Muscular Insight from the Scoliosis


The muscles of the spine are exquisitely orchestrated to act on the vertebral column to adapt.  There lies a wealth of information playing among the postural muscles as they attempt to adjust the spine to the demands of the environment.  The most prominent influence on involuntary muscle function is the stretch reflex.  This mechanism of muscular control permeates all the activities of skeletal motor function.  It is a reliable expression of integrated motor and sensory demands.  In our efforts to understand tensions and contractions of the skeletal muscles, we must understand the need for such an encompassing mechanism. To understand the action of the spinal muscles and their relationship to the scoliosis, we must first investigate the origin of the lateral curves. Haderspeck explains,
 

"The study findings suggest that as the progression of idiopathic scoliosis results from trunk neuromuscular system malfunctions, the malfunctions are more likely to be in the neural systems that control trunk muscle contractions and body weight support strategies than in the functional capabilities of the muscles themselves."1

Reuber ads that,

"Thus when the motor control system fails to provide appropriate amounts of lateral asymmetry in muscle contraction forces, the intervertebral discs within a lateral curve are subjected to nearly constant moments tending to tilt them further laterally and so to increase the scoliosis curve."2
 
and,

"Scoliosis progression seems not to be caused by asymmetry in muscle contractions; rather it may be caused by a lack of adequate asymmetry."2

Muscular influence on the scoliosis is a necessity, says Reuber

"An important implication of the analysis is that lateral asymmetry in trunk muscle contraction forces probably is a necessary consequence of the presence of a lateral curve in the spine.  We will argue this semi-quantitatively.  The Computer Tomography data obtained by Aaro and Dahlborn (1981) in patients with scoliosis showed that, corresponding to a curve of 40 degrees there is a lateral offset of the apical vertebra that usually is at least 1 cm.

". . . increased convex side muscle contraction force, and hence myoelectric activity, is biomechanically necessary if the lateral bending moments on the spine are to be balanced by the muscles in the presence of lateral offsets of the vertebrae within a curve.

"This lateral offset alters trunk mechanics in at least two ways. First, it produces a lateral bending moment on the vertebrae within the curve due to the weight of the body segments above the apex.  A reasonable figure for the magnitude of the weight involved is 200N. Thus this lateral bending moment is on the order of 200N times 0.01m, or 2Nm in a patient with a 1 cm apical vertebra lateral offset.

"To balance lateral moments on the spine in upright positions of the trunk, this 2Nm moment must be compensated for.  This compensation usually would be accomplished through muscular contraction.  If the muscles that compensate for it lie 5 cm lateral of the spine (supposed, for example, that the motor control system selects primarily the erector spinae to compensate), then in relaxed standing a convex side contraction of about 40N is required to compensate,  while the concave side muscles remain at rest.  In response to a l5Nm flexion-resist moment, for example, a normal subject with posterior back muscles lying 5 cm posterior to the spine, would have to contract these muscles with a force of 150Nm each side.  A patient who must balance this flexion moment and at the same time balance to 2Nm body-weight-offset moment through muscular contractions would have to develop 130N contraction force in her concave side muscles and 170N contraction force in her convex side muscles.  If lateral moments are to be balanced by the muscles, the lateral offset of the spine demands that muscle contraction force be laterally asymmetric."2

At rest, "normal" muscle activity virtually ceases (except for the presence of tone).  With the addition of gravity while sitting, standing and during movement, the postural muscles are recquired to maintain the spine and body weight in an erect or equilibrated posture.  For the scoliotic spine this adids increased demands on the side of the convexity.  This is expressed as a sustained increase in muscular contraction. This effort is apparently not sufficient during the progression of the scoliosis.  Haderspeck states,

". . . trunk muscles do have the inherent ability to correct the lateral spine curves, but natural neural signals directing them to do so apparently are deficient."1

Stretch may play an important role in postural muscles, to meet the demands of imbalance as Portnoy has concluded,

"Apparently, minor displacements of the center of gravity are responsible for these variations and our observations show that the sacrospinalis responds promptly to these slight changes in the center of gravity.

"The discordant reports on the constancy of function of postural muscles in the standing at ease position can be explained on this basis.

"During leaning forward, the sacrospinalis, hamstrings and gastrocnemii show a progressive increase in activity similar to that seen when a muscle undergoes isometric tension.  Since during this motion there is little lengthening of these muscles, isometric tension is truly approximated, and it appears that stretch is a powerful activator of these postural muscles.  The relative importance of stretch is also apparent on the records obtained from sacrospinalis during lateral flexion of the trunk in which the sacrospinalis of the extended side is ore active than that of the flexed side."3

Zuk found greater electrical activity on the convex side of the curvature and makes some startling assertions,

"It is therefore concluded that the amplitude and frequency of potentials reflect the strain to which a muscle is subjected, rather than the amount of work it performs.  In other words, a weak muscle lifting a certain load, will show greater electrical activity than a strong muscle lifting a bigger load.  It is suggested that the increased electrical activity is therefore a sign that is reaching the limit of its power and may be a sign of weakness rather than strength, because strong muscles will need to use only a few of their fibers to perform a given task but weak muscles will have to use more of their fibers and the contractions will have to be more frequent.  In other words, greater electrical activity in muscles on the convex side of the scoliosis does not prove that these muscles are stronger unless the conditions under which they are working are taken into consideration.

