At the beginning of the 20th century, Charles Sher-rington put into place a conceptual framework concerning movement regulation and initiation that held sway for a considerable amount of time. The central component of this framework was the reflex—sensory input leading to the initiation of the movement and also providing for its ongoing control by means of feedback. The most studied of these reflexes was perhaps the stretch reflex, but a large variety of other postural reflexes involving somatic, vestibular, and even visual sensory afferents were also elaborated over the course of time. In this viewpoint, reflexes were not obligatory; instead, they could be gated and regulated by higher centers in the nervous system, thus providing for the idea of a hierarchical organization. Another important component of this framework was the concept of synergy—a group of muscles acting together in a common purpose in a particular movement and being controlled together as a unit.

In this conceptualization, limb motion from one posture to another involved two opposing groups of muscles, agonists and antagonists, each activated as a unit. Another important discovery, developed by Elwood Hennemann in the middle of the 20th century, provided an explanation for the orderly recruitment of individual motor units within the agonist and antagonist muscles. Specifically, according to Hennemann's size principle, motor units are recruited in a stereotypic fashion, according to their size. The pattern of muscle activation producing a limb movement was thought to be quite simple. There is an initial burst of agonist activity, arising from descending commands and providing for the movement's initiation. This is

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followed by a burst of antagonist activity and a second (smaller) burst of agonist activity, providing for the movement's arrest and stabilization and arising at least in part from the stretch reflex.

The primacy of reflexes in controlling movement was not acknowledged universally, however. For example, although some investigators initially thought that locomotion was the result of a chain of reflexes, it was ultimately demonstrated that the locomotor pattern of alternating, rhythmic activation of flexors and extensors of each of the limbs was the result of a central pattern generator. These investigations also showed that this central pattern generator for locomotion was located in the spinal cord and that rhythmic motion could be elicited from deafferented preparations (i.e., in the absence of sensory feedback).

Thus, there were two competing viewpoints. According to one, movements were regulated essentially and importantly by feedback mechanisms dependent on sensory information generated by the movement. A diametrically opposite position held that movements were accomplished open loop in a feedforward fashion, and that the delays in feedback loops were too long for afferent information generated during the movement to modify the ongoing trajectory.

The conceptual framework from which we view movement regulation has changed dramatically in the past 20 years, but feedback and feedforward regulation remain central to the issue. One major change is the recognition that movements cannot be controlled by feedback alone and that they also cannot be performed accurately using simply feedforward. Thus, movement regulation involves a hybrid of feedforward and feedback controllers.

Furthermore, our concept of what needs to be regulated by the central nervous system has become more sophisticated. Thus, it is now clear that the physical properties of the plant—the muscles and the skeletal system—introduce appreciable complexities. Similarly, movement regulation usually involves the synthesis of sensory information derived from different modalities, such as vision, somatosensation, and the vestibular system. Sensory information from these different modalities is combined into a common frame of reference, introducing additional computational burdens. Finally, there is a growing appreciation that the control system regulating movements is not "hardwired" but, rather, that it is subject to modification by experience and learning. We discuss each of these points in the following sections.

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