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