June 2005). 4. R. M. Alexander, J. Exp. Biol. 160, 55 (1991). 5. G. A. Cavagna, N. C. Heglund, C. R. Taylor, Am. J. Physiol. 233, R243 (1977). 6. J. Drake, Wired 9, 90 (2001). 7. S. Stanford, R. Pelrine, R. Kornbluh, Q. Pei, in Proceedings of the 13th International Symposium on Unmanned Untethered Submersible Technology (Autonomous Undersea Systems Institute, Lee, NH, 2003). 8. T. Starner, J. Paradiso, in Low Power Electronics Design (CRC Press, Boca Raton, FL, 2004), p. 45–1. 9. G. A. Cavagna, M. Kaneko, J. Physiol. 268, 647 (1977). 10. S. A. Gard, S. C. Miff, A. D. Kuo, Hum. Mov. Sci. 22, 597 (2004). 11. Supporting material is available on Science Online. 12. Because it is a prototype, there has been no attempt to reduce the weight of the backpack—indeed, it is substantially ‘‘overdesigned.’’ Further, the 5.6 kg includes the weight of six load cells and one 25-cmlong transducer, each with accompanying brackets and cables, as well as other components that will not be present on a typical pack. In future prototypes, we estimate that the weight will exceed that of a normal backpack by no more than 1 to 1.5 kg. 13. Under high-power conditions (5.6 km hourj1 with 20and 29-kg loads and 4.8 km hourj1 with a 38-kg load), power generation on the incline was the same as on the flat. Under low-power conditions (4.8 km hourj1 with 20and 28-kg loads), electricity generation on the incline was actually substantially greater than that on the flat (table S1). 14. R. Margaria, Biomechanics and Energetics of Muscular Exercise (Clarendon, Oxford, 1976). 15. R. A. Ferguson et al., J. Physiol. 536, 261 (2001). 16. G. A. Cavagna, P. A. Willems, M. A. Legramandi, N. C. Heglund, J. Exp. Biol. 205, 3413 (2002). 17. A. Grabowski, C. T. Farley, R. Kram, J. Appl. Physiol. 98, 579 (2005). 18. J. M. Donelan, R. Kram, A. D. Kuo, J. Exp. Biol. 205, 3717 (2002). 19. J. M. Donelan, R. Kram, A. D. Kuo, J. Biomech. 35, 117 (2002). 20. J. S. Gottschall, R. Kram, J. Appl. Physiol. 94, 1766 (2003). 21. Because this savings in metabolic energy represents only 6% of the net energetic cost of walking with the backpack (492 W) (table S3) (17, 18), accurate determinations of the position and movements of the center of mass, as well as the direction and magnitude of the ground reaction forces, are essential to discern the mechanism. This will require twin–force-platform single-leg measurements, as well as a complete kinematics and mechanical energy analysis (19, 20). The energy analysis is made more complex because the position of the load with respect to the backpack frame and the amount of energy stored in the backpack springs vary during the gait cycle. Finally, electromyogram measurements are also important to test whether a change in effective muscle moment arms may have caused a change in the volume of activated muscle and hence a change in metabolic cost (20, 27, 28). 22. K. Schmidt-Nielsen, Animal Physiology: Adaptation and Environment (Cambridge Univ. Press, Cambridge, ed. 3, 1988). 23. This assumes that electronic devices are being powered in real time. If there were a power loss of 50% associated with storage (such as in batteries) and recovery of electrical energy, then these factors would be halved. 24. When not walking, the rack can be disengaged and the generator cranked by hand or by foot. Electrical powers of È3 W are achievable by hand, and higher wattage can be achieved by using the leg to power it. 25. R. Kram, J. Appl. Physiol. 71, 1119 (1991). 26. A. E. Minetti, J. Exp. Biol. 207, 1265 (2004). 27. A. A. Biewener, C. T. Farley, T. J. Roberts, M. Temaner, J. Appl. Physiol. 97, 2266 (2004). 28. T. M. Griffin, T. J. Roberts, R. Kram, J. Appl. Physiol. 95, 172 (2003). 29. This work was supported by NIH grants AR46125 and AR38404. Some aspects of the project were supported by Office of Naval Research grant N000140310568 and a grant from the University of Pennsylvania Research Foundation. The authors thank Q. Zhang, H. Hofmann, W. Megill, and A. Dunham for helpful discussions; R. Sprague, E. Maxwell, R. Essner, L. Gazit, M. Yuhas, and J. Milligan for helping with the experimentation; and F. Letterio for machining the backpacks.
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