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PURPOSE: To investigate effects that independent alterations in limb mass and moment of inertia about a transverse axis through the hip have on metabolic and mechanical power of walking and peak electromyography (EMG) amplitude. It was hypothesized that increases in metabolic cost would parallel increases in mechanical power, and that EMG amplitude would increase with greater limb mass or limb moment of inertia. METHODS: Metabolic and mechanical power and lower-extremity EMG were measured on 14 healthy adults walking at 1.5 m x s. Four leg-loading conditions were employed: 1) no load (NL) on the legs; 2) a baseline load (BSLN) condition, with a mean of 2.0 kg per leg distributed on the proximal and distal shank; 3) a load condition with a mean of 2.0 kg per leg distributed on the proximal and distal shank, such that lower-extremity moment of inertia was increased 5% about the hip (MOI5) from the BSLN, but having the same lower-extremity mass as BSLN; and 4) a load condition with a mean of 2.8 kg per leg, concentrated proximally on the shank to increase total lower-extremity mass by 5% (Mass5) from BSLN, but having the same moment of inertia as BSLN. Total subject mass was constant between conditions, as unused leg loads were carried in a waist belt. RESULTS: Changes in mechanical power paralleled changes in metabolic cost as hypothesized. Energy cost increased significantly (4.2%) from NL to BSLN, and from BSLN to MOI5 and Mass5 (3.4 and 4.0%, respectively). EMG did not effectively explain changes in metabolic cost. CONCLUSION: Independent alterations in limb mass and moment of inertia about the hip joint influence energy cost similarly. [ABSTRACT FROM AUTHOR]