Functional degrees of freedom.

Mechanical degrees of freedom (DOF) are defined as the minimum number of independent coordinates needed to describe a system's position. The human musculoskeletal system has many mechanical DOF through which countless movements are accomplished. In the motor control field, one of the aspirations is to understand how the many DOF are organized for movement execution-the so-called DOF problem. Natural movements are characterized by the coordination of the DOF such that few vary independently. The concept of functional degrees of freedom (fDOF) is introduced to describe the very limited DOF of purposeful, coordinated movements. Deterministic (i.e., constraint satisfaction) and statistical (i.e., principal component analysis) approaches are used to determine fDOF. In contrast to DOF as a mechanical descriptor, fDOF emphasizes the mechanisms of human movements and corroborates our search for the solution to the DOF problem.