When balancing, instability can occur when the object being balanced moves at a rate that is beyond the abilities of human motor control. This illustrates that responsiveness of motor control is limited and can be investigated by changing the dynamics of the task. In this study, the responsiveness of trunk motor control was investigated by changing the seat stiffness of an unstable seat. At decreasing levels of seat stiffness the probability of successfully balancing on the seat, speed of the seat, speed of the trunk relative to the seat (trunk-seat) and muscle activation of five trunk muscles were assessed. Also, across the different stiffness levels, the relation between trunk muscle activation and seat speed was determined. As hypothesized, with decreasing seat stiffness the probability of success decreased, seat speed and trunk-seat speed increased, and both agonist and antagonist activation increased. This shows that limits in the responsiveness of trunk motor control were reached during seated balancing. Furthermore, in line with our hypothesis, a positive relation was found between trunk muscle activation and seat speed. It appears that the central nervous system regulates trunk stiffness (via muscle coactivation) in relation to the dynamics of the task, possibly to maintain adequate responsiveness.