To Crunch Or Not to Crunch: an Evidence- Based Examination Of

To Crunch or Not to Crunch: An Evidence- Based Examination of Spinal Flexion Exercises, Their Potential Risks, and Their Applicability to Program Design Bret Contreras, MA, CSCS1 and Brad Schoenfeld, MSc, CSCS2 1Auckland University of Technology, Auckland, New Zealand; and 2Global Fitness Services, Scarsdale, New York

SUMMARY he crunch and its many varia- living such as tying one’s shoes rather tions have long been consid- than ‘‘wasted’’ on crunches and other THE CRUNCH AND ITS MANY VAR- eredastapleexerciseinfitness flexion-based abdominal exercises. Op- IATIONS HAVE LONG BEEN CON- T programs. These exercises involve ponents of the theory counter that an SIDERED A STAPLE EXERCISE IN dynamic flexion of the spine in the alarming discrepancy exists between FITNESS PROGRAMS. HOWEVER, sagittal plane and are performed to laboratory results and what is occur- RECENTLY, SOME FITNESS PRO- increase abdominal strength and de- ring in gyms and athletic facilities FESSIONALS HAVE QUESTIONED velopment (124), particularly in the around the world with respect to total THE WISDOM OF PERFORMING rectus abdominis and obliques mus- flexion cycles and spinal injury and cite FLEXION-BASED SPINAL EXER- culature. Strength and conditioning a lack of evidence showing any detri- CISES, SUCH AS THE CRUNCH. coaches frequently include such exer- ments. Therefore, the purpose of this CONCERNS ARE USUALLY PREDI- cises as a component of athletic article will be 3-fold: First, to review CATED ON THE BELIEF THAT THE routines designed to enhance sporting the relevant research pertaining to the SPINE HAS A FINITE NUMBER OF performance (45). risks of performing dynamic spinal flexion exercises; second, to explore BENDING CYCLES AND THAT EX- Recently, however, some fitness pro- the potential benefits associated with CEEDING THIS LIMIT WILL HASTEN fessionals have questioned the wisdom spinal flexion exercises; and third, to THE ONSET OF VERTEBRAL DE- of performing flexion-based spinal ex- discuss the application of these findings GENERATION. THIS ARTICLE WILL ercises, such as the crunch (23,75,110). to exercise program design. SEEK TO REVIEW THE RESEARCH Concerns are usually predicated on the PERTAINING TO THE RISKS OF belief that the spine has a finite number PERFORMING DYNAMIC SPINAL of bending cycles and that exceeding FLEXION EXERCISES AND WILL this limit will hasten the onset of disc KEY WORDS: DISCUSS THE APPLICATION OF damage (75). Proponents of the theory spinal flexion; crunch; trunk flexion; THESE FINDINGS TO EXERCISE claim that spinal flexion therefore spinal biomechanics PERFORMANCE. should be saved for activities of daily

8 VOLUME 33 | NUMBER 4 | AUGUST 2011 Copyright Ó National Strength and Conditioning Association OVERVIEW OF DEGENERATIVE (21). Discs tend to progressively de- biomechanics and their impact on disc DISC DISEASE teriorate with age, with a majority of pathophysiology, including the use of The intervertebral discs form cartilagi- discs showing signs of degeneration by animal and human in vivo (i.e., within nous joints between adjacent vertebrae, the time a person is 70 years old (79). the living) models, animal and human which stabilize the spine by anchoring Age-related degeneration involves a re- in vitro models (i.e., within the glass), the vertebrae to one another. The discs duction in proteoglycan and collagen and computer-based in silico models also facilitate multiplanar spinal move- levels (114), a 5-fold reduction in the (63). In particular, in vitro research has ment and help absorb vertebral shock. fixed charge density (a measure of implicated repetitive lumbar flexion as Discs have 3 distinct portions: an outer mechanoelectrochemical strength) of the primary mechanism of disc herni- layer annular fibrosus, a central nucleus GAGs in the nucleus (60), and a 2-fold ation (protrusion of disc material pulposus, and 2 hyaline cartilage end decrease in hydration between adoles- beyond the confines of the annular plates (64). The annulus, which has an cent discs and discs of 80 year olds lining) and prolapse (a bulging of inner and outer component, consists of (129), which diminishes the disc’s nucleus pulposus through annulus multiple layers of fibrocartilage, primar- height and load-bearing capabilities fibrosus) because evidence shows that ily a combination of type I and type II (8,22). Men tend to