BackgroundThe metabolic cost of locomotion is a key factor in walking and running performance. It has been studied by analysing the activation and co-activation of the muscles of the lower limbs. However, these measures do not comprehensively address muscle mechanics, in contrast to approaches using muscle moments and co-contraction. Research questionWhat is the effect of speed and type of locomotion on muscle moments and co-contraction, and their relationship with metabolic cost during walking and running? MethodsEleven recreational athletes (60.5 ± 7.1kg; 169.0 ± 6.6cm; 23.6 ± 3.3 years) walked and ran on a treadmill at different speeds, including a similar speed of 1.75m.s-1. Metabolic cost was estimated from gas exchange measurements. Muscle moments and co-contraction of ankle and knee flexors and extensors during the stance and swing phases were estimated using an electromyographic-driven model. ResultsBoth the slowest and fastest walking speeds had significantly higher metabolic costs than intermediate ones (p < 0.05). The metabolic cost of walking was correlated with plantarflexors moment during swing phase (r = 0.62 at 0.5m.s-1, r = 0.67 at 1,25m.s-1), dorsiflexors moment during stance phase (r = 0.65 at 1.25m.s-1, r = 0.67 at 1.5 and 1.75m.s-1), and ankle co-contraction during the stance phase (r = 0.63 at 1.25 and 1.75m.s-1). The metabolic cost of running at 3.25m.s-1 during the swing phase was correlated with the dorsiflexors moment (r = 0.63), plantarflexors moment (r = 0.61) and ankle co-contraction (r = 0.60). Discussion and conclusionFluctuations in metabolic cost of walking and running could be explained, at least in part, by increased ankle antagonist moments and co-contraction.