'Core' Strength & Posture

Athletic Woman With Kettlebells

Developing and Enhancing the 'Core'

To accurately and effectively prescribe exercises that target the structural anatomy, it is important to define the anatomical components that are typically termed the 'core'. Additionally, it is also essential to understand and recognise the role that core musculature has in relation to generating efficent and powerful movements. Therefore, the anatomical core has been defined by Willardson (2013) as the trunk region, which includes segments of the skeletal system [i.e vertebral column, thoracic cage, pelvic girdle and shoulder girdle], associated passive tissues (ligaments and cartilage), and the active muscles that cause control or prevent motion of the body. Furthermore, the nervous system regulates the relative activation of these muscles, and exercises should involve muscles that are required during the performance of sports specific skills.

In this regard, the term 'core' has been used by fitness professionals in conjunction with the term 'functional' (Boyle, 2004). The 'functional' term is used with specific reference to exercise activities that are relevant to task performance or the transferability towards the sports skill. However, the functionality of an exercise is often subjective, with exercises that involve the actions of the upper and lower body possessing greater transferability and also functionality.

In the popular media, the term 'core' is often used within commercial marketing to promote an exercise device or training method that predominately 'targets' the abdominal muscles. In such marketing schemes, the main focus is on the potential aesthetic benefits rather than the performance or functional benefits. This then creates a scientific need for greater scientific objectivity in the methods used to effectively develop the core muscles, with less emphasis being placed on exercises that predominantly focus on aesthetic benefits that has limited transferability to dynamic movements. The integration of total body segmentation centred exercises that involve multiple muscle groups may facilitate greater transferability. These exercises [as discussed later] require the muscles to shorten or lengthen to cause or control movement or isometric actions of the 'core' [no muscle action occurs but muscles are under tension loading]. Furthermore, these types of exercises are generally performed in a standing or task related posture and possess similar kinematic and kinetic characteristics to sport or task related skills.

However, total body exercises that train the 'core' muscles are only one component of a conditioning programme, and these exercises should be based upon the individuals needs of the performer.

  

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Figure 1. The anatomical core

Defining the Anatomical Core    

Attempting to define the anatomical core is problematic due to the inconsistency within scientific publications, with varying definitions based on the field of study (Willson et al. 2005).   Additionally, the term 'core exercises' further complicates the definitions due to the different physical development settings. For example, [1] exercises that are integrated within a resistance training programme including the back squat, deadlifts and standing overhead presses are different to exercises [2] that target the muscles that develop spinal stability or improve torque transfer from the lower to the upper extremities. This improved torque transfer is essential for throwing a javelin as the lower extremity and the ‘core’ muscles work together.  

The ability to throw an object with high velocity is not merely dependant on the muscles of the athlete's arm. Rather, the torque and angular velocity is progressively developed from the lower extremities up through the ‘core’ and ultimately through the athlete's arm as the objective is released. In addition to this is the timing of joint movements that efficiently transfer torque and angular velocity from the lower to the upper extremities. Thus, the core is comparable to a bridge that forms a passageway between the lower and upper extremities. The ‘core’ muscles must be physically conditioned to generate appropriate spinal stability while allowing for efficient dynamic transfer of torque and angular velocity. An important consideration is that there is a crossover as certain resistance exercises (standing overhead press, back squat, and overhead squat) require isometric and dynamic actions of several core muscles including the erector spinae group and the gluteal complex. 

For this page, ‘core’ exercise will be defined as any exercise that stimulates neuromuscular recruitment patterns to ensure a stable spine while permitting efficient and powerful movements (McGill et al., 2003). Therefore, by this definition, ‘core’ stability is the contribution of the passive and active tissues that create the optimal combination of spinal stability and movement capability (Panjabi, 1992).
 

 
Image by Roman Yusupov

Functional ‘Core’??