exhibit more disc these pathologies proceed progres- collagen (39). The annulus serves to degeneration than women, which is sively from the inside outward through resist outward pressure, also known as thought to be because of a combination nuclear migration toward the weakest tensile or hoop stresses, during axial of increased trunk strength, increased region of the annulus, the posterolat- compression and to stabilize the verte- resistance lever arms that heighten eral portion (62,127). spinal forces and stresses, increased bral joint during motion (138). The Most in vitro studies on spinal bio- heavy loading, and increased distance annulus also serves to contain the inner mechanics that are applicable to for nutrient travel (79). nucleus, which is a gel-like structure the crunch exercise have used cervi- composed of a mixture of chondrocytes, Intervertebral disc degeneration can cal porcine models (30,35,36,70,123). collagen, elastin, and proteoglycans manifest from a structural disturbance These models involve mounting spinal (130). Proteoglycans serve to resist in the annulus, nucleus, or end plate (7). motion segments in custom appara- compressive loading because of their Aging, apoptosis, collagen abnormalities, tuses that apply continuous compres- glycosaminoglycan (GAGs) content vascular ingrowth, mechanical loading, sive loads combined with dynamic (114). Glycosaminoglycans are long- and proteoglycan abnormalities can all flexion and extension moments. Total branch polysaccharides that attract contribute to disc degeneration (71). As bending cycles have ranged from 4,400 and bind to water and provide osmotic discs degenerate, focal defects arise in the to 86,400, with compression loads pressure. The nucleus functions as cartilage end plate, the nuclei become equating to approximately 1,500 N. a ‘‘water pillow,’’ helping to cushion increasingly more consolidated and fi- Considering that Axler and McGill the vertebrae from axial loads and brous, and the number of layers in the (13) found that a basic crunch varia- distribute pressures uniformly over ad- annulus diminishes (119). This has been tion elicited around 2,000 N of com- jacent vertebral end plates (111). The shown to alter disc height, spinal bio- pression, the amount of compression in end plates contain primarily type II mechanics, and load-bearing capabilities the various studies is reasonable for collagen (55), are less than 1 mm thick, (99) and ultimately can lead to spinal making comparisons with the crunch and contain fibers that extend into the stenosis—an advanced form of degener- exercise. In each of the aforementioned disc (138). In addition to preventing the ative disc disease that causes compres- studies, a majority of the discs experi- nucleus from protruding into adjacent sion of the contents of the spinal canal, enced either complete or partial her- vertebrae, the end plates also help to particularly the neural structures (93). niations, particularly to the posterior absorb hydrostatic pressure caused by End plate calcification also contributes to annulus. This suggests a cause-effect spinal loading (26,81) and allow for disc degeneration by decreasing nutrient relationship between spinal flexion and nutrient diffusion (131). diffusion that interferes with the pH disc damage. The results of the studies balance and increases inflammatory Degenerative disc disease is a multifac- are summarized in the Table. responses in the nucleus (34). Yet despite torial process involving genetic, me- a clear association between degenerative Although the aforementioned studies chanical, biological, and environmental spinal changes and an increased in- seem to lend credence to the potential factors (59). The first common signs of cidence of lower back pain (LBP) (65), risks of repeated spinal bending, there disc degeneration often appear between many afflicted individuals are neverthe- are several issues with attempting to 11 and 16 years of age, with approxi- less asymptomatic (19,20,139). extrapolate conclusions from a labora- mately 20% of teenagers displaying mild tory setting to the gym. First and disc degeneration (79). However, minor DOES SPINAL FLEXION CAUSE foremost, the studies in question were signs of degeneration, such as mild cleft DISC INJURY? performed in vitro, which is limited by formation and granular changes to the A variety of research approaches the removal of musculature and does nucleus, appear in disc of 2 year olds have been used to elucidate spinal not replicate the in vivo response to the

