Low back pain is a common disabling musculoskeletal disorder, whose prevention and treatment are problematic. The main reason is that current imaging techniques do not identify the source of pain in the vast majority of cases. The diagnosis of low back disorder is, therefore, often based on non-specific signs, such as deep tissue pain and altered motor patterns. Unfortunately, there is little agreement on how these patterns change with pain. While most authors agree that force generated by the muscles is usually diminished (the mechanism is unknown), electromyographic (EMG) recordings are equivocal -both hyper-and hypoactivity have been reported (Ahern et al. 1988). The contradictory results may reflect two major disadvantages connected with clinical studies. First, as a direct consequence of the difficulty of localizing the primary tissue damage, the clinical studies may involve heterogeneous populations of patients with different primary afflictions (affecting intervertebral disks, ligaments, facet joints, muscles, etc.). Second, in clinical studies there is no healthy 'norm' to which the patients could be compared. Data from the individual's pain-free history are rarely available and there is a certain inter-individual variability in motion patterns even within the healthy population. Uncontrolled factors such as these are responsible for the current poor understanding of the

It was concluded that the axial rotation is achieved through the activities of agonists, and return to neutral position is because of elastic recoil controlled by agonistic muscles. A range of approximately 10-15 degrees on either side of the anatomical midsagittal plane involves little muscle effort, but beyond this region, the osteoligamentous structures become stiff and require increasing effort to execute axial rotation.

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The authors investigated the emergence of independent control of body segments in bimanual tasks involving either voluntary or involuntary trunk motion by tracking the transition from an ego- to an exocentric mode of postural control during childhood (i.e., from body-referenced orientation to externally referenced action). A paradigm combining a seated manual task and various trunk manipulations described the coordination strategies used by 24 children at different ages (2 to 9 years) and by adults. The following questions were asked: (a) When do children begin to dissociate upper limb movements from those of the trunk? (b) What segmental strategies are exhibited by each age group (2-3, 4-6, and 7-9 years, and adults)? Kinematic analyses revealed that younger children (2-6 years) used either the trunk or the support surface as reference to orient the limbs. Older children (7-9 years) began to use a gravitational reference frame similar to that of adults; they uncoupled upper limb motion from the trunk in either voluntary or imposed conditions. Young children patterned the forearm trajectory after the initiating segment (support surface or the trunk), thus reducing the degrees of freedom during the dual task. Echoing previous reports, 7-9 years of age appears to be a critical period in which children master postural control and develop an internal representation of body scheme.

Phasic activity in the human back muscle erector spinae (ES) was studied during repetitive hand movements. The hand movements were elicited voluntarily by the subject or induced passively by the experimenter through a servomotor or through cyclical electrical stimulation of muscles acting about the wrist. The aim of the study was to determine whether the rhythmical activation of ES was of supraspinal, intersegmental or segmental origin. When voluntary rhythmical hand movements were performed as fast as possible, cyclical ES EMG bursts occurred at the same frequency. This frequency was significantly higher than that reached when the task was to contract the back muscles as rapidly as possible. This suggests that the ES activity during the fast hand movements was not generated by direct commands descending to the ES muscles from the motor area of the cerebral cortex responsible for voluntary back muscle activation. During imposed rhythmical hand movements, ES EMG bursts remained entrained to the hand movements, even when movement frequencies far exceeded those attainable voluntarily either for the hand or the back. This showed that ES EMG responses could be evoked by the hand movements even when these were not generated by descending neural commands. Two alternative mechanisms of ES activation were considered: (a) propriospinal transmission of afferent input entering the spinal cord from the upper extremity; (b) afferent input from ES and other trunk muscles, responding to local oscillations transmitted mechanically from the hand to the lower back. Activation of ES via proprioceptive signals from the forearm was unlikely since (a) simultaneous electrical stimulation of wrist extensor and wrist flexor muscles did not result in repetitive ES EMG bursting; (b) cyclical vibration of the wrist extensors did not evoke ES EMG bursting; (c) when the forearm was constrained and the hand was moved passively, the lower trunk accelerations and cyclical ES EMG both occurred at a harmonic of the hand movement frequency. We conclude that the repetitive ES EMG bursting during hand movements was probably due to a local segmental reflex rather than to descending commands. Remote mechanical oscillations of the trunk caused by hand movements evidently elicited proprioceptive reflexes in ES that presumably contributed to trunk stabilization.