Abdominal Crunches Are/Are Not a Safe and Effective Exercise

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DOI: 10.1519/SSC.0000000000000263
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Abstract
THE ABDOMINAL CRUNCH IS A WELL-KNOWN EXERCISE PERFORMED BY GENERAL AND ATHLETIC POPULATIONS FOR THE PURPORTED BENEFITS OF IMPROVING FITNESS ATTRIBUTES, SPORT PERFORMANCE, AND CORE MUSCLE FUNCTION. DESPITE THE BENEFITS, PARTICIPATION MAY INCREASE ONE'S RISK FOR LOW BACK PAIN. WHILE A CLEAR VERDICT ON THE RISK-TO-BENEFIT RATIO REMAINS ELUSIVE, A DISCUSSION OF THE AVAILABLE SCIENTIFIC EVIDENCE (OR LACK THEREOF) SHOULD GIVE PRACTITIONERS THE ABILITY TO DETERMINE THE UTILITY OF THIS EXERCISE FOR THEIR CIRCUMSTANCE. WE WANT TO HEAR FROM YOU. VISIT NSCA-SCJ.COM TO WEIGH IN ON THE POINT/COUNTERPOINT QUICK POLL.
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COLUMN EDITOR: Andrew J. Galpin, PhD, CSCS,
NCSA-CPT
Abdominal Crunches
Are/Are Not a Safe and
Effective Exercise
Brad J. Schoenfeld, PhD, CSCS*D, NSCA-CPT*D, CSPS*D, FNSCA
1
and Morey J. Kolber, PT, PhD, CSCS*D
2,3
1
Department of Health Sciences, Lehman College, Bronx, New York; and
2
Department of Physical
Therapy, Nova Southeastern University, Fort Lauderdale, Florida; and
3
Boca Raton Orthopaedic Group, Boca Raton, Florida
ABSTRACT
THE ABDOMINAL CRUNCH IS A
WELL-KNOWN EXERCISE PER-
FORMED BY GENERAL AND
ATHLETIC POPULATIONS FOR
THE PURPORTED BENEFITS OF
IMPROVING FITNESS ATTRIB-
UTES, SPORT PERFORMANCE,
AND CORE MUSCLE FUNCTION.
DESPITE THE BENEFITS, PARTIC-
IPATION MAY INCREASE ONE’S
RISK FOR LOW BACK PAIN.
WHILE A CLEAR VERDICT ON THE
RISK-TO-BENEFIT RATIO RE-
MAINS ELUSIVE, A DISCUSSION
OF THE AVAILABLE SCIENTIFIC
EVIDENCE (OR LACK THEREOF)
SHOULD GIVE PRACTITIONERS
THE ABILITY TO DETERMINE THE
UTILITY OF THIS EXERCISE FOR
THEIR CIRCUMSTANCE. WE
WANT TO HEAR FROM YOU. VISIT
NSCA-SCJ.COM TO WEIGH IN
ON THE POINT/COUNTERPOINT
QUICK POLL.
POINT
The crunch has long been consid-
ered a staple exercise for work-
ing the abdominal musculature.
Despite its widespread inclusion in
strength training programs, however,
the crunch has recently come under
scrutiny as a potentially dangerous
movement that should be avoided by
the general public. This claim is based
on the hypothesis that vertebral discs
have a finite number of bending cycles
and surpassing this limit ultimately
leads to disc damage (15).
Evidence that the crunch is deleterious
to spinal health has primarily been
derived from ex vivo (outside the living)
research using cervical porcine models.
These models involve mounting spinal
motion segments in hydraulic devices
that apply continuous compressive
loads in combination with repeated
dynamic flexion and extension cycles
(7–9,20). After applying bending cycles
that range from 4,400 to 86,400 com-
bined with ;1,500N compression loads,
partial or complete herniations have
been noted in the posterior annulus of
most discs analyzed. Given that the
crunch has been shown to produce
;2,000N of spinal compression (4)—an
amount greater than the forces applied
in the research–this has been held up as
evidence that the crunch predisposes
the discs to injury.