Strength and Conditioning Journal | www.nsca-lift.org 9 To Crunch or Not to Crunch

Table Summary of in vitro studies on spinal biomechanics reporting spinal compression forces applicable to the crunch exercise

Type of Number of Amount of Number of Number of Study Spine Subjects Compression, N Cycles Herniations Callaghan and McGill (30) Porcine cervical 26 260–1,472 86,400 15 Drake et al. (35) Porcine cervical 9 1,472 6,000 7 Tampier et al. (123) Porcine cervical 16 1,472 4,400–14,00 8 Drake and Callaghan (36) Porcine cervical 8 1,500 10,000 8 Marshall and McGill (70) Porcine cervical 10 1,500 6,000 4 human spine during normal movement most prominent in flexion and exten- damage results when fatigue failure (98,141–143,147). As with all living sion, which may mitigate the ability to outpaces the rate of adaptive remodel- tissue, the vertebrae and its supporting draw applicable conclusions to human ing, which depends on the intensity of structures remodel when subjected to dynamic spinal exercise. load, the abruptness of its increase, and applied stress (24). Consistent with Furthermore, the studies in question the age and health of the trainee (2). Wolff’s and Davis’s Laws, deformation attempted to mimic loading patterns of Provided that dynamic spinal exercise of cellular tissues are met by a corre- occupational workers by subjecting spi- is performed in a manner that does not sponding increase in the stiffness of the nal segments to thousands of continuous exceed individual disc-loading capac- matrix, which in turn helps to resist bending cycles, which is far beyond what ity, the evidence would seem to suggest future deformation (102,103). The is normally performed in the course of a positive adaptation of the supporting vertebrae and intervertebral discs are a dynamic exercise program. Typical tissues. In support of this contention, no exception because they have been core strengthening routines use a limited Videman et al. (136) found that shown to adaptively strengthen when number of dynamic repetitions, and on moderate physical loading resulted in exposed to progressive exercise completion of a set, trainees then rest for the least disc pathology, with the (2,24,66,92). Cadaveric tissue does a given period before performing an- greatest degeneration seen at extreme not have the capacity to remodel. other set. Thus, total bending cycles per levels of activity and inactivity. Another important point to consider session ultimately amount to a fraction of In addition, the role of genetics needs to when interpreting results of in vitro those used in the cited research proto- be taken into consideration. Despite the studies involving cyclic spinal loading cols, and these cycles are performed commonly held belief that spinal de- is that natural fluid flow is compro- intermittently rather than continuously. generation is most often caused by the mised. Van der Veen et al. (132) found Rodacki et al. (97) found that despite the wear and tear from mechanical loading, that although porcine lumbar motion moderate values of compression associ- this seems to play only a minor role in segments showed outflow of fluid dur- ated with the traditional crunch, the the process (17). Instead, it has been ing loading, inflow failed to occur dur- transient nature of the load (i.e., the short shown that approximately 74% of the ing unloading, thereby decreasing peak period of compressive spinal force) variance is explained by hereditary disc height and interfering with normal did not induce fluid loss. In fact, factors (15). Battie et al. (17) identified disc biomechanics. abdominal flexion exercise was actually specific gene forms associated with disc In vitro comparisons are further com- found to be superior to the Fowler’s degeneration that hasten degenerative plicated by the use of animal models. position (a semi-recumbent position vertebral changes in the absence of Although animal models do have used in therapy to alleviate pressure on repetitive trauma. Hereditary factors, structural similarities to the human the spine) with respect to spinal unload- such as size and shape of the spinal spine (146,29), especially the porcine ing, presumably mediated by a greater structures, and biochemical constituents cervical spine in comparison with the fluid