J. Biomech. http://doi.org/bs27 (2016)

Attaching a backpack elastically, rather than stiffly, to its carrier is known to reduce peak forces and loads on joints. But whether that comfort comes at an additional energy cost when walking has been less clear, with experimental results seeming contradictory. Based on a simple model, Dejun Li and colleagues have now found that although there is a difference for rigid and flexible packs, it is smaller than previously predicted.

Li et al. showed that for typical walking speeds and pack masses of 5–35 kg, the difference in energy cost between stiff and elastic suspension can be positive or negative, depending on suspension stiffness, load and walking speed. For realistic scenarios the differences are, however, always less than 10%.

These findings are in qualitative agreement with earlier models, but the predicted quantitative changes in energy cost to the walker are significantly lower, and closer to experimental results. The improvement, the authors say, is due to their taking into account the efficiency of the muscle performing mechanical work, which is greater for downward motions than it is for upward ones.