Department of Human Evolutionary Biology, Harvard University
Saturday Afternoon, Galleria North
The ability to carry objects has played an important role in human evolution, from holding infants to transporting tools and other resources. Modern humans use a variety of carrying strategies and technologies, but differences in the metabolic costs of these methods are not well understood. It remains unclear which carrying methods are most economical and how human ancestors might have used them.
One simple technology for reducing energetic costs is to employ carrying structures that absorb, store, and return elastic energy. Throughout Asia springy bamboo poles are commonly used to balance loads across the shoulder. This study develops and tests a model for how poles may permit humans to carry heavy objects with great economy.
We examined this question under controlled lab conditions with Western subjects and in the field in Sichuan, China with porters who habitually use this technique. We show that the pole system can be modeled as a driven harmonic oscillator, allowing us to calculate the optimal relationship between step frequency and the natural frequency of the pole. This produces a stable, out-of-phase relationship between the vertical motion of the center of mass of the load and body. Predicted versus observed results suggest that people tune the pole and their gait to reduce metabolic costs compared to conventional carrying strategies. Preliminary data from China support the hypothesis that people may choose poles or modify their gaits to achieve a cost-saving phase relationship. These results are used to estimate energetic savings in hunter-gathers who also use carrying poles.
Funding was provided by The Fairbank Center, Harvard University, an NSF-GRFP, and the Hintze Family Charitable Foundation.