Although humans clearly expend more energy to walk with an extra load, it is unclear what biomechanical mechanism explains contribute to that increase. One possible contribution is the mechanical work performed on the body center of mass (COM), which simple models predict should increase linearly with added mass. The work should be performed primarily by the lower extremity joints, although in unknown distribution, and cost a proportionate amount of metabolic energy. We therefore tested normal adults (N=8) walking at constant speed (1.25 m/s) with varying backpack loads up to about 40% of body weight. We measured mechanical work (both performed on the COM and joint work from inverse dynamics), as well as metabolic energy expenditure through respirometry. Both measures of work were found to increase approximately linearly with carried load, with COM work rate increasing by about 1.40 W for each 1 kg of additional load. The joints all contributed work, but the greatest increase in positive work was attributable to the ankle during push-off (about 45 - 60% of stride time), and the knee in the rebound after collision (12 - 30% stride). The hip performed increasing amounts of negative work, near the end of stance. Rate of metabolic energy expenditure also increased approximately linearly with load, by about 7.6 W for each 1 kg of additional load. The ratio of the increases in work and metabolic cost yielded a relatively constant efficiency of about 16%. The metabolic cost not explained by work appeared to be relatively constant with load and did not exhibit a particular trend. Most of the increasing cost for carrying a load appears to be explained by positive mechanical work, especially about ankle and knee, with both work and metabolic cost increasing nearly linearly with added mass.