Relationship between metabolic cost, muscle moments and co-contraction during walking and running

Background: The 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 question: What 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? Methods: Eleven recreational athletes (60.5 +/- 7.1 kg; 169.0 +/- 6.6 cm; 23.6 +/- 3.3 years) walked and ran on a treadmill at different speeds, including a similar speed of 1.75 m.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. Results: Both 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 0.5 m.s(-1), r = 0.67 at 1,25 m.s(-1)), dorsiflexors moment during stance phase (r = 0.65 at 1.25 m.s(-1), r = 0.67 at 1.5 and 1.75 m.s(-1)), and ankle co-contraction during the stance phase (r = 0.63 at 1.25 and 1.75 m.s(-1)). The metabolic cost of running at 3.25 m.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 conclusion: Fluctuations in metabolic cost of walking and running could be explained, at least in part, by increased ankle antagonist moments and co-contraction.