The neutral spine principle. 중립지대, 중립자세에 대한 정의와 개념

[문형철]

정확한 정의와 개념은 엄밀한 사고의 기초다.

The neutral spine principle.pdf

In any aspect of life to have principles can aid in the simplification of complex scenarios. All too often, principles can be easily mislaid when the detail of a situation becomes consuming. Such micromanagement, whilst in itself not problematic, in the absence of principles

becomes extremely confusing; the outcome commonly being paralysis by analysis. 

Some examples of useful (though not universally accepted) principles in bodywork and movement therapies could include the SAID principle (Baechle and Earle, 2000; Chek, 2001), the principle of movement emanating from the core (Gracovetsky, 1988; Chek, 2001; Richardson et al., 2004), the form principle (Baechle and Earle, 2000; Chek, 2001), the principle of structure function inter-relationship (Ward, 1997), the principle of balance or the Yin-Yang principle (Hicks et al., 2004), or the topic of this editorial, the neutral spine principle (Baechle and Earle, 2000; Chek, 2001; Lee, 2004; McGill, 2002, 2007).

What exactly is a principle?

The word ‘‘principle’’ (according to http://www. thefreedictionary.com/principle) may be defined as: 

(1) A basic truth, law, or assumption

(2) A basic or essential quality or element determining intrinsic nature or characteristic behavior

(3) A rule or law concerning the functioning of natural phenomena or mechanical processes

Like any assumption, a principle should be tested as far as it allows. There are various ways to test a principle. It can be isolated and tested in isolation; how many people with non-neutral curves have back pain, versus how many with neutral curves, and how many from each group are in pain. Alternatively, it can be tested in a real-world environment with multiple other interacting factors. Either of these environments may or may not reveal the truth or the falsehood of the assumption that maintaining a neutral spinal position is optimal; in which case, the only thing that can serve us is the experience of using it. Ultimately, it may be worth considering that unless a better principle replaces the principle under scrutiny, that principle remains in the ascendency. 

What exactly is neutral?

Neutral literally means unpolarised. When the spine moves into flexion it is moving out of neutral, when it moves into extension, it moves out of neutral. Indeed any ‘‘motion vector’’ which moves the spine away from its optimal postural position could be considered a non-neutral spine. In this way, it might be easier to classify what neutral isn’t rather than what it is!

- 중립은 문헌적으로 극성이 없음을 의미. 척추가 굴곡할때, 그것은 중립에서 벗어남, 척추가 신전할때 중립을 벗어남. 

Where exactly is neutral?

As far as any joint is concerned, the neutral position may be defined as one in which the joints and surrounding passive tissues are in elastic equilibrium and thus at an angle of minimal joint load (McGill, 2007). Other factors that may be considered as part of the definition include: the holding of a position in space in which translation of load is optimal through the structures of weight bearing, and/or where the length-tension relationships about the motion segment(s) are balanced, and/or where the optimal instantaneous axis of rotation can be maintained within the motion segment(s). Describing neutral provides a similar challenge to defining posture; ‘‘the position from which movement begins and ends’’ being as good as any. So this is why Panjabi (1992) and others have opted to describe a ‘‘neutral zone’’.

- 어떤 관절이 관여함에 따라, 중립위치는 관절과 주위 수동조직이 탄성평형과 최소관절부하의 각로 정의될 수 있음. 

The neutral zone 

The neutral zone can be defined as a small range of movement near the joint’s neutral position where minimal resistance is given by the osteo-ligamentous structures (Lee, 2004). In other words, it is a constantly moving position of a living joint, which is characterized by creating the least possible stress to the surrounding passive subsystem of that joint.

- 중립존은 관절의 중립위치의 근처에서 움직임의 작은 범위로 정의함. 

Out of neutral

What happens if the joint is out of neutral? In the first instance, very little should ‘‘happen’’; simply the tissues on one side of the joint will be in a relatively shortened/compressively loaded position, while the tissues on the opposite side of the joint will be in a relatively lengthened, distractively loaded position.

- 만약 관절이 중립위치를 벗어나면 어떤 일이 일어날까? ...

Under natural conditions, the result of this is that the nervous system is made aware of this imbalance via the type 1 mechanoreceptors, which communicate directly with the tonic (inner unit) musculature around the joint and encourage a return toward neutral. However, under not-sonatural conditions of forced concentration in front of a computer screen, or at a desk, deadlines, targets, or under the influence of social or pharmaceutical drugs (among many other examples), the nervous system may not respond in the way it should, i.e. to correct the imbalance.

