Mechanisms underpinning self-selected walking speed (SSWS) are poorly understood. The present study investigated the extent to which SSWS is related to metabolism, energy cost, and/or perceptual parameters during both normal and artificially constrained walking. Fourteen participants with no pathology affecting gait were tested under standard conditions. Subjects walked on a motorized treadmill at speeds derived from their SSWS as a continuous protocol. RPE scores (CR10) and expired air to calculate energy cost (J.kg^-1.m^-1) and carbohydrate (CHO) oxidation rate (J.kg^-1.min^-1) were collected during minutes 3-4 at each speed. Eight individuals were re-tested under the same conditions within one week with a hip and knee-brace to immobilize their right leg. Deflection in RPE scores (CR10) and CHO oxidation rate (J.kg^-1.min^-1) were not related to SSWS (five and three people had deflections in the defined range of SSWS in constrained and unconstrained conditions, respectively) (p > 0.05). Constrained walking elicited a higher energy cost (J.kg^-1.m^-1) and slower SSWS (p < 0.05) versus normal walking. RPE (CR10) was not significantly different between walking conditions or at SSWS (p > 0.05). SSWS did not occur at a minimum energy cost (J.kg^-1.m^-1) in either condition, however, the size of the minimum energy cost to SSWS disparity was the same (Froude {Fr} = 0.09) in both conditions (p = 0.36). Perceptions of exertion can modify walking patterns and therefore SSWS and metabolism/ energy cost are not directly related. Strategies which minimize perceived exertion may enable faster walking in people with altered gait as our findings indicate they should self-optimize to the same extent under different conditions.

• Minimum energy cost (J.kg^-1·min^-1) was closely associated with, but did not occur at, SSWS.

• Perceptual RPE (CR10) and CHO utilisation rate markers did not find a determinant for SSWS.

• People should self-optimise walking to the same extent under different conditions.