influx rate than when sustaining that build or break down the disc can human lumbar spine, numerous ana- a static recumbent posture (97). highly influence disc pathology, as can tomical and physiological variations It should also be noted that after gene-environment interactions (17). nevertheless exist (130). Of particular, an exercise bout, spinal tissues are In a case-control study involving 45 relevance to flexion studies is the fact allowed to recuperate until the next monozygotic male twin pairs, Battie that the absolute ranges of motion are training session, thereby alleviating et al. (16) found that subjects who spent smaller in porcine subjects compared disc stress and affording the structures more than 5 times more hours driving with humans (10). These variations are time to remodel. Exercise-induced disc and handled more than 1.7 times more

10 VOLUME 33 | NUMBER 4 | AUGUST 2011 occupational lifting showed no in- of abdominal muscle coactivation (12). compromise cell function and possibly creases in disc degeneration compared Additional research is needed to shed initiate apoptosis (27,52,71,130). with their twin siblings and, although further light on this topic with partic- Postures involving flexion of the spine values did not reach statistical signifi- ular attention focused on evaluating are superior to neutral and extended cance, actually displayed fewer lower the effects of IAP on compressive postures in terms of promoting in- lumbar disc herniations. In addition, forces in subjects performing spinal creased fluid exchange in the disc, Varlotta et al. (133) found that the flexion exercise including the crunch. especially the nucleus pulposus (5). relative risk of lumbar disc herniation It should also be noted that some One deficiency of neutral posture is before the age of 21 years is approxi- epidemiological studies show an in- that it favors diffusion in the anterior mately 5 times greater in subjects who creased risk of spinal injuries in athletes portion of the disc over the posterior have a positive family history. Further- involved in sporting activities that re- portion. Flexed postures reverse this more, physically active individuals seem quire repeated spinal flexion. Injuries to imbalance by stretching the posterior to experience less back pain than the spinal column, including disc de- annulus, thereby decreasing the dis- sedentary individuals (44,78). generation and herniations, have been tance for nutrients to travel. The Moreover, the studies in question do found to occur with greater frequency in posterior region of the disc contains not necessarily replicate spinal motion gymnasts, rowers, and football players a region that is deficient of nutrient during dynamic lumbar flexion exer- (120,122,135,144). Furthermore, elite supplement from all sources (69), and cise. For example, the traditional athletes experience such injuries more flexion reduces the thickness of the crunch exercise involves flexing the frequently than nonelite athletes posterior portion of the disc by 37%, trunk to approximately 30° of spinal (88,120). However, a cause-effect re- which ensures sufficient supply of flexion so that only the head and lationship between spinal flexion and glucose to the entire posterior region shoulders are lifted from the floor, injury in these athletes has not been of the disc (5). Flexion increases making the thoracic spine the region of established, and the ballistic nature of diffusion of small solutes and fluid flow greatest flexion motion (105,117). Fur- such sporting activities has little appli- of large solutes. This is important ther, Adams and Hutton (6) showed cability to controlled dynamic abdom- considering that disc degeneration has that taking a flexed lumbar spine from inal exercises. been linked to inadequate metabolite an end range of flexion at 13° to 11° of BENEFITS OF SPINAL FLEXION transport (51,83) and that populations flexion, a 2° differential, resulted in EXERCISES adopting flexed postures show less a 50% reduction in resistance to If dynamic flexion exercises in fact do incidence of disc disease (40). The bending moment and therefore a 50% not pose a significant injury risk in the crunch exercise produces tensile reduction in bending stress to the absence of spinal pathology, then the stresses on the posterior annulus—in posterior annulus and intervertebral natural question is whether performing