Passive Tissues


In the gymnasiums, the term ‘core’ is predominantly related to only a few muscle groups, specifically the abdominals. However, it is important to be aware that other passive tissues including bones, cartilage and ligaments play a significant role. For example, the skeleton provides the structural framework of the body and is a mechanism (via a system of levers) that permits, controls or prevents motion through the neurologically regulated production of muscular torque. The muscular-skeletal system is comprised of bones that are attached by ligaments at the joints. These joints function as axes around which opposing muscular and gradational torques act. Essentially the ‘core’ of the body is stabilised via muscular tension, which allows an effective foundation for forceful and powerful dynamic actions of the upper and lower extremities. 

The skeletal elements of the anatomical ‘core’ include bones of the pelvic girdle, consisting of the left and right hip bones (os coxae) and sacrum. The pelvic girdle is attached to the torso at the sacroiliac joints, and the lower extremities are connected to the pelvic girdle at the hip joints (Figure 2). Thus, the anatomical core denotes the kinetic association through which torque and angular velocity are transferred from the lower to the upper extremities. 

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Figure 2. The vertebrae column 

The vertebral column comprises of 33 vertebrae including 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused), and 4 coccygeal (fused). Therefore, there are 24 movable vertebral segments (C1-to-L5), with the greatest movement capability in the cervical and lumbar regions due to the changes in the orientation of the facet joints at the cervicothoracic (C7-to-T1) and thoracolumbar (T12-L1) junctions. Movement of the vertebrae column includes flexion and extension in the sagittal plane, lateral flexion and reduction in the frontal plane, and rotation in the transverse plane. 

Movement terminology is often led by the terms lumbar or trunk to denote the primary region of movement. For example, when an athlete performs an abdominal crunch the action involves lumbar flexion and performing a medicine ball throw involves lumbar rotation. However, Floyd (2009) indicates that core movements represent the result of several smaller-scale actions occurring at multiple facet joints between the vertebrae. 

When studying the facet joints between the vertebrae, about one-to-two degrees of movement in each plane (sagittal, frontal, and transverse) is possible without passive resistance from the ligaments) and intervertebral discs. This unrestricted range of movement is termed by McGill (2007) as the neutral zone. The ability to maintain the lumbar spine within the neutral zone during resistance training is suggested to prevent disproportionate stress on the passive tissues and facilitate activation of the ‘core muscles. The bracing of the vertebral column via muscular tension is central to maintaining the neutral zone and maximising spinal stability (Panjabi 1992).

The maintenance of spinal stability under various loads and postures is determined on the athlete's ability to maintain the lumbar spine within the neutral zone. With the lumbar spine neutral, the muscles can efficiently provide the majority of stabilising support. Equally, when the lumbar spine is outside the neutral zone (i.e. in a flexed posture) the spinal extensor muscles are neurologically inhibited from developing tension with the passive tissues (cartilage, ligaments, facet joints) providing the majority of the stabilising support, which increases the risk of injury to these structures.

When we consider the passive tissues in isolation it should be noted that they have limited ability to stabilise the spine. For example, Cholewicki, McGill, and Norman (1991a) reported that the lumbar portion of the spine without muscular support would collapse under a compressive load of approximately 9 kg. This is not sufficient to support body weight, let alone the additional loads incorporated during resistance training, sports skills, and physical activities. Therefore, the activation of the core muscles is vital to meet spinal stability requirements during the performance of all physical activities.
       

'Core' Muscles

The muscles provide the torque necessary to cause movement (e.g., concentric muscle actions), to control movement (e.g., eccentric muscle actions), or to prevent movement (e.g., isometric muscle actions). In addition to the abdominal muscles, several other muscles are considered part of the core and provide stabilising stiffness and dynamic movement functions. A key point is that there is not a single most important core muscle that fulfils these functions in all static postures and movement scenarios.

Muscles are required to provide the torque necessary to generate, control or prevent movement. In addition to the abdominal muscles, many other muscles are considered part of the ‘core’ and pro-vide stabilising rigidity and dynamic movement actions. Unfortunately, the transversus abdominis has been consigned as being the most significant spinal stabiliser This incorrect concept originated from research that showed that the transversus abdominis was the first ‘core’ muscle activated before an arm-raising task (Hodges and Richardson 1997). However, this study was restricted to assessing one simple movement task. More complex movement tasks emphasise different activation patterns for the ‘core’ muscles, depending on posture, external loads, and breathing actions. Because of this, fitness instructors should consider the importance of any ‘core’ muscle as being task-specific, and the relative meaning can change directly (Arokoski et al. 2001; McGill 2001; McGill et al. 2003). There is an unending range of postures and external loads that act through the force of gravity to create resistive loads on the spine and associated ligaments, facet joints, and discs. To preserve spinal stability, these resistive loads must be offset with equal and opposite muscular actions.

