Is the center of gravity controlled during upper trunk movements?

The question was addressed in this study as to whether the kinematic synergy responsible for equilibrium control during upper trunk movements may involve an actual evaluation of the weight of the body segments. Five adult subjects were asked to bend the upper trunk forward or backward to an angle of about 35 degrees, first without any load and then with a 10 kg load fixed to their shoulders. The center of gravity (CG) shift in the sagittal direction which occurred at the end of the movement was compared versus without a load. Two patterns of CG shift were identified. In the first pattern shown by three subjects in forward and three in backward trunk bending, the CG shift remained the same, while in the other pattern (two subjects for forward and two for backward trunk bending), the CG shift increased by an amount corresponding to the purely mechanical effect of the load. These results indicate that the actual weight of the segments may be evaluated on the basis of sensory inputs and may be responsible for a change in the kinematic synergy which preserves the CG control during the upper trunk movement.