There is great interest among the general population in core training exercises. No matter what the goal of this core training, it must be ensured that these exercises are performed as safely as possible. Exposing the general population to potentially harmful exercises should be avoided at all costs. Since approximately up to 85% of people will experience low back pain (LBP) during their lifetime (1), and it is the leading cause of limited physical activity in people 45 years of age and younger (1), exercise professionals must have an understanding of the research examining how to safely and effectively train the core musculature.
Research has suggested that the core musculature be trained differently than the muscles of the limbs (20). Limb muscles (biceps brachii, hamstrings, etc.) are commonly used to move the segments to which they are attached, so training them as prime movers may be appropriate. However, McGill (20) suggests that most often during human movement, the function of the core musculature is to co-contract, stiffen, and prevent motion rather than produce it. Proficient human movement involves the limb muscles generating power that must be transferred through a stiffened core so that the entire body can be moved efficiently. It is believed that if proper core stabilization is not maintained, when power is developed from the ball and socket joints (hips and shoulders), the spine will bend or lose its neutral alignment. This spinal movement is considered an “energy leak,” as the power generated from the limbs is absorbed proximally in the soft tissues of the spine and not transferred distally as efficiently as possible (20). The detrimental effects of an energy leak such as a repeated spinal flexion during hip flexion movements have been shown to lead to injuries such as posterior disc herniation (5). Therefore, to make core training as safe as possible for any population, the core muscles should be trained as stabilizers rather than prime movers (20). This concept is supported by research examining low back loads during different abdominal exercises (2) as well as by a recent training study comparing long-term isometric core training to a more dynamic training program (15).
Spinal posture has been found to be a key determinant in the amount of compressive load that the spine is able to withstand before injury occurs, as in a flexed posture, the spine has a much lower yield point and ultimate compressive strength (9). This means that when there is flexion of the spine in addition to the compression, the load that the spine can withstand decreases and the risk of injury increases substantially (5,9). Therefore, the safest way to train the core musculature is to ensure that the spine remains in a neutral position when any load is introduced to the body.
LBP can be caused by an acute injury or can be due to chronically poor movement patterns that buildup over time (1). Although core training may not be able to prevent LBP due to blunt traumas such as car accidents, certain types of LBP are preventable by improving the quality of movement (12). A fundamental and effective way to prevent LBP is to maintain a neutral lordotic curve in the lumbar spine (12,19,26) as the stresses and strains on the low back tissues are increased in nonneutral postures (8). Strain applied repeatedly or for a prolonged period of time to a tissue will eventually result in tissue failure and pain (26); therefore, continually adopting an improper posture in any environment (i.e., workplace, gym, activities of daily living) may cause injury (19). However, identifying and correcting spinal postures both statically and during movement may help relieve the pain and prevent further damage (19). Exercise professionals can now quantify and track the movement of the lumbopelvic complex easily in clients using inexpensive and readily available technologies (i.e., smartphone application) that have been shown to identify those at risk of injury (6). Corrective exercises can then be undertaken with these individuals to avoid injury.
MOVEMENTS OF THE SPINE
The spine moves in 4 directions: flexion, extension, lateral flexion (right and left), and axial rotation (right and left). Research supporting the injury risk from excessive movement of the spine in any one direction is presented below.
FLEXION AND EXTENSION
Lumbar flexion and extension both have distinct mechanisms of different injuries. Repeated lumbar flexion and extension have been shown to cause a stretching of the passive tissues (5,12) which can lead to small microtraumas that can buildup over time to cause tissue damage (27). These conditions are worsened when high compressive loads are introduced to many repetitions of flexions or extensions (5). Therefore, greater compressive loads can lead to a greater risk of injury when combined with nonneutral sagittal plane positions of the lumbar spine. Continued exposure to such movements and postures over time may lead to the development of chronic conditions, where viscoelastic tissues begin to degenerate.
