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Postural control is influenced by cognition. In most studies, variability of motor output has been considered regardless of variability in patterns of joint coordination. Uncontrolled manifold framework has been applied to decompose the joint's variance in two components. The first component leaves position of the center of mass in anterior-posterior direction (CoMAP) unchanged (VUCM) while the second component is in charge of variations of CoM (VORT). In this study, 30 healthy young volunteers were recruited. The experimental protocol consisted of three random conditions: quiet standing on a narrow wooden block without a cognitive task (NB), quiet standing on a narrow wooden block with an easy cognitive task (NBE), and quiet standing on a narrow wooden block with a difficult cognitive task (NBD). Results showed that CoMAP sway in NB condition was higher than both NBE and NBD conditions (p =.001). VORT in NB condition was higher than NBE and NBD conditions (p =.003). VORT in NB condition was higher than NBE and NBD conditions (p =.003). VUCM was unchanged in all conditions (p = 1.00) and synergy index in NB condition was smaller than NBE and NBD conditions (p =.006). These results showed that postural synergies increased under dual-task conditions. [ABSTRACT FROM AUTHOR]