While on the surface these findings
may seem to provide compelling
evidence for a direct relationship
between spinal flexion and disc dam-
age, caution must be used when at-
tempting to extrapolate results from
ex vivo research to practical in vivo
settings. For one, inherent differences
exist between animal and human
models that limit generalizability
between the 2. With respect to the
spinal flexion models used, the abso-
lute range of motion of the porcine
spine is smaller than that of humans
during both flexion and extension ac-
tions (3), which compromises general-
izability to dynamic spinal flexion
Copyright ÓNational Strength and Conditioning Association Strength and Conditioning Journal | www.nsca-scj.com 61
Copyright ªNational Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
exercise. It is also important to note
that spinal tissue in living humans
adapts to the stress of progressive
exercise by getting stronger and, thus,
is able to withstand greater applied
stressors over time (5,16,18). In addi-
tion, the number of continuous load-
ing cycles used in the body of research
far exceeds those employed in traditional
programming for the crunch exercise. In
contrast to many thousands of repeated
flexion and extension cycles, typical
abdominal strengthening protocols
involve a fraction of these repetitions.
Moreover, many hours of recovery are
afforded after an exercise bout, allowing
sufficient time for spinal tissues to recu-
perate and remodel. Finally, the research
in question took the spinal segments to
the end range of flexion. It has been
shown that reducing the range of flexion
from 13 degrees to 11 degrees causes
a;50% decrease in bending stress to
the posterior annulus (2). Importantly,
the crunch is a limited range movement
that works the spine nowhere close to its
end range flexion capacity and, thus, re-
sults in much less stress on the
discs (11,19).
To the author’s knowledge, no studies
to date have been performed to deter-
mine whether a cause-effect relation-
ship exists between performance of
the crunch and spinal injury. Damage
to the vertebral discs from exercise
occurs when fatigue failure outpaces
the ability of the tissue to effectively
remodel, which is predicated on factors
that include genetics, the interaction
between load and posture, how rapidly
the load is increased, and the age and
health of the individual (1). Given the
adaptive nature of the discs, a case can
be made thatperformance of the crunch
actually has a positive effect on tissue
remodeling provided that the exercise
is performed in a fashion that does
not exceed disc loading capacity.
Although some claim that static
abdominal exercise provides all the ben-
efits of dynamic spinal flexion, this may
not necessarily hold true in practice. It
has been shown that spinal flexion pro-
motes nutrient delivery to the interver-
tebral discs (12,13), which has been
speculated to occur through a pumping
action that heightens transport and dif-
fusion of molecules into discs. Impor-
tantly, age-related reductions in spinal
nutritional status have been linked to
compromised cellular function, which
can lead to disc degeneration and pos-
sibly even apoptosis (6,14,21).
Dynamic spinal flexion strength/power
is also relevant to many athletic endeav-
ors including wrestling, baseball, tennis,
gymnastics, soccer, swimming, and
track and field. The principle of speci-
ficity dictates that optimizing perfor-
mance should include exercises that
directly work the muscles in the man-
ner that they are used in a given activ-
ity. The crunch seemingly would be
a viable exercise in this regard.
Finally, performance of the crunch may
promote greater abdominal muscle
hypertrophy compared with static core
exercises. Dynamic concentric and
eccentric actions have been shown to
elicit distinct morphological adaptations
at the fiber/fascicle level, including dif-
ferences in regional specific muscle
growth (10). Eccentric actions seem to
be particularly important to the hyper-
trophic response (17), possibly related to
exercise-induced muscle damage.
As a rule, there are no “bad” exercises,
just improper prescription and applica-
tion for a given individual. Based on
logical rationale, it seems prudent that
those with existing spinal conditions
including disc herniation, disc prolapse,
and/or flexion intolerance avoid perfor-
mance of dynamic spinal flexion exer-
cises. However, for those with healthy
spines, the crunch would seem to be
a safe and effective exercise when load-
ing and volume are prescribed within
the scope of individual abilities.
Brad J. Schoenfeld is an Assistant Pro-
fessor in the Exercise Science Program at
CUNY Lehman College and Director of
their Human Performance Laboratory.
REFERENCES
1. Adams MA and Dolan P. Could sudden
increases in physical activity cause
degeneration of intervertebral discs?
Lancet 350: 734–735, 1997.
2. AdamsMAandHuttonWC.Theeffectof
posture on diffusion into lumbar intervertebral
discs. JAnat147: 121–134, 1986.
