Q: I have a few options for carrying my laptop and documents to school and work. Which is best for my posture and overall health?
A: One way to determine the effect of a load on the human body is to measure how far the load pulls a person off their center of gravity. When this is done to test backpacks, shoulder straps, and briefcases, we learn a few important lessons.
A couple of researchers from the University of Illinois at Chicago published an article in 2008 summarizing some experiments they did to examine the effect of a number of types of bags on postural sway.
What is postural sway?
- When a person attempts to stand still, the body isn’t still at all. It has to make constant little corrections in a person’s balance in order to maintain a still posture.
- Sometimes the body isn’t so good at this. For instance, when someone is drunk, it decreases their ability to stand still because the nervous system is depressed and doesn’t react as quickly. That’s why standing still is one way to field test for a drunk person.
- When someone has a concussion, this can also be detected by examining their ability to stand still.
- Scientists can actually measure postural sway by having a person stand on a pressure plate that measures their center of pressure. As their body makes thousands of little self-corrections, these tiny shifts in their COP can be measured by the pressure plate.
In this experiment, the researchers used this method to measure how the body adjusts to various carrying loads.
- The idea is that certain bags or loads are going to pull the body off its COP to a greater degree.
- When this happens, the body has to flex and strain its muscles to maintain a consistent balance. The heavier or worse the load, the more the body has to strain to maintain balance.
- If the load is too heavy or placed in the wrong way, over time this will result in bad posture, muscle strain, chronic pain, and perhaps injuries.
This experiment was very lengthy and complex, so only 10 subjects could be recruited for it.
- The subjects had to be free from any musculoskeletal or balance problems to be in the study.
Each subject was given a series of loads to carry with their dominant hand and were then asked to stand on the pressure platform without shoes.
The loads were:
- Single strap bag on one shoulder (not across the chest but down from one shoulder)
- No bag at all
- Additionally, each bag was filled with two levels of weight:
- 10% of a subject’s body weight
- 20% of a subject’s body weight
- Finally, subjects were instructed to stand either with their feet together, or their feet a natural distance apart.
Thus, each subject stood on the pressure plate a LOT of times to go through all the different types of bags, weights, and stances.
Once on the pressure plate, the subjects were asked to stand as still as possible for 45 seconds while looking straight ahead.
Short breaks were given between loads so the subjects didn’t get too strained or tired.
ANALYSIS AND RESULTS
The subject’s postural sway was measured for each bag, weight, and stance.
Effect of load weight:
- Unsurprisingly, the heavier the load was, the more the body’s weight was displaced.
- This was especially true for the briefcase. Carrying a heavy briefcase (20% of body weight) had the strongest effect on center of pressure displacement.
- This means if you have an extremely heavy load, a briefcase is not the best choice.
- For backpacks and single-strap bags, a heavier load did increase body weight displacement, but not to the same degree.
Effect of type of bag:
- For lighter weights, there wasn’t a huge degree of difference between a backpack and a single strap bag. Both these bags keep the weight fairly close to the body so it doesn’t pull the body too far off its balance.
- The briefcase was worse than the other two in terms of postural sway, but at the 10% of body weight level it wasn’t a huge difference.
- For heavier loads (as mentioned above), the briefcase had an extremely strong and negative pull on the body.
Effect of stance:
- Keeping the feet together was bad for posture and displacement.
- When carrying a bag, it’s best for posture to keep the feet a natural distance apart.
- Not everyone agrees on the exact number, but the researchers suggested that people should only keep about 10% of their body weight in their bags, no matter what bag it is.
- When the weight is light (10% of body weight), the backpack and the shoulder strap were the best, but the briefcase wasn’t too far behind.
- It’s when the weight goes up (20% of body weight) that we start to see the worst effects. When the load is heavy, the briefcase has an extremely negative effect on posture.
- What do we learn from this? TIPS:
- Try to keep the weight of your bag around 10% of your body weight or less (e.g., a 180-pound man should have an 18-pound bag or less).
- If you do, then it doesn’t matter as much whether you have a backpack, shoulder strap bag, or briefcase (although the backpack and shoulder strap are better).
- At heavier weights (20% of body weight), a briefcase is very bad for your posture. This may result in muscle strain, injuries, and chronic pain.
Zultowski, I., & Aruin, A. (2008). Carrying loads and postural sway in standing: The effect of load placement and magnitude. Work, 30(4), 359-368. Link: https://www.ncbi.nlm.nih.gov/pubmed/18725699