Stability Control of Grasping Objects With Different Locations of Center of Mass and Rotational Inertia

ABSTRACT

The objective of this study was to observe how the digits of the hand adjust to varying location of the center of mass (CoM) above or below the grasp and rotational inertia (RI) of a handheld object. Such manipulations do not immediately affect the equilibrium equations while stability control is affected. Participants were instructed to hold a handle, instrumented with 5 force–torque transducers and a 3-D rotational tilt sensor, while either the location of the CoM or the RI values were adjusted. On the whole, people use 2 mechanisms to adjust to the changed stability requirements; they increase the grip force and redistribute the total moment between the normal and tangential forces offsetting internal torques. The increase in grip force, an internal force, and offsetting internal torques allows for increases in joint and hand rotational apparent stiffness while not creating external forces–torques that would unbalance the equations of equilibrium.

Keywords:

• center of mass
• grasping
• motor control
• stability

ACKNOWLEDGMENTS

This work was in part supported by National Institutes of Health grants AG-018751, NS-035032, and AR-048563. The authors would like to thank Denny Ripka for mechanical assistance. They also appreciate the technical assistance provided by Jim Metzler. The authors thank the anonymous journal reviewers for valuable comments.