- 중립조건하에서, 

If this occurs then, across time, the shortened compressed tissues will undergo dehydration, contracture, will shorten and become less able to translate loads, while the lengthened, tractioned tissues will also undergo dehydration, creep, will lengthen and will lose tensile strength (McGill, 2002), and will become less effective at passively restricting excessive movement at the joint, and consequently may lose mechanoreceptive efficacy. 

Neutral zone concerns

This raises some concerns with respect to the concept (or perhaps just the definition) of a ‘‘neutral zone’’. If the neutral zone is defined as a small range of movement near the joint’s neutral position where minimal resistance is given by the osteoligamentous structures (Lee, 2004), then a spine which has been held in a position of relative flexion for 15 or 20 years, for example, will have a neutral zone that has migrated anteriorly, compared to someone who has maintained a neutral spine during that same time frame.

The neutral spine principle

The neutral spine principle is a rehabilitative and performance conditioning principle (Lee, 2004; Comerford and Mottram, 2001; McGill, 2002; Chek, 2001; Baechle and Earle, 2000) which suggests that in early stage rehabilitation and the learning phase, postural conditioning, strength-endurance, and strength development phases of conditioning, the capacity to both maintain a neutral spine, and to be able to dissociate the spine from the hips, is a foundational movement skill.

Why should being in a neutral spine be of any benefit?

Different authors and researchers have attempted to estimate how the loading of the motion segment (the disc and facet joints) should be shared in a functional spine. Adams et al, (2006). describe weight bearing through the spine and indicate that some early research suggested that the zygapophyseal (facet) joints carried approximately 20% of the load and the disc 80%. More recent studies have suggested that the facet joints can bear up to 40% of the applied load, while other researchers have suggested that, in the lumbar spine at least, the facet surface orientation means that no weight can be borne through these structures. This will be further discussed below (Figures 1 and 2).

The implication, then, is that if too much of the loading is passed through the disc, it will break down ahead of the facets and vice versa.

Load sharing

Very simply put, if any one part of the biomechanical chain is utilized in favor of sharing the load, it will undergo greater cumulative microtrauma andismost liable toundergo changes in tensile strength and eventual degenerative change. A simple example of this is the lumbo-pelvic rhythm, where overuse of a lumbar strategy (flexion through the spine in forward bending) will increase risk of lumbar injury, whereas those with a hip strategy (flexion through the hip in forward bending) will increase risk of hip injury (see Figure 5 below).

Similarly, within the spine, if one component is used to

bear load alone or take greater load than it is designed to,

the result will be greater cumulative microstress and the

potential of subsequent degenerative change to that overstressed

component.

The concept of cumulative microtrauma, or cumulative

trauma disorders, or repetitive strain injuries has been

thoroughly discussed in the literature; including a recent

JBMT editorial (Wallden, 2009). Such a process of accumulating

microscopic stress in the tissues can result in

a decline in tensile strength (McGill, 2002), which culminates

in greater vulnerability to injury from ever decreasing

loads, such as lifting a small child, to weeding a flower bed,

to sneezing, to tying a shoe-lace, to sleeping in a draught

(where consequent changes in muscle tone may exert

a compressive load through the spine).

In practice, it is most commonly these kinds of scenarios

that are presented by patients as the causative factor in

their back pain, yet the discerning therapist would

presumably recognize the highly implausible nature of

these claims; especially if they recognized that the disc will

withstand loading at greater intensities than the bones

themselves can handle and, indeed, remained intact after

all the vertebrae had been reduced to a series of crush

fractures in research on monkeys (Gracovetsky, 2003). So

how is it that picking up a pencil, or bout of hay fever can

rupture a healthy disc?

This seems implausible based on our knowledge of disc

strength.

Nevertheless, a pencil or a sneeze may become the

proverbial straw that breaks camel’s back in an unhealthy

disc. That last little bit of stress to an already severely

compromised and weakened tissue means that even

a pencil or a draught can become ‘‘the last straw’’ as far as

the spine is concerned.

How might such a process of cumulative microtrauma

result in such a significant weakening of the immense

tensile strength of the annulus of the disc, for example?

This is an important question, as it is clear that the disc

is an incredible translator of mechanical forces. So, how

could simply sitting with a flat-backed posture, even for

a number of years, result in a breakdown of the annulus

fibrosis; a structure which not only has its own immense

strength, but that is swaddled in paraspinal ligaments

which can withstand loads of up to 1260 kg (Kerr, 1999) per

square centimeter; a tensile strength greater than steel?

Compare this to the 5 kg per square centimeter (Kerr, 1999)

that muscle can translate and it gives you a feel for the raw

strength of these passive structures. And bear in mind that

the muscles are able to absorb forces as much as 33 times

bodyweight in sprinting according to Lees (1999).