flexion, the posterior annulus has been ligaments. Thus, both the location these movements confers benefits over shown to extend up to 60% of its and degree of flexion will have a signif- and above static-based exercises. The original height (90)—and tensile stress icant impact on spinal kinetics. following potential benefits can be has shown to exert a protective effect on Finally, although abdominal exercises identified. disc cells by decreasing the expression of catabolic mediators during inflam- create compressive forces by way of First, spinal motion has been shown to mation (107). By enhancing nutrient muscular contraction, they also in- facilitate nutrient delivery to the in- uptake and limiting inflammatory-based crease intra-abdominal pressure (IAP) tervertebral discs (50,51). The mecha- catabolism, regimented flexion exercise (32). Three-dimensional biomechani- nism of action is theorized to be related may actually confer a positive effect on cal models predict reductions in com- to a pumping action that augments the long-term spinal health and pro- pressive forces of approximately 18% transport and diffusion of molecules into mote disc healing in the periphery (9). when IAP is factored into spinal flexion discs. Motion causes more fluid to flow efforts (118). Hence, IAP produced out of the disc, which is reversed when In fact, research suggests that spinal during spinal flexion exercise may the spine is unloaded (5). Fluid flow is flexion and extension exercises can be serve to moderate compressive forces, better at transporting large molecules, valuable in reducing LBP (38,43,96). helping to unload the spine and whereas diffusion is better at trans- Although pain or lack of pain is not facilitate fluid absorption in the discs porting smaller molecules (128). This necessarily an indicator of spinal health, (97). Because in vitro research models has a particular significance for spinal it nevertheless is interesting to speculate to date have not incorporated IAP, tissue given that age-related decreases in that spinal flexion movements may conclusions drawn may be limited with disc nutritional status is considered actually confer therapeutic benefits pro- respect to the safety of spinal flexion a primary cause of disc degeneration, vided exercise does not exceed the exercises. However, it should be noted leading to an accrual of cellular waste adaptive capacity of the tissue. that the unloading effects of IAP products, degradation of matrix mole- In addition, spinal flexion exercises may be diminished with high levels cules, and a fall in pH levels that further may help to improve functional spinal

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flexibility and thereby reduce the onset (42,72), muscle hypertrophy has specific of protein synthesis, is significantly gre- of LBP. Multiple studies have found that relevance to athletes who require exten- ater in eccentric contractions compared a lack of sagittal plane spinal flexibility is sive core strength. Moreover, muscle with isometric contractions. This may associated with an increased incidence hypertrophy of the rectus abdominis is be because of heightened metabolic of LBP (28,37,73,89). Resistance exer- also integral to aesthetic appearance of stress, greater muscle damage, or a com- cise has been shown to serve as an active the abdominal musculature and is there- bination of both. form of flexibility training, helping to fore highly desired by bodybuilders and improve joint mobility within a func- other fitness enthusiasts. PRACTICAL APPLICATIONS tional range of motion (14,80,106), and The hypertrophic superiority of dy- Taking all factors into account, it would spinal flexion exercises have been namic movement can be partly attrib- seem that dynamic spinal flexion ex- shown to increase sagittal plane spinal uted to the eccentric component, ercises provide a favorable risk to mobility (38). Improved flexibility asso- which has been shown to have the reward ratio provided that trainees ciated with resistance training has been greatest effect on promoting muscle have no existing spinal injuries or attributed to increased connective tissue development (41,49,53,100). Eccentric associated contraindications, such as strength, increased muscular strength, exercise has been linked to a preferen- disc herniation, disc prolapse, and/or and improved motor learning, and/or tial recruitment of fast twitch