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Purpose: The metabolic cost of walking is greater in old compared to young adults. This study examines the relation between metabolic cost, muscular efficiency, and leg muscle co-activation during level and uphill walking in young and older adults., Procedures: Metabolic cost and leg muscle activation were measured in young (22.3 ± 3.6 years) and older adults (74.5 ± 2.9 years) walking on a treadmill at six different slopes (0.0-7.5% grade) and a speed of 1.3 ms(-1). Across the range of slopes, 'delta mechanical efficiency' of the muscular system and antagonist muscle co-activation were quantified., Main Findings: Across all slopes, older adults walked with a 13-17% greater metabolic cost, 12% lower efficiency, and 25% more leg muscle co-activation than young adults. Among older adults, co-activation was weakly correlated to metabolic cost (r=.233) and not correlated to the lower delta efficiency., Conclusion: Lower muscular efficiency and increased leg muscle co-activation contribute to the greater metabolic cost of uphill slope walking among older adults but are unrelated to one another., (Copyright © 2014 Elsevier Ltd. All rights reserved.)