3. Alini M, Eisenstein SM, Ito K, Little C, Kettler
AA, Masuda K, Melrose J, Ralphs J, Stokes I,
andWilkeHJ.Areanimalmodelsusefulfor
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Eur Spine J 17: 2–19, 2008.
4. Axler CT and McGill SM. Low back loads over
a variety of abdominal exercises: Searching for
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5. Brickley-Parsons D and Glimcher MJ. Is the
chemistry of collagen in intervertebral discs
an expression of Wolff’s Law? A study of
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1976) 9: 148–163, 1984.
6. Buckwalter JA. Aging and degeneration of
the human intervertebral disc. Spine (Phila
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7. Callaghan JP and McGill SM. Intervertebral
disc herniation: Studies on a porcine model
exposed to highly repetitive flexion/extension
motion with compressive force. Clin Biomech
(Bristol, Avon) 16: 28–37, 2001.
8. Drake JD, Aultman CD, McGill SM, and
Callaghan JP. The influence of static axial
torque in combined loading on
intervertebral joint failure mechanics using
a porcine model. Clin Biomech (Bristol,
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9. Drake JD and Callaghan JP.
Intervertebral neural foramina
deformation due to two types of
repetitive combined loading. Clin
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10. Franchi MV, Atherton PJ, Reeves ND, Fluck
M, Williams J, Mitchell WK, Selby A,
Beltran Valls RM, and Narici MV.
Architectural, functional and molecular
responses to concentric and eccentric
loading in human skeletal muscle. Acta
Physiol (Oxf) 210: 642–654, 2014.
11. Halpern AA and Bleck EE. Sit-up exercises:
An electromyographic study. Clin Orthop
Relat Res 145: 172–178, 1979.
12. Holm S and Nachemson A. Nutritional
changes in the canine intervertebral disc
after spinal fusion. Clin Orthop Relat Res
169: 243–258, 1982.
13. Holm S and Nachemson A. Variations in
the nutrition of the canine intervertebral
disc induced by motion. Spine (Phila Pa
1976) 8: 866–874, 1983.
14. Horner HA and Urban JP. 2001 Volvo
Award Winner in Basic Science Studies:
Effect of nutrient supply on the viability of
Point/Counterpoint
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62
Copyright ªNational Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
cells from the nucleus pulposus of the
intervertebral disc. Spine (Phila Pa 1976)
26: 2543–2549, 2001.
15. McGill S. Core training: Evidence translating
to better performance and injury prevention.
Strength Cond J 32: 33–46, 2010.
16. Porter RW, Adams MA, and Hutton WC.
Physical activity and the strength of the
lumbar spine. Spine (Phila Pa 1976) 14:
201–203, 1989.
17. Roig M, O’Brien K, Kirk G, Murray R,
McKinnon P, Shadgan B, and Reid WD.
The effects of eccentric versus concentric
resistance training on muscle strength and
mass in healthy adults: A systematic review
with meta-analysis. Br J Sports Med 43:
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18. Ruff C, Holt B, and Trinkaus E. Who’s
afraid of the big bad Wolff?: “Wolff’s law”
and bone functional adaptation. Am J Phys
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19. Sands WA and McNeal JR. A kinematic
comparison of four abdominal training
devices and a traditional abdominal crunch.
J Strength Cond Res 16: 135–141, 2002.
20. Tampier C, Drake JD, Callaghan JP, and
McGill SM. Progressive disc herniation: An
investigation of the mechanism using
radiologic, histochemical, and microscopic
dissection techniques on a porcine model.
Spine (Phila Pa 1976) 32: 2869–2874,
2007.
21. Urban JP and Roberts S. Degeneration of
the intervertebral disc. Arthritis Res Ther 5:
120–130, 2003.
COUNTERPOINT
The abdominal crunch, hereafter
referred to as a “crunch,” may
not be safe for all. The relative
safety of a crunch is not something that
can be narrowed down to a dichoto-
mous answer. The general and athletic
populations are both heterogenous
groups of people, each with different
needs and individual risk factors. Nev-
ertheless, certain exercises such as the
crunch may indeed be harmful to select
individuals with certain medical condi-
tions (past or current) or risk profile.