"It is concluded that the development of scoliosis is due to muscle imbalance, the weaker muscles being on the convex side of the curve.  It is suggested that the increased electrical activity in the muscles on the convex side is secondary to the scoliosis, being part of the body’s attempt to compensate for the curvature."4

Dupuis described the structures of the spine as "stablizers" and placed them into one of four categories:

"Passive Stabilizers"
Passive stabilization is provided by the shape and size of vertebrae and by the size, shape and  orientation of facet joints that link them.

"Dynamic Stabilizers"
Dynamic stabilization is provided by viscoelastic structures, such as the ligaments, capsules and annulus fibrosus.  The cartilage of the facet joints also acts as a damper.

"Active Stabilizers"
Active voluntary or reflex stabilization is provided by the muscular system that governs the lumbar spine, such as the major motors (psoas, quadratum lumborum, erector spinae, and abdominal wall muscles) and the postural muscles (multifidii, interspinosii, intertransversii, and rotatores).

"Hydrodynamic Stabilizers"
Hydrodynamic stabilization is provided by the turgor of the nucleous pulposus."5

In the erect posture of the average human skeleton, the center of gravity is such that most of the weight bearing is accomplished by the skeletal structures themselves with very little expenditure of muscular energy.  Contrastingly, the curvature of the scoliotic spine is not balanced solely by the skeletal structures, but requires the additional energy expended by the postural muscles.  The adaptation of the skeleton, including bone deformation, does not efficiently distribute weight bearing and therefore depends on the active resistance of the spinal muscles. Evidently, the scoliosis progresses without adequate neural feedback to halt or reduce it.  Many are implicating a neural or proprioceptive problem but cannot pinpoint the origin of such. We may speculate about the contribution that vertebral subluxations can make to short circuit the nervous system, and there is mounting clinical evidence that points to the vertebral subluxation.  But the mechanism involved may be a biomechanical compensation to the vertebral subluxation rather than the obvious.  In the vast majority of people, idiopathic scoliosis begins without symptomatic spasms and appears to begin without resistance to the progression of curvature.  It is apparent at this time in research that the muscles of postural stabilization are active and responsive to proprioceptive demands of the scoliosis, but are "unwilling" to stop curve progression until some antecedent need is fulfilled.  It is assumed that the apparent "neural deficiency"
for musculoskeletal controls associated with idiopathic scoliosis is a primary causative factor.  But the neural deficit may be "intentional" as the nerve system sees a need for structural adaptation.  The primary insult being obscure.  In the theory proposed by White and Panjabi, it is summarized by this statement,

"The precipitating condition may be an abnormal or malaligned facet, a discreet traumatic episode, . . . or any number of other possible embarrassments that upset the delicate balance."6

We do know that the curves begin without adequate resistance from muscles. Reuber explains,

"Except when experiencing significant acceleration, the trunk, like all other physical bodies, must be in mechanical equilibrium. The net result bending moment acting on any cross-section must then be zero, so that something must balance the bending moment created by the offset weight.  We assume that normally this lateral moment is balanced totally by lateral asymmetry in trunk muscle contraction forces. If the trunk muscle contraction forces are not sufficiently asymmetric, then at least a portion of the balancing moments must be provided by passive resistance of the soft tissues of the motion segments contained within the scoliosis curve. Those motion segments will increase their lateral tilts in order to provide the lateral moment resistance necessary for equilibrium."2

The hypomobility of spinal misalignment may be disruptive to the constant neurological afferentation of the C.N.S. which normally occurs under gravity. Deafferentation as a contributing factor, due to aberant mechanreceptive firing is not hard to imagine.

Haderspeck simplifies it,

"From a biomechanical viewpoint, a scoliosis may tend to get worse because the spine is too slender, because the soft tissues of the spine allow excessive lateral flexibility, or because abnormal loads continuously act on that spine.