An intervertebral disc (IVD) injury most commonly occurs posteriorly due to a spinal flexion mechanism. This is due to the increased pressure on the posterior side of the disc as the spine goes into flexion (5,18). Anterior disc herniations are not as common but have been known to occur among populations such as gymnasts who experience extreme levels of extension (25,28). IVD injuries are progressive conditions with a mechanism of injury that commonly involves compression with the spinal flexion (posterior IVD herniation) or extension (anterior IVD herniation) (5). Callaghan and McGill (5) showed that, as the compressive load increased, there was much greater incidence of injury to the IVD during repeated flexion and extension movements. This indicates that the risk of IVD injury increases during a movement that involves flexing or extending the spine when external loads are added to the movement.
Spinal flexion not only creates a posterior shear force on the IVD, but it also increases the extensor muscle activity and load (10). The increased muscle activity has been shown to cause compressive stresses on the IVD in the lumbar spine (10). In comparison to an upright posture, an anteriorly positioned thorax resulted in significant increases in load and stress on the lumbar spine (10). McGill et al. (21) also assessed whether changes in hip flexion angle or lumbar curvature affected lumbar extensor muscle fiber direction and thus its ability to support anterior shear loads. Muscle angle orientation was not affected with 30° of hip flexion if a neutral lumbar position was maintained; however, in a flexed lumbar position, these angles were altered. Full lumbar flexion compromised the ability of lumbar extensors to support shear force, making the handling of heavy loads in a flexed lumbar position much more dangerous.
There are also spinal pathologies that are believed to be caused by excessive, repeated extension of the lumbar spine. These include the spondylitic disorders (spondylolisthesis and spondylolysis), which are a common cause of LBP in young athletes (25,28). Although this work is focused on a general fitness population, many former athletes may have developed long-term injuries from these activities (3). Those who have been gymnasts, cricket bowlers, divers, weightlifters, wrestlers, and football linemen may be at increased risk of developing these disorders as these activities can frequently require extension of the lumbar spine while in loaded positions (25). Spondylolisthesis is recognized as forward slippage of one vertebra on another, whereas spondylolysis occurs when there is a fracture to the posterior part of the vertebrae known as the pars interarticularis (25,28). Therefore, avoiding overt and excessive extension of the lumbar spine, while loaded, is important for preventing LBP.
ROTATION AND LATERAL FLEXION
Movements that involve twisting of the spine are common in work-related, athletic, and domestic tasks. Pure axial rotation has not been shown to be a direct cause of injury (18), but rarely do movements involve just rotation. Generally, there is a combined loading that includes flexion-extension, static axial torque, or static compression (7). Axial torque, which leads to axial rotation, has been shown to significantly increase the likelihood of injury (7). Those who experience axial rotation repetitively with combined loading are at a high risk of several pathologies, including disc herniation (7). Marshall and McGill (18) determined that pure axial torque (twist) did not lead to any damage to the disc; however, when axial torque was combined with flexion-extension, there was a large increase in tissue deformations that are known to lead to disc herniation. Therefore, compulsory movements that involve combined twisting and bending of the spine create a risk for low back injury and should be avoided.
Asymmetric loading of the spine with lateral lifting and lowering is also believed to lead to LBP. Huang et al. (11) examined the pulling force during lateral lifting and lowering from a standing position and found that peak force occurred in a laterally flexed position in the opposite direction of the load, which may impose an increased risk of injury to the spine. Marras et al. (17) identified the laterally bent posture and lateral bending velocity as risks related to low back disorders. Additionally, Kumar et al. (14) found that strength declines as an individual assumes a laterally flexed spinal posture. Research invariably concludes that lateral bending or frontal plane movement of the spine compromises the integrity of the spine; therefore, it may be wise to avoid exercises that involve repeated spinal lateral bending beyond that of which is tolerable.
PROGRESSION AND REGRESSION
Exercise modification is necessary so that exercise professionals may tailor exercises to fit an individual's specific needs while still maintaining an appropriate level of challenge. Exercise progression increases the difficulty when the basic exercise no longer provides a challenge. Theoretically, exercises that are properly progressed will continually improve the efficiency of movement patterns, while also increasing the margin of safety (the distance between applied loads and the failure tolerance of the tissues) (24). Conversely, exercise regression decreases the difficulty so that an individual can learn the appropriate muscular activation strategies in a way that decreases the chances of developing compensations in their movement patterns leading to injury (16). It has been suggested that a safe and effective exercise plan should be of low risk and high demand (23); therefore, a trainer should have the ability to make small variations in the difficulty of an exercise to ensure that the client can handle the increases in demand without placing them at risk for injury.