Different ‘core’ muscles possess fibres that are aligned in varying directions that produce ample spinal stability through simultaneous activation of antagonistic muscles on either side of the trunk while allowing for spinal motion (if necessary). Therefore, the best approach for developing the ‘core’ muscles is through a variety of different exercises that involve a combination of stabilising (e.g., isometric muscle actions) and dynamic actions. The functional significance of each ‘core ‘muscle is determined by the cross-sectional area, fibre alignment, and immediate stabilising or dynamic actions. For example, the longissimus and iliocostalis of the erector spinae group extend several vertebral divisions and possess large moment arms, making them suitable for large torque production for trunk extension. Since muscular torque is equal to the product of muscular force and the moment arm, a large moment arm increases the potential spinal stabilising and movement production functions of a muscle because it increases the amount of muscular torque that can be produced. For example, during the implementation of a Romanian deadlift, the longissimus and iliocostalis (Figure 3) perform isometrically to fix the pelvic girdle in an anterior tilt, which allows the gluteus maximus and hamstring muscles to control the alternating extension and flexion actions of the hips. The accurate visual cue when instructing this exercise would be for the individual to create a “hinge” at the hips.
 

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Figure 3 . Muscles of the erector spinae group. 


Equally, other ‘core’ muscles (e.g., rotatores, intertransversalis, interspinalis) have various proprioceptors, making them suitable for detecting rotation of specific intervertebral facet joints (McGill 2007). These muscles primary function is position transducers that enable activation of larger superficially located muscles to meet spinal stabilising demands. Additionally, other ‘core’ muscles are suited for transferring torque and angular velocity from the trunk to the extremities. Therefore, the ‘core’ muscles can be separated into three classifications: (1) global core stabilisers, (2) local core stabilisers, and (3) upper and lower extremity core limb transfer muscles (Table 1-to-4).

Table 1. Global Core Stabilisers

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Table 2. Local Core Stabilisers

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Table 3. Upper Extremity Core Limb Transfer Muscles

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Table 4. Lower Extremity Core Limb Transfer Muscles

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Applied Standpoint

 

From an applied standpoint, the local core stabilisers cannot be independently trained from the global core stabilisers. A previous study by Cholewicki and Van Vliet (2002) assessed the relative input of several’ core’ muscles to lumbar spine stability during seated (i.e., trunk flexion, trunk extension, lateral trunk flexion, trunk rotation) and standing (trunk vertical loading, trunk flexed 45 degrees while holding a weight) isometric tasks. Muscle activity was evaluated in the rectus abdominis, external and internal oblique abdominis, latissimus dorsi, erector spinae, multifidus, psoas, and quadratus lumborum. The central finding was that several different muscles contributed to lumbar spine stability depending on the direction and magnitude of the load. Additional, no single muscle group contributed more than 30 per cent to lumbar spine stability, regardless of the physical activity task. However, removal of the erector spinae contribution (global core stabiliser) resulted in the largest reduction in lumbar spine stability during each physical task.
 
A further study by Arokoski et al., (2001) compared rectus abdominis, external oblique abdominis, longissimus thoracis, and multifidus muscle activity during 16 physical tasks performed in prone, supine, seated, and standing postures. The main finding was that the multifidus (local core stabiliser) and longissimus of the erector spinae group (global core stabiliser) demonstrated similar activity patterns and simultaneous function.  Thus, both local and global ‘core’ muscles are necessary for producing adequate spinal stability for complex movement tasks. 

