Degrees of freedom problem

In neuroscience and motor control , the degrees of freedom problem or motor equivalence problem states that there are multiple ways for humans or animals to perform a movement in order to achieve the same goal. In other words, under normal circumstances, no simple one-to-one correspondence exists between a motor problem (or task) and a motor solution to the problem. The motor equivalence problem was first formulated by the Russian neurophysiologist Nikolai Bernstein: "It is clear that the basic difficulties for co-ordination consist precisely in the extreme abundance of degrees of freedom, with which the [nervous] centre is not at first in a position to deal." Although the question of how the nervous system selects which particular degrees of freedom (DOFs) to use in a movement may be a problem to scientists, the abundance of DOFs is almost certainly an advantage to the mammalian and the invertebrate nervous systems. The human body has redundant anatomical DOFs (at muscles and joints), redundant kinematic DOFs (movements can have different trajectories, velocities, and accelerations and yet achieve the same goal), and redundant neurophysiological DOFs (multiple motoneurons synapsing on the same muscle, and vice versa). How the nervous system "chooses" a subset of these near-infinite DOFs is an overarching difficulty in understanding motor control and motor learning. The study of motor control historically breaks down into two broad areas: "Western" neurophysiological studies, and "Bernsteinian" functional analysis of movement. The latter has become predominant in motor control, as Bernstein's theories have held up well and are considered founding principles of the field as it exists today. In the latter 19th and early 20th centuries, many scientists believed that all motor control came from the spinal cord, as experiments with stimulation in frogs displayed patterned movement ("motor primitives"), and spinalized cats were shown to be able to walk.