Moreover, crunches may potentially
increase one’s risk for injury to the lum-
bar spine because of the nature of
repetitive flexion, rises in lumbar intra-
discal pressure and ensuing muscle im-
balances that may occur as a result of
a biased exercise program. Lastly,
crunches performed incorrectly may
be responsible for injuries of the lum-
bar, thoracic, or cervical spine.
There are medical conditions that
would be a concern with respect to
performing the crunch. Several condi-
tions come to mind (e.g., diastasis recti,
osteoporosis [due to risk of compres-
sion fracture (21)], and various hernia
subtypes); however, the focus of this
column will be primarily on pathology
of the lumbar spine intervertebral disc,
hereafter referred to as “disc pathol-
ogy.” Although various subtypes of disc
pathology exist, intervertebral disc her-
niations (posterior, central, and pos-
terolateral) and tears of the posterior
annulus are the primary concern. The
reasoning for this concern is fairly
straight forward with respect to the
clinical and biomechanical evidence.
From a clinical research perspective,
there is no question the nucleus pulpo-
sus (NP) (center of intervertebral disc)
moves in response to loading and that
flexion movement or positions (tradi-
tional crunches are strictly flexion-
biased movement) of the lumbar spine
induce a posterior-directed movement
of the NP in vivo (1,3,4,7,9,10,14). In
addition to the pattern of nucleus
movements identified among human
subjects (in vivo), in vitro evidence sug-
gests flexion is associated with a poste-
rior migration of the NP, as well
(11,15,20). The concern over influenc-
ing posterior-directed movement of
the NP resides in the fact that symp-
tomatic disc herniations are primarily
the result of posterior-induced migra-
tion of the NP (6). It would be errone-
ous to assume that everyone who does
an abdominal crunch will develop disc
pathology. However, those with previ-
ously diagnosed disc pathology or con-
current low back pain may indeed be at
risk for recurrence or exacerbation.
Moreover, positions or movements
that require flexion, and those requiring
abdominal activation, have been
shown to produce a rise in lumbar in-
tradiscal pressure (18,19). Specific to
the crunch (supine crooklying position
with contraction of abdominals to
a limited range), evidence suggests that
an intradiscal pressure increase ranging
from 40-108% may occur (19). In-
creases in pressure combined with
a flexion-biased movement would
seemingly present a cumulative risk.
In addition to biomechanical evidence,
there is a considerable body of evi-
dence that has linked specific move-
ments or positions to worsening
a symptomatic disc herniation. Invari-
ably flexion-biased activities are often
the source (6,23). Moreover, evidence
suggests that individuals who have
a condition associated with worsening
from flexion movements will have
a poor outcome and experience wors-
ening of symptoms with activities that
focus on repeated flexion (17). Further-
more, evidence has been consistent cit-
ing a worsening of one’s clinical
presentation with repeated flexion
when a confirmed disc pathology is
present, based on the diagnostic gold
standard of discography (6,23). Thus, it
seems reasonable that a crunch, despite
having limited flexion when compared
with a traditional sit-up, would worsen
symptoms arising from disc pathology.
Although there are no studies specifi-
cally implicating abdominal crunches
as an etiological cause of a specific per-
son’s disc herniation, an absence of evi-
dence does not imply an evidence of
absence. For example, a systematic
review published in 2003 concluded
that there is no evidence to support
the use of parachutes for preventing
mortality during free-fall from a plane
(22). Should we abandon the use of
parachutes in the lay population?
Given standards of research and sub-
ject protection, most would agree that
a study designed to determine whether
indeed a particular exercise could “her-
niate” a disc would be unethical.
Last, the abdominal crunch may per-
petuate trunk muscle imbalances
associated with and predictive of low
back pain. Evidence, that is, both pro-
spective and retrospective has shown
that imbalances of the flexor-to-
extensor ratio in the trunk is a risk
factor for low back pain (2,13,16). Spe-
cifically, when the flexor strength
dominates the extensors, individuals
are more likely to develop low back
Strength and Conditioning Journal | www.nsca-scj.com 63
Copyright ªNational Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
pain (11). Moreover, individuals with
low back pain often have existing im-
balances beyond that of asymptomatic
person’s, further suggesting risk (13).