muscle flexion intolerance. However, several neuromuscular coordination (80). At fibers (85,112,121) and perhaps re- caveats need to be taken into consid- the same time, dynamic strengthening cruitment of previously inactive motor eration to maximize spinal health. of the supporting musculature and units (77,84). Given that fast twitch First and foremost, because hereditary ligamentous tissue may attenuate spinal fibers have the greatest growth poten- factors have a tremendous impact on hypermobility in those afflicted, which tial, their recruitment would necessar- the disc degeneration, it is difficult to has also been implicated as a cause of ily contribute to greater increases in know the precise amount of volume, LBP (119). Hence, a case can be made muscle cross-sectional area. intensity, and frequency sufficient to that a well-designed resistance training stimulate soft tissue strengthening adap- Eccentric exercise is also associated program that includes dynamic spinal tations without exceeding the recovery with greater muscle damage, which flexion may bestow a preventative effect ability of the spine. It has been theorized has been shown to mediate a hypertro- against LBP. However, it should be that a ‘‘safe window’’ of tissue mechan- phic response (77,108). Muscle damage noted that some studies have failed to ical loading exists that facilitates healthy induced by eccentric exercise upregu- reveal significant differences in the maintenance of spinal discs (119). There lates MyoD messenger RNA expression sagittal plane spinal flexibility between is evidence supporting this theory (57) and has been implicated in the pain free subjects and those with LBP because it pertains to spinal compres- release of various growth factors that (94), and 1 study indicated that lumbar sion (145); however, further research is spinal flexibility is associated with disc regulate satellite cell proliferation and needed to determine whether this degeneration (48). Moreover, we cannot differentiation (126,137). applies to other types of spinal loading necessarily determine a cause-effect re- In addition, dynamic muscle actions including flexion. lationship between poor spinal flexibility have been shown to induce significantly An epidemiological study by Mundt and an increased risk of injury. Further greater metabolic stress than static et al. (82) found that participation in research is warranted to draw pertinent contractions (25). Specifically, the sports such as baseball, softball, golf, conclusions on the topic. buildup of metabolites, such as lactate, swimming, diving, jogging, aerobics, Finally, flexion-based spinal movements hydrogen ion, and inorganic phosphate, racquet sports, and weight lifting are help to optimize hypertrophy of the has been shown to mediate a hypertro- not associated with increased risk of rectus abdominis muscle. The crunch phic response (101,109,116), and some lumbar disc herniation, and they even exercise and its variations have been researchers have speculated that meta- may offer a protective effect against shown to target the rectus abdominis to bolic stress may be more important herniation. Kelsey et al. (58) reported a much greater extent than the other than high force development in opti- similar findings with respect to disc core muscles. McGill (74) found that mizing muscle development (113). The prolapse. Many of these sports involve a variant of the crunch activated 50% stress-induced mechanisms theorized to a high frequency of spinal motion of maximal voluntary contraction increase muscle hypertrophy include including flexion, which casts doubt (MVC) of the rectus abdominis but only alterations in hormonal milieu, cell on the theory that humans have 20%, 10%, 10%, and 10% of MVC of the swelling, free radical production, and a limited number of flexion cycles. external obliques, internal obliques, trans- increased activity of growth-oriented Unfortunately, there is no way to verse abdominis, and psoas major, transcription factors (108). Russ (104) determine when an individual’s train- respectively. Given that a direct associ- displayed that phosphorylation of Akt, ing volume and/or intensity falls out- ation has been noted between muscle a protein kinase associated with mTOR side this range and thus predisposes the cross-sectional area and muscle strength pathway signaling and thus regulation spine to localized overload injury.

