Stability in force-production tasks

Exerting a force on a mechanical system can induce mechanical instability. To overcome that instability, humans may take advantage of their upper limb mechanical impedance (e.g., hand stiffness). The authors investigated what stiffness is required to maintain static stability and how humans can achi eve that stiffness in the context of the task of pushing on a pivoting stic k. Results showed that the stiffness required is in the range of measured h uman upper limb stiffness. To avoid an ill-posed problem, one can better ex press the requirements for stability as a simple geometrical criterion rela ted to the curvature of the potential energy field at the hand. A planar mo del of the upper limb revealed that individuals can use both hand rotationa l and translational stiffness to stabilize a stick. Although hand rotationa l stiffness does not participate in producing the axial force on the stick, it can significantly contribute to achieving a limb stiffness appropriate for maintaining static stability. Hand rotational stiffness can be importan t for the design of hand tools, because humans can increase it only by augm enting grip force, a biomechanical factor associated with cumulative trauma injuries of the upper extremities.