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© 2018 Sims, Onambélé-Pearson, Burden, Payton and Morse. The disproportionate body mass and leg length of Achondroplasic individuals may affect their net oxygen consumption (VO 2 ) and metabolic cost (C) when walking at running compared to those of average stature (controls). The aim of this study was to measure submaximal VO 2 and C during a range of set walking speeds (SWS; 0.56-1.94 m·s-1, increment 0.28 m·s -1 ), set running speeds (SRS; 1.67-3.33 m·s -1 , increment 0.28 m·s -1 ) and a self-selected walking speed (SSW). VO 2 and C was scaled to total body mass (TBM) and fat free mass (FFM) while gait speed was scaled to leg length using Froude's number (Fr). Achondroplasic VO 2FFM x and VO 2FFM were on average 29 and 35% greater during SWS (P < 0.05) and 12 and 18% higher during SRS (P < 0.05) than controls, respectively. Achondroplasic CTBM and CFFM were 29 and 33% greater during SWS (P < 0.05) and 12 and 18% greater during SRS (P < 0.05) than controls, respectively. There was no difference in SSW VO 2TBM or VO 2FFM between groups (P > 0.05), but CTBM and CFFM at SSW were 23 and 29% higher (P < 0.05) in the Achondroplasic group compared to controls, respectively. VO 2TBM and VO 2FFM correlated with Fr for both groups (r = 0.984-0.999, P < 0.05). Leg length accounted for the majority of the higher VO 2TBM and VO 2FFM in the Achondroplasic group, but further work is required to explain the higher Achondroplasic CTBM and CFFM at all speeds compared to controls. New and Noteworthy: There is a leftward shift of oxygen consumption scaled to total body mass and fat free mass in Achondroplasic adults when walking and running. This is nullified when talking into account leg length. However, despite these scalars, Achondroplasic individuals have a higher walking and metabolic cost compared to age matched non-Achondroplasic individuals, suggesting biomechanical differences between the groups.