"The studies show that postural offsets of upper body segment weights of several different kinds can promote lateral curve increases of several different natures."1

and,

"Application of the weights of upper body segments to a laterally curved spine can cause significant curve increases.  The amounts of these increases depend on the initial spine configuration and the nature of trunk righting responses to the weight application."1

The expected response is summed up in this statement,

"The neural mechanisms controlling trunk posture must sense the existence of any non-zero moments on the spine and signal the trunk muscles accordingly.  The trunk muscles must then contract appropriately to bring the spine lateral moments back to zero."1

There are instances where a muscle may cause curvature of the spine.  Acute muscle spasm can accompany subluxation of a vertebra.  If this is within the large postural muscles, then it may begin a lateral curve in the spine.  This can only occur when the "body" engages muscles for the correction of the vertebral subluxation (the primary target), that by their anatomical relationship, inadvertently effect the curves.  This is secondary to the vertebral subluxation and certainly does not account for the majority of muscular contractions associated with scoliosis.  As implied by Triano and Luttges,

"If the CG is displaced, either as a result of a muscle action or structural alteration, a change in magnitude of the moment generated about the lumbar spinal joints will result.  A concurrent adjustment in shunt muscle action from paraspinal and other trunk muscles will then be required to maintain equilibrium."7

And Haderspeck has shown that,

"Contractions of the muscle actions needed to maintain the trunk upright significantly alter the effects of an initial trunk muscle contraction "1

Studies by Yarom conclude that,

"Alterations due to cause and those due to effect are too interwoven and complex to allow a clear picture or meaningful differences to emerge from examinations of spinal muscles after full development of the deformity."8

There are parallels joining the structural and neuromuscular requirements of the scoliosis and the vertebral subluxation.  One such parallel is the necessity for the stabilization of the scoliosis in postural performance as compared to the constant provocation (to the nervous system) for correction of the vertebral subluxation.  These needs are demonstrated by the difficulty with which the body adapts to normal demands under these two conditions.  The muscles seem to attempt to "balance" these conditions.  We see reflex muscular contractions stabilizing the scoliotic spine and similarly we find vigilant muscular tension associated with the vertebral subluxation.

There is a basic assumption that has been made by most researchers and that is the idea that the function of the spinal musculature operates only to influence the posture and general condition of the spine, and is not designed to affect the vertebral motion segment in any specific fashion.  This has been an unspoken and clearly unchallenged assumption in that there has been almost no reference to particular muscle action in the literature.  But the need for selective muscle action is seen in the independent position and fixation of a vertebra.  Klausen suggests that,

"From a discussion of the mechanical effect of gravity on the individual joints, and on the spine as a whole, it is concluded that the short, deep muscles of the back must play an important part in stabilizing the individual joints, and that the long back muscles or the abdominal muscles are responsible for the stabilization of the spine as a whole."9

Donisch suggests the same when he states that,

". . . the transversospinal muscles adjust the motion between individual vertebrae.  The experimental evidence confirms the anatomical hypothesis that the multifidi are stabilizers rather than prime movers of the whole vertebral column.

"These adjust small movements between individual vertebra while movements of the vertebral column probably are performed by muscles with a better leverage and mechanical advantage."10

The mechanism here is also reflex in nature, as noted by Grieve:

"Dee, in a well-referenced summary, has reviewed the structure and function of joint innervation, and makes the suggestion that articular reflexes may have an important role in maintaining articular integrity - ‘possibly distributing the load in a suitable way and not merely acting to resist gross forces that may cause subluxation or dislocation, although this is doubtless, an important function.’"11

When describing the "osteopathic lesion", which is analogous to the vertebral subluxation, Denslow  explains,

"Reflex muscle activity, similar to stretch or postural reflexes, is seen in areas of ‘lesion’ in the erector spinae muscles when the subject is relaxed, the extremities in symmetrical position, and the head and face in the midline.  Adjacent normal areas do not show motor unit activity.

"The action potential findings in the lesion area have characteristics identical with the stretch reflex in their response to various tensions and to head position.  However, they were observed when the subject was completely relaxed, in the ‘resting’ period of the respiratory cycle and with the head in the midline.  The activity occurred in local areas where palpation revealed apparent abnormality in the tissue.

"Normal relaxed muscle is soft and resilient; it is not tender to moderate pressure.  In the thoracic region the, extensor muscle masses can be palpated as columns or segments which become firmer on voluntary contraction.  Muscle in a ‘lesion’ segment is abnormally firm; it is resilient and has a characteristic which simulates local spasm or contraction even in the relaxed subject.  The muscle columns feel indurated and ropey and roll under the fingers.  There is abnormal tenderness and deep pressure which may cause considerable pain.  Such lesion areas are frequently unilateral and limited to one or two spinal segments.

"That this activity was local, and not merely a part of a generalized neuromuscular hypertonus is shown by the absence of activity in an adjacent normal area observed simultaneously."12

It is elementary that the function of the spinal musculature is under the healthful influence of the nervous system.  Although complicating factors may add muscular activity without obvious purpose or productivity, it is unequivocal that skeletal muscles reflect the deliberate intentions of the nervous system.