Thus, the often encouraged notion that the local core muscles are the most important for spinal stability is inappropriate. With reference to different spinal stabilising techniques, abdominal hollowing has often been practised in rehabilitation programs (Richardson and Jull 1995). Abdominal hollow-ing emphasises the activation of the transversus abdominis to pull the abdominal wall posteriorly toward the vertebral column. This manoeuvre is also often performed in a somewhat non-functional position (Figure 4). A second commonly performed stabilising exercise is termed abdominal bracing. Abdominal bracing has been reported to be superior to abdominal hollowing due to the co-contraction of the abdominal muscles. Abdominal bracing involves a conscious emphasis on maintaining tension in the abdominal muscles. A study by Grenier and McGill (2007) reported 32 per cent less stability with abdominal hollowing resulted than abdominal bracing. This was caused by reductions in the moment arm for the internal and external obliques and rectus abdominis as the abdominal wall was pulled posteriorly.  

 

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Figure 4 . Abdominal Hollowing 

The abdominal bracing technique also produces intra-abdominal pressure, which further contributes to spinal stability due to increasing compressive force between adjacent vertebrae (Cholewicki, Juluru, and McGill 1999; Cholewicki et al. 1999). The abdominal cavity is surrounded by the ‘core’ muscles with an abdominal hoop forming the walls with the diaphragm forming the ceiling, and the pelvic floor muscles forming the floor. In particular, the abdominal hoop is created by fascial connections between the rectus abdominis anteriorly, external oblique abdominis, internal oblique abdominis, transversus abdominis, and the lumbodorsal fascia posteriorly (Figure 5). The lumbodorsal fascia is similar to an external weightlifting belt by providing spinal stabilising support and contributing to the transfer of torque and angular velocity during the performance of sports skills (McGill 2007).  For example, the latissimus dorsi originates on the lumbar vertebrae and pelvic girdle via the lumbodorsal fascia and inserts on the humerus. During the windup stage of a baseball pitch, the latissimus dorsi transfers torque and angular velocity from the trunk to the upper extremities. The series of ‘core’ muscle activation that enables the “guiding” of torque and angular velocity between segments of the body is regulated by the nervous system.

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Figure 5. Thoracolumbar Fascia

Neural Activation and Integration

The nervous system regulates the specific pattern and intensity of ‘core’ muscle activation to stabilise the spine, and it also enables the transfer of torque and angular velocity between various skeletal sections. The nervous system coordinates a seamlessly integrated steering of muscular torque through the skeletal kinetic chain, enabling efficient and powerful movement patterns. The optimum performance of motor skills is not dependent on absolute muscular torque production. Absolute muscular torque production is not appropriate without the neurologically coordinated steering of torque that enables precise storage and recovery of muscular elasticity. Skeletal muscles possess an elastic property that allows for the storage and recovery of energy. The contractile force of the muscles is heightened through the elastic recoil of the muscles in the performance of sports skills. However, the ability to couple this elastic recoil is dependent on movement efficiency. In other words, technique is more important than absolute strength for successful sports performance (see stretch training and plyometric pages).

This is why training muscles in isolation does not necessarily transfer to improved sports performance. Resistance training for dynamic sports must include ground-based movements that incorporate multiple muscles groups focusing on stabilising and dynamic functions. With inclusion of ground-based movements, there is a greater prospect of a positive transfer between resistance train-ing movements and sports skill performance. This is because the central nervous system receives sensory feedback from proprioceptors regarding muscle length, muscle tension, joint position, and the rate of joint rotation (Holm, Indahl, and Solomonow 2002). An important point is that the nervous system must instantaneously meet spinal stability and breathing requirements. The rhythmic action of breathing may impair spinal stability through the temporary relaxation of the ‘core’ muscles. This is why during maximal lifts, breathing may briefly cease altogether with the Valsalva manoeuvre, whereas weightlifters attempt to exhale against a closed airway. For healthy individuals with no contraindications (i.e. high blood pressure)  this manoeuvre can be advantageous by in-creasing intra-abdominal pressure and therefore increase the compressive forces between adjacent vertebrae to preserve spinal stability.