Blanchard and Glasgow (4) suggested a model for exercise progression and regression (Figure 1). In Figure 1, horizontal axis represents time, and the vertical axis represents the level of difficulty of the exercise. The most controlled level of any exercise is labeled “A” in Figure 1. At this level, the client should be focusing on only one fundamental task, and other variables should be limited. For core exercise, this should be training the core musculature to stabilize the lumbar spine in a neutral lordotic position. Proprioceptive sense of this neutral lordotic position would be the primary goal of a core exercise at this level. This would also be a fundamental goal of every progression of a core exercise as the triangle labeled A is on top of every progression. The level “A” exercise can then be slowly progressed linearly (moving up the triangle) by increasing the duration of the hold, number of repetitions, etc. The next step “B” would be to add an extrinsic component to the exercise to further complicate the basic task. Further progressions to the exercise could then be performed separately or added to the previous progression (“C,” “D,” etc.). Once an exercise variation is mastered, a progression needs to be introduced to ensure constant challenge and improvement. The exercise progression model will be revisited at the end of the article to give more specific ideas for implementing the exercises presented in this article.
DURATION OF STATIC POSTURE
Although increasing the duration of a static hold is presented as a way to progress within the basic exercise, there is some concern in regard to the duration that isometric contractions are held. Kell and Bhambhani (13) investigated muscle oxygenation (MOx) during prolonged static posture and found that maintaining a static posture may lead to decreased MOx and increased fatigue—factors that can lead to injury and pain. It is believed that there is a correlation between MOx levels and LBP, which makes it vital that MOx is maintained during an isometric contraction. It has been determined that a 30-second isometric hold will reduce lumbar extensor MOx (22). Thus, muscular rest after a 30-second isometric hold is recommended to maintain adequate MOx levels and to ultimately reduce the risk of injury (13,22). Progressing isometric exercises by simply increasing the duration of the hold may not be the best strategy. Progressions could instead be introduced by increasing the difficulty of the exercise or by increasing the number of repetitions of shorter isometric holds.
MOVEMENT VERSUS MOMENT
Progressing isometric exercises requires an understanding of the difference between a “movement” and “moment.” A flexion movement is a kinematic term and defines the act of bending the spine forward or of flexing the spine. A flexion moment is a kinetic term and is the act of creating a moment or torque. Moments can be created independent of whether or not movement occurs. Pushing an immovable object requires the spine to stiffen with anterior muscle activation to avoid energy leaks or spine movements. Hence, these muscles should be producing a flexion moment without actual movement. For those who want to avoid back injury or pain, enhancing stiffness with flexion moment training while avoiding flexion movements is essential to control and ultimately eliminate the microtraumas that lead to pain (20). With this knowledge, it can be assumed that it is safest to progress core exercises with moments rather than movements. This article will provide examples of progressions of isometric core exercises that challenge the core musculature in each of the 3 principal planes: sagittal, frontal, and transverse.
The following figures show exercises with their corresponding progressions and regressions. Note, all steps needed to perform an exercise are depicted with letters (a, b, c), whereas roman numerals (i, ii, iii) indicate progressions within an exercise (difficulty increases with larger numbers).
ISOMETRIC CORE TRAINING EXAMPLES
SAGITTAL PLANE EXERCISES
One option for regressing the plank (figure 2) includes placing the forearms on a raised surface (i); the higher the surface you use, the greater the regression. The basic form of the plank is performed with the feet on the floor, a level trunk, and the elbows directly under the shoulders (ii). There are many ways to progress this exercise, some of which include walking the forearms slightly forward from the trunk (iii) to increase the moment arm distance or placing the feet on a raised surface (iv).
The starting position for the wheel rollout (figure 3) has the individual kneeling with their hands on the wheel (ia). The movement involves rolling out as far as possible while maintaining stability and a neutral lumbar spine as the moment arm distance is increased (ib), and then returning to the starting position by rolling back. Performing this exercise on the feet rather than the knees will further increase the moment arm distance and the difficulty (iia–iib). If this exercise is too difficult to perform on the wheel, the same concept can be achieved in a more stable position on the hands (not shown).
