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Fun-Sized Hikers Can Carry Heavy Load; Physics Explains How Heavy Backpack Weight No Challenge For Smaller Hikers

Professor Michael O'Shea came up with a model that provides an accurate estimation of the backpack weight a given hiker will be able to carry. Photo courtesy of Shutterstock

The more chivalrous among us are always ready to provide a helping hand to our less bulky co-hikers. Helping our female counterparts by carrying their backpacks will certainly earn us some brownie points, but it turns out that they don’t actually need any help.

The observation that smaller hikers could easily carry heavier loads was made by Outward Bound, Kansas State University physics professor Michael O'Shea, who routinely leads students on extended backpacking trips. Using the principle of the ant that despite its size can carry a load 10 to 50 times its body weight, the professor developed a model to determine the weight that hikers should carry. His article appears in journal The Physics Teacher.

The model uses the assumption that backpackers have to carry their weight as well as the weight of the backpack while hiking. Putting these variables in the model provides an accurate estimation of the backpack weight a given hiker will be able to carry and an example of how real-world modeling examples can be used in the classroom.      

Most people often carry weights corresponding to their own size. But this is wrong, says O’Shea. "Online advice from several sources was somewhat misleading in suggesting that pack weight should be a certain percent of a person's weight," he said in a press statement. Giving the example of the ant, he says that as the size of any animal increases, strength increases more slowly than body weight.

He used this information along with body scaling proportions obtained from other research and incorporated it in to his new model. The model needs variables such as the hiker's entire load — backpack plus body weight — to determine the maximum backpack weight for an individual of a given size.

"Overall strength of an individual does not determine how heavy a backpack a person can comfortably carry," O'Shea said.

The assumptions that the model makes is that hikers being compared have similar body-fat percentages, and thus that increase in size does correspond to a proportional increase in strength. In spite of this, it provides a more streamlined estimation of the hiker’s carrying capacity. 

This model not only helps hikers make informed decisions about the weight they need to lug, but it also helps in the classroom.

"Students should be able to see how some aspects of complex systems, in this case the frame of a human being, can be modeled in a relatively simple way to extract useful information," he said.

Source: O’Shea, M. Backpack Weight and the Scaling of the Human Frame. The Physics Teacher. 2014.