured 2.75 times more trapezius activity than the same load on two straps, and wearing a single strap bag across the body instead cut the reading by about 63 per cent.
Carrying a bag on one shoulder makes that shoulder work roughly two to three times harder than carrying the same load on two straps. In a University of Hertfordshire walking study at 10 per cent of body weight, a shoulder strap resting on the loaded side measured 8.31 per cent of maximum voluntary contraction at the trapezius. A two strap rucksack measured 3.02 per cent, slightly below carrying nothing at all. If a one strap bag is what you already own, the correction is free: a 2017 study found that dropping the same strap across the body, so the bag sits at the opposite hip, cut trapezius activity by about 63 per cent. Two straps when the load is heavy, across the body when it is not.
In this article
- What does one shoulder do to the load?
- Does wearing the same bag across your body help?
- Which shoulder should a one strap bag sit on?
- How do the carrying methods compare, measured?
- How heavy was the load in these studies?
- What should you change tomorrow morning?
- Frequently asked questions
What does one shoulder do to the load?
It concentrates it. A bag hanging from one shoulder has to be held there, and the muscle doing the holding is the upper trapezius, running from the base of the skull to the point of the shoulder. A 2015 study by Hardie, Haskew, Harris and Hughes at the University of Hertfordshire put numbers on it: The Effects of Bag Style on Muscle Activity During Walking.
Twelve female students walked in five conditions carrying 10 per cent of their own body weight, while surface electrodes read the left trapezius as a percentage of maximum voluntary contraction.
- No bag at all: 3.57 per cent
- Two strap rucksack: 3.02 per cent
- One strap rucksack: 6.42 per cent
- Shoulder strap on the loaded side: 8.31 per cent
Read the first two lines again. A two strap rucksack carrying 10 per cent of body weight asked slightly less of that muscle than walking with nothing. The same weight over one shoulder asked 2.75 times more. It is a twelve person study on a laboratory floor and should be read at that size, but it is a measurement, which is more than most bag advice has.
Does wearing the same bag across your body help?
Substantially, and this is the finding to use if you are buying nothing. Effects of Same Sided and Cross Body Load Carrying, in the Journal of Musculoskeletal Science and Technology in 2017, had twenty healthy adults stand for five minutes with a single strap bag loaded to 10 per cent of body weight. The bag was identical in both conditions. Only the way it hung changed: first on one shoulder, then across the torso with the weight at the opposite hip.
- Right upper trapezius: 333.45 on one shoulder, 124.60 across the body. About 63 per cent lower.
- Left erector spinae: 124.20 on one shoulder, 78.70 across the body. About 37 per cent lower.
Both differences reached significance at p equals 0.01. The reason is mechanical: on one shoulder the strap must be pinned in place and the trunk leans to counterbalance, while across the body your torso traps the strap and the load sits nearer your centre line. The test bag itself was 40 x 7 x 30 cm and 600 g empty.
3.4 L, 28 x 15 x 10 cm, 175 g empty. An adjustable chest or shoulder strap, a padded back panel and a quick-access front zip pocket, in water-repellent nylon.
Which shoulder should a one strap bag sit on?
Your non dominant one, with the bag crossing to the opposite hip. In Unilateral Shoulder Bags: Can They Be Worn in a Way to Reduce Postural Asymmetry?, in the Internet Journal of Allied Health Sciences and Practice in 2012, Qureshi and Shamus had 65 students stand on a postural scale analyser wearing a 10 pound bag two ways.
Draping it from the left shoulder across to the right hip, for the 63 right hand dominant participants, significantly improved how evenly weight passed through both legs, at p below 0.05. So: strap on the non dominant shoulder, bag at the opposite hip, dominant arm free. Three studies, one direction: the further the load moves from a single shoulder towards your middle, the less any one muscle has to do.
How do the carrying methods compare, measured?