To perform the dead bug (figure 4), begin by laying supine on the BOSU with the hips and knees flexed at 90° and both arms positioned directly upward (ii). Maintain an abdominal brace and neutral lordotic curvatures of the spine (including neck), and hold this position. To increase the difficulty, slowly extend either one leg or one arm at a time and alternate (iii). Further progressing, this exercise is performed by extending the opposite arm and leg simultaneously (iv). If an individual is not able to maintain stability on the BOSU, these same exercises can be performed on the floor with a towel rolled under the lumbar spine to maintain a neutral lordotic curvature (i).
The mountain climber exercise shown in figure 5 is being performed on a suspension trainer, but it can also be performed on surfaces such as sliders or a slide board. The feet insert into the handles of the suspension trainer, and the body is level with the hands directly under the shoulders (a). Alternately driving one knee toward the chest and returning to the starting position (b–c). Options for progressing this movement might include increasing the speed, raising the height of the suspension trainer, or walking the hands slightly forward away from the body. It is important to ensure that the motion is coming entirely from the hip and there is no spine flexion as the knee comes toward the chest.
The starting position for the bodysaw (figure 6) is a basic plank but with the feet positioned on a sliding surface (a). The movement involves pushing against the floor with the forearms to slide the feet backward, going only as far as the abdominal brace is maintained (b), and then sliding the feet forward to where the shoulders pass over the elbows (c). To progress this exercise, decrease the speed of the movement or further increase the moment arm distance by sliding the feet farther back. This exercise is being performed on a slide board, although it can also be performed on sliders or even a towel on a tiled floor.
The sagittal plane body blade (figure 7) exercise can be performed by holding the blade either vertically or horizontally (not shown) and involves oscillating the blade while maintaining abdominal bracing. This exercise can be progressed by moving the arms further away from the trunk (ii) or by performing the same exercise unilaterally to introduce some twisting forces that will require increased transverse plane muscle recruitment as well (not shown).
FRONTAL PLANE EXERCISES
The most regressed form of the side plank (figure 8) shown is with the knees bent at 90° and stacked, with the elbow directly under the shoulder (i). The elbow can also be placed on a raised surface to regress this exercise using the same principle as was shown in Figure 1 (basic plank). Extending the legs with the feet in a wide stance (ii) is the first progression and can be later followed by stacking the feet (iii) to reduce the base of support. Placing the feet on a raised surface (iv) is another option for progressing this movement. Once higher levels of frontal plane stability are achieved, this exercise can be performed with the supporting arm extended (v) and later with the top leg lifted for the highest progression shown here (vi). Additional ways of progressing this exercise might involve placing a weight on the hip or holding a weight with the top arm extended straight up.
To perform a suitcase carry (figure 9) hold a moderately heavy kettlebell in one hand, and walk with abdominal bracing to avoid any lateral trunk bending (i). For progression, hold the kettlebell bottoms-up at shoulder level, making sure not to rest the arm on the chest (ii). The last progression shown is holding the kettlebell with an extended arm directly above the shoulder (iii). Considerations for altering this exercise include the degree of arm abduction, the weight of the kettlebell, and the duration of walking.
To perform the frontal plane body blade (figure 10) hold the body blade directly in front of the chest and oscillate the blade side to side (i), being sure to maintain abdominal bracing. This exercise can be progressed by moving the arms further away from the trunk (ii) or by performing the same exercise unilaterally as was discussed in Figure 6 (not shown).
TRANSVERSE PLANE EXERCISES
The landmine exercise (figure 11) is generally performed with a barbell that is secured at one end and freely movable at the other. Hold the top end with both hands and stand with the feet shoulder-width apart (ia). Apply abdominal brace and, in a controlled manner, rotate the bar from side to side. This exercise can be performed in 2 forms: (i and ii) allowing the hips and shoulders to rotate and the weight to shift or (iii and iv) keeping the hips and shoulders facing forward and only allowing the arms and bar to rotate with no weight shift. One progression (iib and ivb) would be to extend the arms further from the body. In addition, adding weight to the top end of the barbell can increase the challenge of the exercise.
