We investigated co-varied changes in muscle activity during voluntary sway tasks that required a quick shift of the center of pressure (COP). We hypothesized that multi-muscle synergies (defined as task-specific covariation of elemental variables, muscle modes) stabilize a COP location in the anterior-posterior direction prior to a voluntary COP shift and that during the shift the synergies would weaken. Standing subjects performed two tasks, a cyclic COP shift over a range corresponding to 80% of the maximal amplitude of voluntary COP shift at 1 Hz and a unidirectional quick COP shift over the same nominal amplitude. The cyclic sway task was used to define muscle modes (M-modes, leg and trunk muscle groups with parallel scaling of muscle activation level within a group) and the relations between small changes in the magnitudes of M-modes [in the principal component analysis (PCA), the M-mode magnitudes are equivalent to PC scores] and COP shifts. A novel approach was used involving PCA applied to indices of muscle integrated activity measured both within a trial and across trials. The unidirectional sway task was performed in a self-paced (SP) manner and under a typical simple reaction time (RT) instruction. M-modes were also defined along trials at those tasks; they have been shown to be similar across tasks. Integrated indices of muscle activity in the SP-sway and RT-sway tasks were transformed into the M-modes. Variance in the M-mode space was partitioned into two components, one that did not affect the average value of COP shift (V (UCM)) and the other that did (V (ORT)). An index (DeltaV) corresponding to the normalized difference between V (UCM) and V (ORT) was computed. During steady-state posture, DeltaV was positive corresponding to most M-mode variance lying in a sub-space corresponding to a stable COP location across trials. Positive DeltaV values have been interpreted as reflecting a multi-M-mode synergy stabilizing the COP location. The magnitude of DeltaV was larger in SP trials than in RT trials. During voluntary COP shifts, the DeltaV magnitude dropped to zero or even became negative. We conclude that M-mode synergies stabilize COP location during quiet standing, while these synergies weaken or disappear during fast voluntary COP shifts. Under RT conditions, the COP stabilizing synergies were weaker supposedly to facilitate a quick COP shift without time for preparation. The suggested method of M-mode identification may potentially be applied to analysis of postural synergies in persons with impaired postural control such as elderly persons, persons with atypical development, or in the course of rehabilitation after an injury.