The present study focuses on the organization of kinematic synergy and its adaptation to an unstable support surface during upper trunk movements in aging adults. Seven healthy aging adults (49–66 years old) were instructed to bend the trunk forward (the head and the trunk together) by about 40° and to stabilize their final position, in the standard condition (both feet on the ground), and on a seesaw swinging in the sagittal plane. Kinematic synergy was quantified by performing a principal components analysis on the hip, knee and ankle angle changes during the movement. The results indicate that trunk bending was represented by a single component (PC1) in both conditions, indicating a strong coupling between the angle changes during the movement. The results also show a reorganization of the contribution of PC1 to the three angles when the balance constraints are increased in the seesaw condition. It is concluded that kinematic synergy is preserved during trunk bending in aging adults, regardless of the support conditions. It can also be adapted when the balance constraints are increased by changing the ratio between the angles, indicating a modification of interjoint coordination without modifying the movement's trajectory.