The two studies report in different units, so read down each study, not across.
| Carrying method | Measured reading | Study | What it means |
|---|---|---|---|
| No bag | 3.57 per cent MVC | Hardie 2015, walking | The baseline |
| Rucksack, both straps | 3.02 per cent MVC | Hardie 2015, walking | Just under baseline |
| Rucksack, one strap | 6.42 per cent MVC | Hardie 2015, walking | Double the two strap figure |
| Shoulder strap, loaded side | 8.31 per cent MVC | Hardie 2015, walking | Highest of five conditions |
| Shoulder bag, same side | 333.45 amplitude | Lee 2017, standing | Reference row |
| Same bag, across the body | 124.60 amplitude | Lee 2017, standing | About 63 per cent lower, p equals 0.01 |
MVC is maximum voluntary contraction, the share of a muscle's full effort. The 2017 rows are raw electromyography amplitudes from a different protocol, so they show the change between two ways of wearing one bag, not an absolute level. Both samples are small.
How heavy was the load in these studies?
All three sat in the same region. Hardie and colleagues and Lee both used 10 per cent of each participant's body weight, and Qureshi and Shamus used a fixed 10 pound bag, about 4.5 kg.
For an Indian adult that is 6 kg at 60 kg of body weight, 7 kg at 70 kg and 8 kg at 80 kg: a laptop, a charger, a lunch box, a bottle and a change of clothes. Where the figure comes from, and the one bag weight rule India publishes, is in how much should a backpack weigh.
So if your daily load is near that mark, the two strap result is the one to act on. And the bag's own weight counts: at 560 g a 17 litre backpack uses 8 per cent of a 7 kg allowance before you pack it, while a 175 g sling uses 2.5 per cent.
17 L, 43 x 29 x 17 cm, 560 g empty. Padded breathable straps, a dedicated compartment for a laptop up to 15 inch, a rear-access compartment and a front organiser panel, in water-repellent nylon.
What should you change tomorrow morning?
One, use both straps. If the bag on your shoulder has a second strap tucked behind it, using it took the walking reading from 6.42 per cent to 3.02 per cent of maximum effort.
Two, if it only has one strap, wear it across. Non dominant shoulder, bag at the opposite hip, which cut trapezius activity by about 63 per cent in the standing test.
Three, weigh the packed bag once. On a kitchen scale, on a Monday. Above 10 per cent of your own weight, the number to change is what is inside it, not how you hold it.
A tote is a one sided carry by design, so the same variables decide it: how much goes in, and how long it stays on one side. An 8.6 litre bag at 465 g empty caps the load, and switching hands spreads the rest. None of this is medical advice: ongoing shoulder, neck or back pain belongs with a doctor or a physiotherapist.
8.6 L, 45 x 30 x 8 cm, 465 g empty. Two separate compartments with a padded suspended sleeve for a laptop up to 16 inch, dual handles, a full-length top zip, a front slip pocket and a rear zip pocket, in water-repellent nylon.
Frequently asked questions
Is carrying a backpack on one strap bad for you?
It asks roughly twice as much of the loaded shoulder. In the 2015 walking study a one strap rucksack read 6.42 per cent of maximum voluntary contraction against 3.02 per cent for the same bag on both straps.
Is a crossbody bag better than a shoulder bag?
For the same bag and load, the measurements say yes. The 2017 study used one bag in both conditions and found trapezius activity about 63 per cent lower, and erector spinae activity about 37 per cent lower, across the torso.
How heavy should a daily bag be?
All three studies tested at or below 10 per cent of body weight, which is 7 kg for a 70 kg adult. Include the bag's empty weight in that total.
Should I switch shoulders while carrying a one sided bag?
Switching spreads the work rather than removing it, which makes it sensible for a tote. The 2012 improvement came from moving the bag across the body onto the non dominant shoulder, so wear it across first and switch second.
Does a chest strap on a sling make a difference?
The studies did not test chest straps, so there is no figure to quote. They did test load position, and the 2012 and 2017 results point one way: the closer the bag sits to your centre line, the less any single muscle holds.
Can one shoulder carrying cause a lasting postural change?
These studies measured muscle activity and standing weight distribution while carrying, not long term outcomes, so they cannot answer that. They show short term asymmetry of 2 to 3 times in one muscle, which is reason to change how you carry.
Pick the carry, then the bag
A 3.4 L sling made to sit across the body, through to a 17 L backpack on padded breathable straps.
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