J Manipulative Physiol Ther 2005 (Jan); 28 (1): 68–72 ~ FULL TEXT
Howard Vernon, DC, PhD, John Mrozek, DC
In 1976, Sandoz  published an important article entitled “Some physical mechanisms and effects of spinal adjustments.” In this article, Sandoz published a figure which was meant to describe the nature of joint manipulation with respect to where, in the total arc of the motion of a joint, manipulation was proposed to take place. Sandoz's figure is shown in Fig 1. The figure was particularly effective in identifying several phases of a joint's total motion, starting with the active range, defined as the range capable of being voluntarily produced by a person with their own motor power. Sandoz postulated that a further movement could be produced passively, either by the person themselves or by an external agent (ie, therapist), where ‘passive’ implied the imposition of externally applied force.
The outer margins of the figure represent the anatomic limit of motion of a joint, beyond which injury would occur to any of the holding elements and, with severe enough force, to the bony elements themselves. Active and passive motions are clearly shown to be less than, that is, within, the anatomic limit.
Sandoz postulated the presence of a “paraphysiological space,” beyond the passive range, but less than the anatomic limit. It was “into” this space that he postulated that a manipulation occurred. This “space” was described by others as a “zone of end-play,”  the “barrier,”  or “the capsular pattern.”  All of these terms are based on the notion that, at the end of “normal motion,” there exists a zone of elasticity in the joint which can be decreased in a joint which has lost some of its flexibility. The clinical term for this state has, as well, been given various names by all the schools of manipulation. Chiropractors have used the terms “subluxation” and “fixation” : osteopaths use the term “somatic dysfunction”; medical and physiotherapeutic specialists use terms such as “dysfunction,” “barrier,” and “loss of end-play.” All of these terms contain the notion of “hypomobility.” We propose that the generic term for this problem is “joint dysfunction.”
This concept has been incorporated into the description of various palpatory procedures to assess joint motion. The palpatory experience which is proposed to match with normal joint motion is a feeling of smooth motion ending in a feeling of “play” or “spring” at the end of the passive range. Osteopaths used the term “ease” to describe this normal palpatory feel. An abnormal finding would be the feeling described in the term “blockage,” whereby the palpated motion is felt to stop before the expected end-range (perhaps as compared with the opposite side if it is healthy) and be accompanied by a “hard end-feel.” 
Before stating the problem alluded to in the introduction, it is timely to recognize that recent spinal biomechanics experts have introduced the term “neutral zone” to describe the zone within a joint's motion which produces little if no actual stress on the intrinsic tissues and within which minimal muscular activity is required for joint stabilization.  For example, Panjabi et al  determined that the neutral zone for C1-C2 rotation was approximately 28°, whereas the normal full active range is approximately 40°. The notion of an “elastic zone” has also been proposed  which is a zone in normal subjects which extends beyond the “neutral zone” and within which tissues undergo physiological levels of strain which increase but still remain less than sufficient to produce disruption or injury of tissues and within which higher levels of muscle recruitment occur for active stabilization. Klein et al  have described the situation of clinically restricted joint motion as “being stuck in the neutral zone.”
It is tempting to fit these 2 concepts into the model proposed by Sandoz. A rough equivalence might posit that the neutral zone lies within the “active” range, whereas the elastic zone might approximate the limit of the normal active zone.
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