Recent advances in the neuroscience of movement organisation are challenging and expanding some of the traditional models of human movement organisation. The models of fixed, hardwired neural networks that can be defined by anatomical connectivity are being replaced by more dynamic, modifiable models generated and maintained by the CNS that can only be properly defined when functional connectivity is considered (Selverston 1992).

Since Sherrington (1906) defined the stretch reflex concept for movement and posture organisation, a simple reflex structural model for movement and posture has prevailed (Lackner 1988; Woollacott and Shumway-Cook 1990; Baev and Shimansky 1992). Many recent studies on the central nervous system control of movement and posture question this reflex concept and instead propose more dynamic models for posture and movement organisation (Lackner 1988; Gurfinkel et al 1988; Lacquaniti et al 1990). A central representation of body biomechanical characteristics or a BODY SCHEME including the length of limb segments, the sequence of their linkage, the position of limb segments in space and the shape of the body's surface has been proposed (Clement et al 1984; Lackner 1988; Gurfinkel et al 1988; Roll and Roll 1988).

These parameters do not have any specific individual sensory modality but are generated from the transformation of sensory information from virtually all receptor systems of the body (DiZio and Lackner 1986; Gurfinkel et al 1988). A postural control system organised on the basis of an internal model operating with highly integrated information from many sensory systems could possess a much wider range of functional possibilities than a system based on the interactions between single level reflexes (Gurfinkel et al 1988).

The illusory and real effects produced by tendon vibration (Gurfinkel et al 1988; Lackner 1988; Roll and Roll 1988), altered gravity conditions (Clement et al 1984) and cat responses to platform movements (Lacquaniti et al 1990) has lead to this proposal that the CNS of humans and vertebrates generates an internal representation of body geometry. Lacquaniti et al proposed that posture in the cat is maintained by the control of a preferred silhouette image of the geometrical configuration of the limbs rather than by the regulation of the projection of the centre of gravity onto the support surface. This body scheme is thought to be largely an unconscious model of body biomechanics (Gurfinkel et al 1988). Lackner (1988) induced changes in perceived limb length and trunk contour by the manipulation of sensory input and suggested that the body scheme is maintained as a dynamic organisation and that is potentially modifiable.

Feldenkrais (1972) maintained that this body scheme can be brought into active awareness and significantly modified by learning . Movement lessons (Awareness Through Movement) were specifically designed by Feldenkrais to enhance the brain's body scheme. Attention to body segment (eg limb, spinal segment) length, movement, interactions, relation to gravity, timing, ease vs effort during gentle movement sequences improves coordination, control and awareness of movement. If there is pain at one point, this is significantly "appeased" (and not cured or fixed) by improving local and global movement control.

Currently in the diagnosis and treatment of low back pain, for example, attention is primarily focussed to the area of pathology (Jayson 1992). Local motion segment (one joint) biomechanics and muscle strength are the focus. A "movement diagnosis" based on global functional parameters related to the body scheme (eg motor control of the trunk, ie pelvis, spine and head relations and functions), as well as the local movement issues could greatly improve diagnosis and management of low back pain.

Bernstein (1967) proposed that the CNS must create a "motor image" for the performance of movement. This image represented the form of the movement to be achieved, not the temporal sequences of neural impulses that produces it. Berthoz (200) called it a "blue print" for action. That is the movement is stored as an image of the form of the movement and not as a transformation into neural impulses. He also assumed that common actions, like walking, reaching, running, and throwing were organised as synergies to reduce the number of degrees of freedom for the CNS to store and effect. Hence in these actions only a small part of the CNS sets the motion in action.

Similarly, Reed (1988) proposed that animals and people do not move by contracting muscles or displacing their limbs but by coordinating subsidiary actions. The organism controls movements and postures in space rather than displacements in space-time. Muscular contractions and limb displacements are a consequence of intention and action rather than the operating elements of the CNS.

In decerebrate cats, stimulation of the mid brain evoked an entire normal gait pattern (Shik and Orlovsky 1976). Activation of the entire locomotor pattern could be turned on and off like a switch. This means that the neural network is tuned to a complete global motor function and not to individual motor elements such as muscles. Since then single cell and multiple cell central pattern generators for many rhythmical behaviours like locomotion, chewing, scratching, singing, posture , swimming, jumping, feeding and flight have been identified in many vertebrates and invertebrates (Pearson 1993). The neuronal network for the Tritonia escape swim system is shown in figure 1. With activation of the C2 cell, the I cell is inhibited and the DSI cells become excitatory resulting in swimming (Getting 1989). Inhibition of C2 results in the DSI cells functionally inhibiting each other via the I cell resulting in no swimming.

CPG'S have been shown to be enormously modifiable by neuromodulation (mainly amines and peptides) (Selverston 1992; Pearson 1993). Neuromodulators can facilitate, depress, initiate, modify and completely reorganise the motor behaviour of the CPG .

These regions of the brain for motor control are homologous from reptiles to primates suggesting phylogenetically old and unchanged systems (Grillner & Wallen 1985). Do CPG'S for locomotion, breathing and other rhythmical behaviours exist in human brains? Are they as functionally re-organisable as animal CPG'S? What role might they play in musculoskeletal disorders?