However, during most training settings, continual submaximal torque production requires the balancing of breathing and ‘core’ muscle activation to meet spinal stability requirements. Traditionally, the instruction for breathing has been to inhale during the lowering phase and exhale during the lifting phase. However, breathing during exertion rarely includes such a coordinated pattern. Therefore, strength and conditioning coaches should instruct athletes to breathe freely while concentrating on the maintenance of constant tension within the core muscles. As the prescription of resistance exercises advances from simple to complex movement patterns, the nervous system adapts to effectively meet breathing and spinal stability requirements.


The specific combination and intensity of core muscle activation during the execution of any given task is dependent on both feed-forward and feedback mechanisms (Nouillot, Bouisset, and Do 1992). Feed-forward mechanisms involve the anticipatory activation of the core musculature, based on muscle memory from prior performance (Nouillot, Bouisset, and Do 1992). Feedback mechanisms play a role as sports skills are repeatedly practised and refined; the nervous system stores sensory feedback regarding the appropriate combination and intensity of core muscle activation necessary to create sufficient spinal stability and also enable efficient movement.

The intervertebral discs, vertebral ligaments, and facet joint capsules have multiple proprioceptors such as free nerve endings that transmit sensory feedback to the central nervous system regarding position and movement of the vertebral column. This sensory feedback is crucial to stimulate specific neural recruitment patterns of the ‘core’ muscles to meet task demands. During performance of any given physical task, the ‘core’ musculature must be stimulated adequately to produce a stable spine, while not constraining movement. Therefore, a balance exists between rigidity and mobility; the nervous system regulates the activation of the core musculature to allow for sufficient stiffness without compromising movement capability (McGill 2006). Through proper movement training, athletes can enhance the regulation of core muscle activation to improve performance.
 

 
 
 

Assessing Functionality of the ‘Core’

In the popular press and fitness media, core strength and core stability are frequently used interchangeably, but it is important to note that the two are not similar. For example, Kibler, Press, and Sciascia (2006) defined core stability as “the ability to control the position and motion of the trunk over the pelvis to allow optimum production, transfer and control of force and motion to the terminal segment in integrated athletic activities”. This is different from muscle strength which is typically defined as the maximum force output produced by a muscle or group of muscles. Akuthota and Nadler (2004) defined core strength as “spinal muscular control to maintain functional stability. 

With so much misunderstanding regarding the definition of core strength and stability, it can be difficult to measure it. It is important to understand that the ‘core’ itself incorporates several muscle groups with multiple functions. Bergmark (1989) classified the muscles of the ‘core’ as either local or global. Local muscles are referred to as deep muscles with insertion or origin at the spine, and their role is to maintain spinal stability. Global muscles control the external forces on the spine, decreasing the tension on the local muscles. Kibler, Press, and Sciascia (2006) identified several muscle groups of the ‘core’ that includes (but not be limited to) the rectus abdominis, internal and external obliques, transversus abdominis, and erector spinae. It may be these types of definitions that make it difficult to state what core strength and stability are. Regardless of the definition or location used to classify the core, it maintains the stability of the spine in a neutral position during the movement of the extremities (Willson et al. 2005; Bliss and Teeple 2005). Given the volume of research that has been performed on the ‘core’, there appears to be no standardised definition (Hibbs et al. 2008) or means of assessment.

 

Core assessment may include measures of flexibility for the trunk, functional balance, and several forms of trunk strength testing, mainly to establish a connection between the ‘core’ and the risk for injury (Claiborne et al. 2006), especially to the low back (McGill, Childs, and Liebenson 1999). Because the ‘core’ muscles are responsible for spine stability, the examination of the core musculature must be performed with caution due to the possible injury to the spine. Willson et al., (2005) stated that three variables contribute to core stability: [1] intra-abdominal pressure; [2] spinal compressive forces; and [3] hip and trunk muscle stiffness.

Muscular 'core' assessment can be static (isometric) or dynamic. Static 'core' assessment requires individuals to hold a position for a period of time with no movement of the body. This practice is simple to employ and can be completed by individuals of various fitness levels, but it is most appropriate for people who are less physically active. Dynamic 'core' assessment involves movement of the body and is most appropriate for those at a higher level of physical fitness. Dynamic testing usually includes the use of an instrument or special equipment. 
 