Figure 12 shows 3 levels of the bird dog exercise. The first level is performed on the hands and knees (i–iii), the second on the feet and elbows (iv–vi), and the third on the feet and hands (vii–ix). The most regressed form of each level starts with alternately extending either one arm or one leg out at a time, and the final progression is to extend the opposite arm and leg at the same time.
To perform the cable press (figure 13) walk out laterally from a cable machine holding a handle with both hands close to the chest and the feet in a comfortable athletic stance (a). Without any movement of the trunk, slowly extend both arms away from the chest. You can begin by only extending your arms slightly away from your chest (b) and progress to extending them as far as possible (c), being sure to maintain the abdominal brace, and then return back to the starting position. This exercise can be further progressed by increasing the weight on the cable machine or by narrowing the stance (i.e., the base of support).
To perform the around the world exercise (figure 14) hold a kettlebell with both hands starting just above the shoulder (ia) and bring it back and around the head in a circular fashion (ib–id). This movement essentially challenges the muscles of the trunk that control motion in all 3 planes but is only effective as long as the abdominal brace is maintained. Progressing this exercise can be performed by extending the arms further away from the body (iia–iid) or by increasing the weight of the kettlebell.
To perform the push up shoulder touch exercise (figure 15) start with the feet slightly wider than the shoulder width and with the hands directly under the shoulders (ia). Keeping a level pelvis, lift one hand to grab the opposite forearm (ib), and then slowly return the hand back down. For regression, widen the feet and bring the hands closer together. Progressing this exercise can be performed by reaching the hand higher up the opposite arm (ii) or by narrowing the feet (not shown).
The chop (figure 16) is performed by kneeling on one knee (knee furthest from cable machine down) next to the cable machine and hold the bar with both hands above the shoulder level (ia). With a neutral spine and abdominal brace, bring the handle down in a diagonal pattern toward the opposite hip (ib–ic); then return to the starting position. To gradually progress, incrementally narrow the stance in half-kneeling until both legs are inline (iia–iic). Performing this exercise in a lunge position or standing will also make it more difficult, as well as increasing the weight on the cable machine.
To perform the lift (figure 17) kneel on one knee next to the cable machine (the knee closest to the cable machine is down), and hold the bar with both hands at the hip level (ia). With a neutral spine and abdominal brace, bring the handle up in a diagonal pattern toward the opposite shoulder (ib–ic). To gradually progress, incrementally narrow the stance in half-kneeling until both legs are in alignment (iia–iic). Performing this exercise in a lunge position or standing will make it more difficult, as well as increasing the weight on the cable machine.
To perform the bear crawl (figure 18) start on your hands and feet with your knees bent, hands directly under the shoulders and knees directly under the hips. Walk forward on your hands and feet, moving your contralateral limbs together (a–c). It is important to maintain level hips and a neutral lordotic spine throughout the movement. This exercise is not limited to just moving forward and backward, but can be performed in any direction. To regress this exercise, take a small contralateral step forward, pause for a moment, and then return back. Lengthening the time hovering between steps will help prepare an individual for locomotive bear crawls.
EXERCISE PROGRESSION MODEL
As stated earlier, it is extremely important that exercise progressions are implemented properly to ensure that the client is continuously moving toward their fitness goals as safely as possible. Figure 1 presents a model proposed by Blanchard and Glasgow (4) for exercise progression that is very general. Therefore, this section will help the exercise professional apply this model to safely and appropriately progress the exercises presented in this article.
It is suggested that the section marked “A” in Figure 1 is the most controlled level of the exercise where the client has the goal of only focusing on intrinsic factors such as joint proprioception. For core training, the goal of this initial exercise could be to stabilize the lumbar spine in a neutral lordotic position under a sagittal plane load such as that presented in the front plank (Figure 2ii). This simple exercise can then be regressed if the client struggles to maintain a neutral lordotic curvature by putting them on an angle (Figure 2i), as this decreases the amount of moment that the core muscles must match in order to maintain spinal position. You can then slowly progress linearly within this exercise by increasing the moment arm distance. This can be performed by slowly decreasing the angle by getting a smaller box for the client to put their forearms on. It can then further be progressed by increasing that moment arm distance by moving your forearms further away from your feet or by elevating the feet. All of these things will linearly alter the core challenge while still keeping the intrinsic focus of this level “A” exercise.