12 VOLUME 33 | NUMBER 4 | AUGUST 2011 Given that the spine and core muscu- the abdominal musculature as opposed populations, the risks of faster repeti- lature are loaded during nonmachine- to heightening muscular endurance. A tions would appear to outweigh the based exercise performance, such as repetition range of approximately 6–15 potential rewards and thus a slightly during squats, deadlifts, chin-ups, and repetitions is advised for achieving this slower tempo of approximately 2 sec- push-ups, most training can be consid- goal (108). External resistance should onds may be more appropriate with ered ‘‘core training.’’ Therefore, it is be used when necessary to elicit an respect to maintaining spinal health. As best to err on the side of caution and overload response within this target for athletic populations, more research limit the amount of lumbar flexion repetition range. Those seeking im- is needed to show whether explosive exercise to ensure that the tissue provements in local muscular endur- dynamic core exercises lead to positive remains in ‘‘eustress’’ and does not ance would be best served by adaptations that strengthen tissues and become ‘‘distressed.’’ Based on the performing static, neutral posture ex- prevent injury or whether they subject current data, the authors recommend ercises that are held for extended the athlete to greater risk of injury by that a sound core strengthening rou- periods. Specific guidelines will vary adding more stress to the tissues. tine should not exceed approximately dramatically according to the individ- It also is important to consider the 60 repetitions of lumbar flexion cycles ual’s needs and abilities, but a general effects of diurnal variation on spinal per training session. Untrained individ- recommendation for untrained individ- kinetics. During sleep, loading on the uals should begin with a substantially uals would be to perform 3–4 sets of discs is reduced, allowing them to lower volume. A conservative estimate 10- to 15-second holds in multiple absorb more fluid and increase in would be to start with 2 sets of 15 planes. Advanced exercisers seeking volume (129). Fluid is then expelled repetitions and gradually build up increases in static endurance might throughout the day as normal daily tolerance over time. perform 3–4 sets of 60 seconds or more spinal loading ensues. In the early In addition, it is important to allow for in multiple planes, whereas advanced morning, intradiscal pressure is 240% sufficient rest between dynamic spinal exercisers seeking increases in static higher than before going to bed (140), flexion sessions. The time course of power could stick to the 10- to 15- and bending stresses are increased at postexercise muscle protein synthesis second holds but perform more chal- the discs by 300% and at the ligaments lasts approximately 48 hours (67). Train- lenging variations or increase external of the neural arch by 80% because of ing a muscle group before protein resistance to promote further adapta- hydration and absence of creep (4). As synthesis has completed its course can tion. Athletes who engage in sports the day goes on, discs bulge more, impair muscle development (47) and where spinal flexion exercise or other become stiffer in compression, become potentially lead to localized overtraining. inherently dangerous motions for the more elastic and flexible in bending, Thus, the notion that it is optimal to discs, such as spinal rotation, is prom- affinity for water increases, and the risk perform dynamic abdominal exercises inent and volumes of flexion cycles and of disc prolapse decreases (1). After just on a daily basis is misguided. Because the training frequencies above our recom- 30 minutes of waking, discs lose 54% of intervertebral discs are poorly vascular- mendations are exceeded should con- the loss of daily disc height and water ized with low levels of metabolite trans- sider the possibility of excluding spinal content and 90% within the first hour port, their rate of remodeling lags behind flexion exercise from their routines. (95). For this reason, spinal flexion that of other skeletal tissues (69,115), Exercise tempo is another important exercises should be avoided within at which may necessitate even greater time consideration. Several studies have least 1 hour of rising. To be conserva- for recuperation. Taking all factors into shown that repetitions performed at tive, athletes may want to allow account, a minimum of 48 hours should a speed of 1 second elicit greater a minimum of 2 hours or more before be afforded between dynamic spinal muscle activation than those per- engaging in exercises that involve flexion exercise sessions, and it may be formed more slowly (134), and faster spinal flexion. prudent to allow 72 hours or more repetitions may selectively recruit the There is some evidence that spinal depending on individual response. rectus abdominis (87). Given the flexion exercises should