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[Purpose] This study investigated the learning effects of repetitive perturbations on muscle synergies during automatic postural responses (APR) induced by unexpected horizontal surface translations. [Subjects] Nine healthy young subjects participated in this study. [Method] Each subject was instructed to maintain equilibrium without stepping after perturbations in the backward-forward direction under stable or unstable conditions. Each subject practiced 540 trials over a 6-day period. The displacements of seven markers used to calculate center of mass (COM) were captured using a motion analysis system and electromyogram signals were recorded at the same time. [Results] The peak velocity of COM during APR decreased with practice, and was accompanied by increases in muscle activities, namely "knee co-contraction" and "sway-forward" synergies in the backward-perturbed direction under the unstable condition. [Conclusion] The results suggest that the central nervous system enhances particular muscle synergies with practice, specifically the co-contraction of knee joints and the coordinated activity patterns of proximal muscles combined with distal muscles, during APR under mildly unstable conditions. [ABSTRACT FROM AUTHOR]