Once the intrinsic factors of the basic “A” exercise are mastered, Blanchard and Glasgow (4) suggest that these exercises could then be progressed by adding an extrinsic factor. Examples of extrinsic factors are the wheel presented in Figure 3, the BOSU ball presented in Figure 4, the suspension trainer straps presented in Figure 5, or the sliders under the feet presented in Figure 6. All of these exercises present an extrinsic factor that adds to the difficulty of the exercise while still maintaining the same basic goal—stabilize the lumbar spine in a neutral lordotic position under a sagittal plane load. Any of these extrinsic factors could be included under the progressions marked “B,” “C,” or “D” in the model and then be progressed/regressed linearly within the same extrinsic factor by altering the moment arm distance or the amount of sagittal plane torque applied to the core muscles. It should be noted that the extrinsic factors presented in Figures 3–6 (wheel, BOSU ball, suspension strap, foot sliders) would most likely never be used in combination, as was suggested by Blanchard and Glasgow (4), as the fourth level of progression in Figure 1. However, all of these exercises are performed in very nonfunctional positions (prone or supine), so Blanchard and Glasgow (4) suggest a further progression would be to add multiple changes in stimulus/environment to the basic challenge. This could be performed in a standing position and using something such as a body blade (Figure 7) to add further challenge to the core. The moment arm distance can then also be altered within this exercise by increasing the distance of the body blade from the core muscles.
After the client has become proficient in maintaining a neutral lordotic lumbar spine under sagittal plane loads and challenges, they can be progressed to frontal plane exercises (Figures 8–10), followed by transverse plane exercise (Figures 11–18). For each of these planes, a new progression model will begin with the basic intrinsic goal “A” of training the proprioceptors to maintain a neutral lumbar spine under a destabilizing load. These exercises can then be progressed by first adding one extrinsic factor, followed by multiple changes in the environment/stimulus. Using the dead bug exercise (Figure 4), the goal of the basic exercise “A” is to maintain a neutral lordotic curvature in a supine position on the floor (Figure 4i). This exercise could be progressed by adding the extrinsic factor “B” of the BOSU ball (ii). Adding limb movement would be modification “C” and could be used while lying on the floor “AC” (not shown) or in combination with the BOSU ball “ABC” (Figure 4iii and iv). The final progression “D” could add another intrinsic factor by having them catch and toss a ball. If they could do this while stabilizing their core on the BOSU and moving their limbs, this would be progression “ABCD.”
The goal of this article was to enhance the understanding of how to progress isometric exercises designed to target the core musculature. It should be noted that the figures provide only limited examples of these types of training exercises, but there are an infinite amount of additional variations and other positions and movements that can accomplish the same goal. Many of the exercises shown also involve unilateral loading of the spine, which can help the exercise professional to identify and correct any asymmetries that may be present in their clients. However, it is important that these exercises are performed bilaterally (on both sides) to ensure even development of stabilizing musculature. As mentioned prior, some variations of the exercises may not be appropriate for all individuals. For example, the plank progression of walking the forearms forward away from the trunk places stress on the shoulder joint and may be aggravating to those with an existing shoulder pathology. Many exercises which require static holds can also be progressed by increasing the time the exercise is held; however, performing several repetitions while holding a position for no more than 30 seconds is advised (13,22). It should also be recognized that some of the exercises presented in this article are performed in relatively nonfunctional exercise positions, thus further progression would involve weight bearing exercises such as squats, lunges, and weightlifting; however, it is imperative that a neutral lordosis is maintained throughout the course of every exercise. If an individual is unable to maintain a desirable spine position during any movement, it should be regressed until the client can develop the appropriate muscular activation strategies. Once they have mastered the exercise in its regressed form, then it can be appropriately progressed and adapted to the needs of the individual using the model presented by Blanchard and Glasgow (4).
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