Isometric Maximum Strength Testing

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Maximum isometric strength testing of the ‘core’ can be accomplished with a simple handheld dynamometer as described by Magnusson et al. (1995). Trunk flexion isometric strength is assessed while the subject is in a supine position on a treatment table. The dynamometer is then secured with a strap between the subjects upper body and the treatment table. The subject then flexes upward with maximum effort, measuring maximum force production of the anterior core muscles. Trunk extension is assessed the same as trunk flexion except the subject is in a prone position on a treatment table and extends with maximum effort, measuring maximum force production of the posterior core muscles. Isometric strength testing of the core is easy to complete, and handheld dynamometers are inexpensive. The issue with isometric testing is that it can measure only one joint angle at a time, and it must be replicated exactly for test reliability.

Isometric Muscle Endurance

Isometric muscle endurance examinations are another means of testing the core. McGill, Childs, and Liebenson (1999) designed a frequently used core assessment that involves holding one of four postural positions for a while. Position one is a modified Biering-Sorensen test (i.e. back extension). In a prone position, the subject extends the upper body beyond the edge of a table or bench and remains parallel to the floor for as long as possible while the feet are secured (Figure 7). This position assesses the erector spinae muscles of the lower back. The second position assesses the hip flexors and abdominal region. The body is in a supine position with knees bent, feet flat on the floor, and the upper body resting on a block at 60 degrees of hip flexion. When the subject is ready, the block is removed and the subject holds the position for as long as possible with the arms across the chest. The third and fourth tests are lateral planks. The subject lies on the right or left side, supporting the body on the elbow of that side; the hip is elevated into the air; and the feet rest on the floor, heel to toe, with the top foot in front of the bottom foot (Figure 8). This position is held for as long as possible.
 

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Figure 7.  Biering-Sorensen test

As soon as the physical form is broken for any of these tests, time is stopped and recorded. A second muscle endurance test is the prone bridge, which measures both the posterior and the anterior core (Bliss and Teeple 2005). In a prone position resting on the elbows and toes, the participant maintains a neutral hip position and holds this position for as long as possible. Elbow and shoulder fatigue can sometimes develop before the core fails, and thus the true capabilities of the core are not assessed. Similar to isometric strength testing, these tests assess only the muscle in one particular joint position. Since core stability can also be dynamic, isometric testing for strength or endurance may not be a true assessment of the functional stability of the core musculature.

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Figure 8.  lateral plank test

Isoinertial Strength Testing

Isoinertial strength testing evaluates muscle force output at a constant resistance. Resistance training with free weights is considered isoinertial since the amount of weight used does not change throughout the range of motion of the exercise. The two common isoinertial tests currently used to evaluate are the curl-up test and an extensor endurance test.  The curl-up test requires subjects to complete a maximum number of curl-ups at a tempo rate of 45 beats per minute (Willson et al., 2005). The extensor endurance test (Moreland et al., 1997) requires the subjects to complete a maximum number of back extensions at the same tempo rate as the curl-up test while lying prone on a foam wedge. Both these tests are simple to implement and evaluate subjects ‘core’ muscle endurance rather than muscular strength. 

Another commonly performed isoinertial test is the rotational ‘core’ that was developed by Andre et al., (2012). This test uses a pulley system and weight stack rather than an isokinetic dynamometer. Subjects sit on a 50-cm box in front of a pulley trainer. To start the subjects extend their arms in the direction of the pulley trainer and rotate 180 degrees forcefully until their arms are facing away from the pulley trainer.  Resistance is set at 9 per cent, 12 per cent, and 15 per cent of subjects body weight. One set of three repetitions is completed at each resistance loading. Watts are measured with the use of a dynamometer attached to the pulley trainer.
 

Assessing the 'Cores' Functionality

Several functional assessment tests evaluate the core, unfortunately, they do not directly assess the core but only infer whether the ‘core’ is weak or strong based on how well the subjects complete the activity. For example, the Star Excursion Balance Test (SEBT) developed by Bliss and Teeple (2005) requires the instructor to lay out two sets of lines on the floor. The first set of lines run perpendicular to each other. The second set of lines run at a 45-degree angle to the first set. Subjects stand on their dominant leg where both sets of lines intersect and reach out in each direction with their non-dominant leg as far as possible without touching the floor (Figure 9). The furthest distance extended with the toe in each direction is logged. This type of assessment is normally used to determine the effectiveness of a training protocol or rehabilitation. 