In addition, evidence has suggested
that individuals who are athletic or
perform routine resistance training
present similar muscle imbalances
favoring the flexors when compared
with extensors (5,8,12). These imbal-
ances, however, must be interpreted
with caution as the performance of
abdominal crunches alone cannot be
tied to these imbalances and most of
the studies have not presented details
with regard to training patterns. One
may consider the possibility, albeit
theoretical, that trained individuals
may develop a remodeling response
that would afford their disc tissue
a certain remodeling response to the
stresses of a crunch, offering a degree
of protection. Nevertheless, if an
imbalance exists, performing abdomi-
nal crunches in the absence of bal-
anced extensor training would
seemingly perpetuate one’s risk. Thus,
the solution resides in a balanced
training program as opposed to avoid-
ing exercises such as the crunch.
With regard to specific recommenda-
tions, a rule of avoiding crunches is
not supported by the evidence. Cer-
tainly among individuals with a current
or history of disc pathology, these exer-
cises would be considered a precaution
and left to the decision of a healthcare
practitioner. Evidence does support the
position that sustained or repeated flex-
ion is likely to cause a worsening of
symptoms among individuals with
a symptomatic lumbar disc herniation,
as a result of intradiscal pressure in-
creases and the nature of repeated flex-
ion (6,18,19,23). Assuming there are no
precautions to performing abdominal
crunches, a balanced exercise program
that includes both strengthening of the
spinal flexors and spinal extensors would
seemingly mitigate injury risk from mus-
cle imbalances and subject the spine to
more balanced forces. However, this
recommendation may generate the
question of whether extension exercises
are safe and effective.
Conflicts of Interest and Source of Funding:
The authors report no conflicts of interest
and no source of funding.
Morey J. Kolber is a Professor in the
Department of Physical Therapy, Nova
Southeastern University.
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FW, and MacSween A. The response of the
nucleus pulposus of the lumbar
intervertebral discs to functionally loaded
positions. Spine 32: 1508–1522, 2007.
2. Bayramoglu M, Akman MN, Kilinc S, Cetin N,
Yavuz N, and Ozker R. Isokinetic
measurement of trunk muscle strength in
women with chronic low-back pain. Am J
Phys Med Rehabil 80: 650–655, 2001.
3. Beattie PF, Brooks WM, Rothstein JM,
Sibbitt WL, Robergs RA, MacLean T, and
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the nucleus pulposus in supine subjects. A
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Spine 19: 2096–2102, 1994.
4. Brault JS, Driscoll DM, Laakso LL, Kappler RE,
Allin EF, and Glonek T. Quantification of the
lumbar intradiscal deformation during flexion
and extension, by mathematical analysis of
magnetic resonance imaging pixel intensity
profiles. Spine 22: 2066–2072, 1997.
5. Chan RH. Endurance times of trunk muscles in
male intercollegiate rowers in Hong Kong. Arch
Phys Med Rehabil 86: 2009–2012, 2005.
6. Donelson R, Aprill C, Medcalf R, and Grant
W. A prospective study of centralization of
lumbar and referred pain: A predictor of
symptomatic discs and anular competence.
Spine 22: 1115–1122, 1997.
7. Edmonston SJ, Song S, Bricknell RV, Davies
PA, Fersum K, Humphries P, Wickenden D,
and Singer KP. MRI evaluation of lumbar
spine flexion and extension in asymptomatic
individuals. Man Ther 5: 158–164, 2000.
8. Evans K, Refshauge KM, and Adams R.
Trunk muscle endurance tests: Reliability,
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9. Fennell AJ, Jones AP, and Hukins DW.
Migration of the nucleus pulposus within the
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of the spine. Spine 21: 2753–275 7, 1996.
10. Fredericson M, Lee SU, Welsh J, Butts K,
Norbash A, and Carragee EJ. Changes in
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extension movement: A comparison
between L4-5 and L5-S1 levels in normal
subjects. Spine J 2001: 10–17, 2001.
