This paper focused on the relationship between trunk stiffness and postural control during unstable seated balancing. We hypothesized that an increase in trunk stiffness would degrade postural control, and further hypothesized that signal dependent noise (SDN), resulting in increased muscle force variability, was responsible for this impairment. Ten subjects balanced on an unstable seat during four randomized conditions: normal balancing (control condition), trunk muscle co-activation (active stiffness), arm muscle co-activation (attention control), and belt (passive stiffness). Center of pressure (CoP) and EMG data were collected during three 20 s trials. Postural control was quantified by CoP velocity (total path divided by sample time in seconds). Trunk muscle co-activation resulted in significantly higher CoP velocity than the control (P < 0.001) and arm co-activation (P < 0.001) conditions. EMG data confirmed that the trunk co-activation condition had significantly higher muscle activity than the control (P = 0.001) and arm co-activation (P = 0.001) conditions. The belt condition, which increases passive trunk stiffness, showed no degraded postural control, but interestingly produced slightly lower levels of trunk muscle activity than the control condition (P < 0.001). Increased active trunk stiffness from muscle co-activation degraded postural control. Since the arm co-activation condition showed no impairment, attention demands cannot explain this result. Furthermore, since passive trunk stiffness from wearing a belt did not affect performance, it is believed that SDN from increased trunk muscle recruitment, and not an altered postural control strategy from increased joint stiffness, was responsible for the impairment.