There are a few considerations that may shed some light

on this:

First, the nervous system will always migrate the body to

its position of strength. When someone has adopted

a specific posture for a prolonged period of time and in this

instance (where we are using the example of a flat-backed

[hypolordotic] sitting posture), the lumbar erectors will

lengthen by laying down sarcomeres in series and/or become

stronger in their outer-range; which will alter static and

dynamic lengthetension relationships about the lumbopelvic

region, respectively (Chek, 2001; Sahrmann, 2002).

The upshot of this may be that the faulty seated posture

is transposed into activities of daily living (e.g. lifting,

twisting, squatting, walking) and into sports or other more

highly loaded activities.

If we just pick one of these activities, such as walking,

and consider how this may affect the lumbar spine of

someone with a flattened lordosis, it will provide a useful

illustration for how structures as strong as discs, ligaments,

joint capsules, and so on, become affected by cumulative

microstress.

Gracovetsky (1988, 1997) explains that the ground

reaction forces returning through the lower limb after heelstrike

travel into the lumbar spine creating loading through

both the discs and the facets up the length of the spine

ipsilateral to the heel-strike. For example, a left heel-strike

will drive ground reaction forces through the left leg into

the spine on the left hand side compressing the facets on

the left side of the spine, and due to the contralateral

coupling of the arms and legs in gait (Van Emmerick et al.,

1999) there will be a relative left rotation of the lumbar spine creating torsional stress through the oblique fibers of

the annulus fibrosis. This loading of the facets and

stretching of the annulus results in a storing of potential

energy within the viscoelastic collagen fibers which will

recoil to drive the spine into right rotation and, with it,

draw the right leg through its swing phase. This mechanism

that allows this is otherwise known as the spinal engine

(Gracovetsky, 1988) (Figure 3).

The importance of this understanding becomes clear

when we look at both the loading and the concept of load

sharing in the context of human gait.

The average person takes around 10,000 steps per day

(Morris, 1985). This means that the spine undergoes

a compressive load with each heel-strike and toe-off,

somewhere between 1 and 3 times bodyweight. If we take

an average 70 kg adult male, who has sat at a desk for many

years with a flat-backed (hypolordotic) posture and look at

how this 70 kg load may affect his lumbo-pelvic integrity,

we find that multiplying the steps taken on an average day

(10,000) by his bodyweight (70 kg) which may be

further multiplied by between 1 and 3 times due to the

compressive penalty of the up and down sine-wave motion

of gait, we reach a total of somewhere between

10;00070 kgZ700; 000 kg1Z700; 000 kg

.and.

10;00070 kgZ700; 000 kg3Z2; 100; 000 kg

Clearly, this is a lot of loading, but let us not forget that this

is only the walking. If we were to take the kind of person

that may be engaging in the activities prescribed by

Liebenson in this section and issue of JBMT, then with each

step they take, as a runner, they will be loading between 3

and 7 times bodyweight through their spine with each step

(Lees, 1999). If this person also plays sports, or lifts children,

or has a manual job, the loading will be multiplied

dramatically again. Importantly, these are the kinds of

loads that a spine must handle per day. If we want to look

at the same loading across longer periods of time e just

based on the lower figure of 700,000 kg, which is only based

on the walking loads put through the spine, we can see

some startlingly large figures begin to emerge.

700; 000 kg7 daysZ4; 900; 000 kg

700; 000 kg31 daysZ217; 000; 000 kg

700; 000 kg365 daysZ2:5558 kg

700; 000 kg10 yearsZ2:5559 kg

Suddenly, from being impressed at the immense strength of

the discs, ligaments and other tissues of the body, it

becomes painfully clear why slight aberrations in posture

which create greater loading through one of the weightbearing

structures (in this scenario, the posterior disc), can

result in dramatic weakening and diminished tensile

strength leaving the disc exposed to injury from a simple

low load activity, like picking up a pencil.

Weight bearing in the spine

To revisit Bogduk’s (2005) synopsis of weight bearing

through the spine, we can look at how the assumption that

weight bearing occurring through the ‘‘tripod mechanism of

the spine’’ proposed by Kapandji (1974) and others, may be

incorrect. Bogduk (2005) described earlier research in

which load sharing was suggested to fall around a 60:20:20

ratio (disc to facet left to facet right) in the tripod mechanism.

However, Bogduk’s conclusion based on the most

current available evidence was that, in fact, the disc may

be the only weight-bearing structure; the facets remaining

completely uninvolved.

What ramifications does this have for our flat-backed

office worker? It would, at first glance, appear to indicate

that he is ‘‘back to square one’’. If the disc is the only

weight-bearing structure and the disc has ruptured, when

he bent to pick up a pencil, then may be it was simply

‘‘meant to be’’. perhaps a genetic aberration?