The effects of psychological pressure on lower limb muscular activity and center of pressure (COP) were investigated in a standing, postural control task. Healthy male participants (N=18) performed a balancing task by standing on a balance disk with their dominant foot. Participants were requested to stabilize their posture for 30 s (one-trial). After acquisition trials, participants performed 2 non-pressure and 2 pressure trials in counterbalanced order for a performance-contingent cash reward, or punishment. Stress responses were successfully induced as assessed by state anxiety, perceived pressure, mental effort, and heart rates that increased under pressure conditions. The results indicated that the rate of co-contraction between the soleus (SOL) and tibialis anterior (TA) muscles in the dominant leg increased significantly in association with an increment in the EMG amplitude of the SOL under pressure. Moreover, the COP area in pressure trials was significantly smaller than in non-pressure trials. These functional changes in postural control under pressure could have been modified by internal focus of attention, affective states including anxiety, and movement strategies that enhance muscle and joint stiffness in the lower limbs.