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15. Krag MH, Seroussi RE, Wilder DG, and
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Experimental results and theoretical
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18. Nachemson A. Disc pressure
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22. Smith GC and Pell JP. Parachute use to
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Point/Counterpoint
VOLUME 38 | NUMBER 6 | DECEMBER 2016
64
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  • Article
    Objectives: To determine whether parachutes are effective in preventing major trauma related to gravitational challenge. Design Systematic review of randomised controlled trials. Data Sources: Medline, Web of Science, Embase, and the Cochrane Library databases; appropriate internet sites and citation lists. Study Selection: Studies showing the effects of using a parachute during free fall. Main Outcome Measure: Death or major trauma, defined as an injury severity score > 15. Results: We were unable to identify any randomised controlled trials of parachute intervention. Conclusions: As with many interventions intended to prevent ill health, the effectiveness of parachutes has not been subjected to rigorous evaluation by using randomised controlled trials. Advocates of evidence based medicine have criticised the adoption of interventions evaluated by using only observational data. We think that everyone might benefit if the most radical protagonists of evidence based medicine organised and participated in a double blind, randomised, placebo controlled, crossover trial of the parachute.
  • Conference Paper
    INTRODUCTION Numerous structures have been implicated in the etiology of the low back pain (LBP), with the intervertebral disc (IVD) being one of the more common sources.1-5 The IVD is primarily composed of two structures, namely the annulus fibrosus (AF) and the nucleus pulposus (NP).6 A biomechanical principle referred to as the dynamic disc model (DDM) suggests that compression loading of the IVD during movements of the spine causes the NP to migrate within the annulus opposite the direction of compression, thus placing pressure on pain sensitive structures within the annulus.4, 7 The concept of the DDM suggests that the position of the NP may be altered in response to specific directions of loading. While the DDM is recognized as a potential explanation for a patient’s response to movements, one must recognize both the merits and limitations of this principle. The purpose of this poster is to review the available research pertaining to the DDM and where possible draw conclusions regarding the pattern of NP migration. METHODS Articles used in this review were retrieved by two independent researchers using MEDLINE (PUBMED and Ovid), SPORTDiscus, and CINAHL databases and the following key words independently and in combination: intervertebral disc, nucleus pulposus, nucleus migration, disc model, disc loading and dynamic disc model. Results from each researcher were pooled and then cross referenced. Based on the established criteria 13 articles were ultimately retained for use in the review. Articles were retained for the review if they met the following inclusion criteria: 1. The article appeared in a peer review journal, 2. Included humans both in-vivo and in vitro, 3. The study addressed migration of the NP in response to an angular movement or position, and 4. The study had to provide a conclusion as to the direction of migration or lack of migration for the NP. RESULTS This literature review retained a total of thirteen articles that yielded a predictable direction of nucleus migration in the lumbar spine. No studies investigating the DDM in the cervical or thoracic spine above T10 were identified. Table 1 and Table 2 list the results, subjects and methods for in-vitro in vivo studies respectively. A majority of the studies used fluoroscopy, discography or magnetic resonance imaging (MRI) to measure this movement. Four studies used in vitro discs8-11 and nine were performed with in-vivo experimental methods. 12-19 Eleven of the thirteen articles, identified a posterior migration of the nucleus in response to lumbar flexion. 8-10, 12, 14-18, 20, 21 Twelve of the thirteen articles report that the NP migrates anteriorly during extension. 