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Figure 9.  Star Excursion Balance Test

Another functional core test is the single-leg squat test (Kibler, Press, and Sciascia 2006). The subjects are required to perform repeated partial squats to 45 degrees or 60 degrees of knee flexion. The movement of the subject is analyzed, particularly knee position (valgus and varus), using motion analysis software. Instructors should note that the knee should track the foot and if there is any deviation it can suggest a problem with muscle activation and force transfer through the core. 

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Figure 10.  Single-leg squat test

Other Assessments

The Sahrmann core stabilising test (Stanton, Reaburn, and Humphries 2004) requires the subjects to lie in a supine position with the knees bent and the feet flat on the floor. A pressure biofeedback device (PBD) is positioned under their lower back, and the PBD is inflated to a pressure of 40 mm Hg. The subject is then required to complete a series of leg-lifting exercises (Table 5) while not altering the pressure in the cuff by more than 10 mm Hg. A reading greater or less than 10 mm Hg suggests a loss of lumbopelvic stability.

Table 5. Sahrmann core stability test

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Another method of core assessment was developed by Liemohn and colleagues. This test is comparable to Sahrmann, Liemohn and colleagues (Liemohn et al., 2010) who measured core stability while subjects raised one or more limbs into the air. However, subjects were required to be in a kneeling, quadruped, or bridge position on a wobble board. For intervals of 30 seconds, subjects would have to attempt to maintain balance while alternately raising an arm in time with a metronome set at either 40 or 60 beats per minute. Any deviation in balance outside a 10-degree arc (± 5 degrees from centre) was recorded in seconds for the total time the subject was out of balance.

Assessing Muscular Power of the ‘Core’

Tests that have sought to determine core power have applied some type of medicine ball throw (Shinkle et al., 2012).  Shinkle et al., (2012) developed a test that would have subjects complete a series of static and dynamic medicine ball throws from a seated position on a bench. Four throws were completed including a forward throw, a backward throw, and lateral throws to the right and the left using a 2.7 kg medicine ball.  The upper body was stationary for the static throws in which to prevent the core muscles from contributing to the throw. For the dynamic throws, the upper body was free to move, allowing the core muscles to contribute. The feet were not secured during any of the throws. Maximum distance for each throw was recorded. Shinkle and colleagues suggested that the differences between the static and dynamic throws were due to the contribution of the ‘core’. 

Cowley and Swensen (2008) developed another test that required subjects to complete the forward throw of the medicine ball. The throw was performed with subjects sitting on a mat, knees bent at 90 degrees and feet shoulder-width apart. To perform the forward throw, the subject kept their elbows extended, “cradled” the ball with the hands, and leaned back into a supine position. When the subject was ready they contract the abdominals and hip flexors, moving their upper body upward with the arms extended overhead. The shoulders of the subject were not allowed to extend. Maximum throw distance was measured for all throws in each study.

Key Points and Considerations

There several static and dynamic tests available that assess the core. However, the type of test selected is dependent upon the subjects needs and the availability of equipment. Moreover, the form of assessment should be as specific as possible to the sport or activity. For example, isometric testing of the core is appropriate for individuals of all physical fitness levels. This mode of testing does not require special equipment, and it is the most widely used. 

Conversely, the results of isometric testing are difficult to apply to any movement-based activity. For individuals with a higher degree of physical fitness or who participate in sports, dynamic testing would be the preferred mode. The test selection is ultimately dependent on the activity of the person. For example, if trunk rotation is a principal movement required of the activity, then selecting an assessment that involves rotation of the core would be preferred (e.g., medicine ball throw). If the key movement involves flexion or extension, then the assessment tool would assess those movements. For each individual, the dynamic assessment tool should be chosen by the movement requirements of the activity or the sports skill set.