FITNESS
October 29, 2014

Carry That Load

Guess who can carry the heavier backpack: Fullback types or more modestly-sized adults?

When it comes to carrying a backpack, small people can more than hold their own. According to one physicist, a 110-pound adult can tote the heaviest backpack, weighing around 50 pounds.

Michael O'Shea is a professor of physics at Kansas State University who has been leading students on extended backpack trips for 20 years. He noticed that larger (not overweight) students often struggled with backpacks that smaller students could carry comfortably. And he wondered why.

Many insects, like the ant, can carry loads heavier than their own weight. Horses and elephants can't.

Part of the answer seems to be that larger people are already carrying more weight than smaller people are — their own weight. A 160-pound person is carrying 50 more pounds than a 110-pound person.

Yes, larger people are stronger, but as a person gets bigger, their strength doesn't increase by all that much. At some point, the added strength becomes less than what's needed to carry the additional body weight. That leaves less strength available for carrying other loads such as backpacks.

This isn't only true of people. Many insects, like the ant, can carry loads heavier than their own weight. Horses and elephants can't. Strength simply does not rise as fast as size does in the animal kingdom.

Healthy adult hikers are often advised that their backpack weigh no more than 25-30% of their body weight. This means that heavier hikers should be able to carry heavier backpacks. But that does not reflect the reality O'Shea has seen.

So he invented his own formula.

O'Shea's formula estimates that a 110-pound person can shoulder the heaviest pack, 49.6 lbs. This slowly drops as a person's weight goes up, to a 46-pound pack for a 160-pound person. It's at the higher body weights that maximum pack weight really begins to drop, with a 200-pound person only able to carry a 40-pound pack and a 220-pounder a 37-pound pack.

These are only estimates, but O'Shea feels they fit reality better than the standard advice does. His formula accounts for the observation that the 160-pound person can carry more total weight (206 pounds) than the 110-pound person can (159.6 pounds), but he or she can't carry as heavy a backpack.

Think of these estimates as the heaviest backpack an adult of a particular weight should pack for an extended trip. They're calculated for healthy adults who are not overweight and who have a little hiking experience. Highly trained hikers (especially larger hikers) would be able to carry more weight than these figures predict, while novice hikers would need to carry a bit less, according to O'Shea.

Heavy backpacks can certainly be a problem for school children. O'Shea hopes that his estimates will make them less of a problem for adults.

The paper is published in The Physics Teacher.

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