Purpose Intra-limb and muscular coordination during gait are the result of the organisation of the neuromuscular system, which have been widely studied on a flat terrain. Environmental factors, such as the inclination of the terrain, is a challenge for the postural control system to maintain balance. Therefore, we hypothesised that the central nervous system flexibly modifies its control strategies during locomotion on slopes. Methods Ten subjects walked on an inclined treadmill at different slopes (from − 9° to + 9°) and speeds (from 0.56 to 2.22 m s −1 ). Intra-limb coordination was investigated via the Continuous Relative Phase, whereas muscular coordination was investigated by decomposing the coordinated muscle activation profiles into Basic Activation Patterns. Results A greater stride to stride variability of kinematics was observed during walking on slopes, as compared to walking on the level. On positive slopes, the stride period and width present a greater variability without modification of the timepattern of the muscular activation and of the variability of intersegmental coordination. On negative slopes, the stride width is larger, the variability of the stride period and of the inter-segmental coordination is greater and the basic activation patterns become broader, especially at slow speeds. Conclusion Our findings suggest that the control strategy of downhill walking corresponds to a more conservative gait pattern, which could be adopted to lower the risk of falling at the cost of a greater energy consumption. In uphill walking, where metabolic demands are high, the strategy adopted may be planned to minimise energy expenditure. Keywords Coordination • Neuromuscular control • Basic activation pattern • Continuous relative phase Abbreviations CRP Continuous relative phase GRF Ground reaction force EMG Records of the electrical activity produced by skeletal muscles TO Toe off COP Centre of pressure FC Foot contact F x , F y , F z Lateral, fore-aft and vertical component of the GRF d y , d x Lateral and fore-aft position of the COP ω i , θ i Lower-limb segment angular velocity and position φ iN Phase angle ϕ iN CRP angle MARP Mean absolute relative phase DP Deviation phase of the CRP FWHM Full width at half maximum CI Co-activation index NNMF Non-negative matrix factorisation P, W Basic activation patterns and associated weighting components VAF Variability accounted for

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As the largest extant legged animals, elephants arguably face the most extreme challenge for stable standing. In this study, we investigated the displacement of the centre of pressure of 12 elephants during quiet standing. We found that the average amplitude of the oscillations in the lateral and fore–aft directions was less than 1.5 cm. Such amplitudes for postural oscillation are comparable with those of dogs and other species, suggesting that some aspects of sensorimotor postural control do not scale with size.

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