8, 10, 12-18, 20-22 Six of the research studies found an unpredictable migration of the NP when degeneration was present within the disc.9, 10, 14-17 CONCLUSION Among the articles reviewed, there was significant agreement between research studies which suggest the normal disc has a predictable pattern of migration. In order for this reduction to occur the annulus must be intact and the hydrostatic mechanism must be functioning.4, 5, 7 There is sufficient data to conclude that the NP moves posterior with flexion and anterior with extension in normal disc. However, among the articles reviewed, only four out of the thirteen articles that generated a direction of migration used a known population of abnormal discs defined as a disc that has undergone degenerative changes, repetitive use injuries or traumatic injuries. Nine of the thirteen articles used an asymptomatic sample population. Two cadaveric studies did not reveal the state of the subject’s disc. There is an unpredictable migration pattern seen in abnormal or symptomatic discs with six studies that support this finding. Among the six studies three were performed in vivo with a known symptomatic population and they support the above finding of unpredictable movement. This finding is significant to the establishment of extension or flexion based interventions for the management of LBP if they are to be used specifically based on imaging findings. REFERENCES 1. Andersson GB. Epidemiological features of chronic low-back pain. Lancet. 1999;354(9178):581-585. 2. Young S, Aprill C, Laslett M. Correlation of clinical examination characteristics with three sources of chronic low back pain. The spine Journal. 2003;3:400-465. 3. Kuslich S, Ulstrom C, Micheal C. The tissue origin of low back pain and sciatica: A report of pain response to tissue stimulation during operations on the lumbar spine using local anesthesia. Orthopedic clinics of north america. 1991;22(2):181-187. 4. McKenzie RA. The Lumbar Spine Mechanical Diagnosis and Therapy. Wellington: Spinal Publications Limited; 1981. 5. Sehgal N, Fortin J. Internal Disc Disruption and Low Back Pain. Pain Physician. 2000;3(2):143-157. 6. Middleditch A, Oliver J. Functional Anatomy of the Spine. second ed. Philadelphia: Elsevier; 2005. 7. Donelson R, Aprill C, Medcalf R, Grant W. A prospective study of centralization of lumbar and referred pain: a predictor of symptomatic discs and anular competence Spine. 1997;22(10):1115-1122. 8. Seroussi RE, Martin HK, Muller DL, Malcolm HP, . Internal deformations of intact and nucleated human lumbar discs subjected to compression, flexion, and extension loads. Journal of Orthopaedic Research. 1989;7:122-131. 9. Krag MH, Seroussi RE, Wilder DG, Pope MH. Internal displacement distribution from in vitro loading of human thoracic and lumbar spinal motion segments: experimental results and theoretical predictions. Spine. 1987;12(10):1001-1007. 10. Shah JS, Hampson WGJ, Jayson MIV. The distribution of surface strain in the cadaveric lumbar spine. The Journal of Bone and Joint Sugery. 1978;60-B(2):246-251. 11. Gill K, Videman T, Shimizu T, Mooney V. The effect of repeated extensions on the discographic dye patterns in cadaveric lumbar motion segments. Clin Biomech. 1987;2:205-210. 12. Alexander LA, Handcock E, Agouris I, Smith FR, MacSween A. The Response of the Nucleus Pulposus of the Lumbar Intervertebral Discs to Functionally Loaded Postions. Spine. 2007;32(14):1508-1522. 13. Edmonston SJ, Song S, Bricknell RV, et al. MRI evaluation of lumbar spine flexion and extension in asymptomatic indiviuals. Manual Therapy. 2000;5(3):158-164. 14. Brault JS, Driscoll DM, Laakso LL, Kappler RE, Allin EF. Quantification of the lumbar intradiscal deformation during flexion and extension, by mathematical analysis of magnetic resonance imaging pixel intensity profiles. Spine. 1997;22(18):2066-2072. 15. Fennell AJ, Jones AP, Hukins D. Migration of the nucleus pulposus within the intervertebral disc during flexion and extension of the spine. Spine. 1996;21(23):2753-2757. 16. Beattie PF, Brooks WM, Rothstein JM, Sibbitt WL, Robergs RA, Maclean T. Effects of lordosis on the position of the nucleus pulposus in supine subjects. Spine. 1994 19(18):2096-2102. 17. Schnebel BE, Simmons JW, Chowning J, Davidson R. A digitizing techinque for the study of movement of intradiscal dye in response to flexion and extension of the lumbar spine. Spine. 1998;13(3):309-312. 18. Frazy PJ, Song S, Monsas A, et al. An MRI Investigation of Intervertebral Disc Deformation in Response to Torsion. Clinical Biomechanics. 2006;21:538-542. 19. Perie D, Sales de Gauzy J, Curnier D, Hobatho M. Intervertebral disc modeling using a MRI method: migration of the nucleus zone within scoliotic intervertebral discs. Magn Reson Imaging. 2001;19:1245-1248. 20. Fredericson M, Lee S, Welsh J, Butts K, Norbash A, Carragee E. Changes in the posterior disc bulging and intervertebral foraminal size associated with flexion-extension movement: a comparsion between L4-5 and L5-S1 levels in normal subjects. The spine Journal. 2001;2001(1):10-17. 21. Parent E C, Videman T, Battie M. The Effect of Lumbar Flexion and Extension on Disc Contour Abnormality Measured Quantitatively on Magnetic Resonance Imaging. Spine. 2006;31(24):2836-2842. 22. Adams MA, May S, Freeman BJC, Morrison P, Dolan P. Effects of Backward Bending on Lumbar Intervetebral Discs. Spine. 2000;25:431-437.
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