also be Although some core training programs principle of specificity, rapid speeds avoided after prolonged sitting. It has include ultrahigh repetition sets of of movement would also tend to have been shown that discs actually gain crunches, for example, multiple sets greater transfer to athletic activities height after sitting (11,61) and decrease of a hundred repetitions or more, this that require dynamic core power, such lumbar range of motion (31), which type of protocol has little functional as wrestling (54), throwing a baseball reduces slack in the flexion-resisting applicability. After all, when does an (56), tennis (33), gymnastics (91), structures including ligaments and the individual need to continuously flex the soccer (125), swimming (68), and track posterior annulus while increasing the spine in everyday life? It is therefore and field (46). However, an increased risk of injury to those structures (4,18). recommended that flexion-based spi- repetition speed could subject the spinal However, as noted by Beach et al. (18), nal exercises be reserved for impro- tissues to excessive forces that may individual differences in sitting pos- ving strength and/or hypertrophy of lead to injury (6,86). For nonathletic ture lead to large variations in tissue

Strength and Conditioning Journal | www.nsca-lift.org 13 To Crunch or Not to Crunch

response. Some individuals actually tempos, and ranges of motion. It is 2. Adams MA and Dolan P. Time-dependent gain lumbar range of motion from hoped that this article will serve to changes in the lumbar spine’s resistance sitting, which can also increase the risk spark new research in this area. to bending. Clin Biomech (Bristol, Avon) 11: 194–200, 1996. of injury because of viscoelastic creep With respect to program design, basic (76), stress relaxation (3), or fluid loss 3. Adams MA and Dolan P. Could sudden core strength and endurance will be increases in physical activity cause (5), which increases joint laxity (4). realized through performance of most degeneration of intervertebral discs? Considering that approximately 50% of nonmachine-based exercises such as Lancet 350: 734–735, 1997. stiffness is regained within 2 minutes of squats, rows, deadlifts, and push-ups. 4. Adams MA, Dolan P, and Hutton WC. rising after 20 minutes of full flexion That said, targeted core exercises may Diurnal variations in the stresses on the (76), it seems prudent to allow at least serve to enhance sports performance, lumbar spine. Spine (Phila Pa 1976) 12: several minutes to elapse, perhaps 5 or functional capacity, and physique aes- 130–137, 1987. more, before engaging in spinal flexion thetics. Consistent with the principle of 5. Adams MA and Hutton WC. The effect of exercises after a period of prolonged specificity, core program design should posture on the fluid content of lumbar sitting and to walk around to facilitate take into account the individual goals intervertebral discs. Spine (Phila Pa 1976) 8: 665–671, 1983. dehydration of the disc. and abilities of the exerciser with respect 6. Adams MA and Hutton WC. The effect of CONCLUSION to their need for muscular hypertrophy, power, strength, and/or endurance, and posture on diffusion into lumbar intervertebral Based on current research, it is premature discs. JAnat147: 121–134, 1986. to conclude that the human spine has the types of joint actions involved in their sport. A variety of abdominal 7. Adams MA, May S, Freeman BJ, Morrison a limited number of bending cycles. The HP, and Dolan P. Mechanical initiation of claim that dynamic flexion exercises exercises are necessary to sufficiently intervertebral disc degeneration. Spine are injurious to the spine in otherwise work the abdominal musculature, and (Phila Pa 1976) 25: 1625–1636, 2000. these exercises will differ based on healthy individuals remains highly spec- 8. Adams MA, McNally DS, and Dolan P. ulative and is based largely on the training objectives (13). Variety in spinal Stress distribution inside intervertebral extrapolation of in vitro animal data that loading is associated with lower risk of discs: The effects of age and is of questionable relevance to in vivo spinal pathology (136). A balanced degeneration. J Bone Joint Surg Br 78: human spinal biomechanics. Although it multiplanar approach to core training 965–972, 1996. appears that a large number of contin- that incorporates a combination of 9. Adams MA, Stefanakis M, and Dolan P. uous bending cycles may ultimately have isometric and dynamic exercises is Healing of a painful intervertebral disc should not be confused with reversing a detrimental effect on spinal tissues, warranted to prevent any particular disc degeneration: Implications for no evidence exists that a low-volume spinal segment from accentuated stress and to ensure proper spine-stabilizing physical therapies for discogenic back strength-based exercise routine that pain. 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