F. CORE STABILITY: THE SIGNIFICANCE WITH ATHLETIC PERFORMANCE

On reviewing the literature the establishment of a definition of core stability is highly complex. There have been a number of studies completed on the topic of core stability, where a number of approaches have measured different physical attributes such as strength, endurance, body awareness, neuromuscular coordination, fascial line coordination, proprioceptive mechanisms, load absorption, muscular order of recruitment, and transferal of force, where a number of physical components have also been noted to be involved such as; bones, joints, joint capsules, receptive mechanisms, ligaments, muscles, tendons, and fascia (7, 8, 12, 14, 38, 44, 51, 60). This in turn allows professionals to develop a number of different approaches for developing a model for the improvement of core stability and potential enhancement of sports performance.  After review of the literature there is a common notion that if specific “core stability” is enhanced, then there may be an indirect improvement to sports performance. This has been measured a number of times, however only in specific environments, with specific methods, with specific equipment, for specific athletes (8, 41, 44, 49, 51, 54, 60). Therefore an approach to improvement with one method for all athletes is hard to conceive.

Sharrock et al. (54) evaluated the relationship between core stability and athletic performance of female collegiate athletes. In this study the athletes performed one core stability test (double leg lowering), and four performance tests (40 yard dash (speed) / T-test (agility) / vertical jump (power) / medicine ball throw (power). The aim of the study was to derive a relationship between the core stability and performance tests, where they demonstrated a significant relationship exists between the double leg lowering and the medicine ball chest throw test only. They note in their discussion that no known core stability test serves as the “gold standard” for all athletes, where a more appropriate dynamic measure of core stability which mimics complex / explosive / multi-planar movements like those performed in most sports may produce better study results.

Reed et al. (51) performed a systematic review of all known articles pertaining to an association between core stability training on athletic performance measures. They initially found 179 articles in the literature, and culled that list down to 24 key articles relating to age / athletic and sports performance / observational vs. quantitative specific requirements for sufficient comparison needs. They explained some of the challenges of the studies reviewed for areas of research concern such as; core exercise is rarely the sole exercise being performed as it is generally part of a larger training regimen, and many of the studies showing the greatest correlation between testing and performance have a study population of recreationally active students, rather than highly trained competitive athletes. They also mentioned that randomized controlled trials can be difficult to enforce with team sports. Thirteen of the twenty-four studies had a study population of “athletes”, ranging in a number of core training protocols. The general trend of these studies suggested that training tailored to the sport is more successful in significantly improving sport specific measures. They noted that the six studies that incorporated “specifically targeted training techniques” each reported a minimum of one significant improvement in sport specific function. Reed et al. (51) concluded by stating that there is little evidence tying core stability to specific athletic performance, and improvements in general performance are not directly attributable to specific core training alone. They finalised that sports with strong core components (golf / swinging a implement / running) demonstrate the greatest improvements from core training.

As seen above core stability definition and assessment can vary dramatically, and therefore the results of testing under different circumstances may vary considerably. A “Delphi” approach was used by Majewski-Schrage et al. (44), where an amalgamative core stability definition was produced based on the opinions of a number of professionals from a number of different areas in physical sciences. The definition stated that core stability was “the ability to achieve and sustain control of the trunk region at rest and during precise movement”. Based on this definition it is easy to conceive that a number of physical components are at play, where a number of different physical attributes may affect appropriate functioning in a number of environments. Some of the more popular approaches will be discussed to gain an understanding of the total effect that core stability training can have on sports performance.

One of the original and broadly known concepts represented in the literature was the three-system model originally produced by Panjabi (48). Panjabi (48) discussed three physical subsystems which all contribute to spine / core stability. The first is the “passive or osseo-ligamentous” subsystem, which is dynamically active in monitoring spine position. It does not produce spine motion, but produces reactive forces at end ranges of motion that resist spine motion. This subsystem has a large amount of mechanoreceptors, which provide sensory information on position and movement. This information comes from facet joint capsules and ligaments situated in the vertebral column. The second subsystem is termed the “active or muscular” subsystem. It is comprised of the muscles and tissues (tendons and fascial components), which stabilise the core (which also involve the production of intra-abdominal pressure), and produce and transfer force between the thoracic and pelvic regions. The third subsystem termed the “neural” subsystem is responsible for monitoring and adjusting muscle forces, where the requirements for stability can change instantaneously based on postural adjustments and external loads accepted by the body (48). It is said that all three systems work in an inter-related fashion, where numerous factors listed can affect the proper function of the entire system.

Diagram 2. Adapted Model of Core stability. Panjabi (39)

The Panjabi (48) model of core stability has also been re-adapted by Bergmark (5). Bergmark (5) discussed a broader approach to the second subsystem listed by Panjabi (39), where he divided the active muscular subsystem into two components or groups. He stated that there was a local group of muscles, which had a primary role of stabilising the core, and a global group of muscles, which had a role of transferring force through the body and enhancing intra-abdominal pressure. It has been noted that these two groups have been used previously to provide ‘supposed’ isolated prescription, where there is the local group known to be the smaller / deeper muscles responsible for spine stability, and the global group primarily involved in force production. Willardson (60) noted through extensive research that this was an ineffective strategy for exercise prescription as the two groups were not independent from each other; rather they were heavily interconnected with multiple muscles having both local and global functions also via their tendons, and fascial regions. A diagrammatic example is shown below explaining the interconnected process, commonly known as the “abdominal wall”. It is an interconnected cross-sectional “ring” or “hoop” of muscles around the trunk. Emphasis should be placed on the fact that the muscles are all contained within the same interconnected fascia (dark region). This demonstrates that when one or more muscles contract there is a resultant effect across all connected parts, whether for stability or prime moving function (9).

Diagram 3. Superior Cross-Sectional View of the Abdominal Wall. Brukner et al. (9)

The “serape effect” takes force production through the body one step further.  Muscle co-activation in the upper and lower extremities is integrated through the fascial system termed the “serape effect” (14). Myers (45) well known for his extensive work on the study of myofascial lines, identifies the use of force production through the interconnected mechanical lines of pull of corresponding muscles. Myers (45) terms the serape effect using “functional lines”, which give power and precision to the movements of the limbs by increasing the length of the lever arm via kinetic chains linking them across the body to the opposite limb from the shoulder to the pelvic girdle and vice versa. The muscles associated on the lines have both stabilising and force producing effects being in both deep and superficial regions. All functional movement, which the body produces, is said to occur through the use of the serape effect across fascial and or functional lines (45). A myriad of soft tissues work in unison to develop momentous power via force transferal throughout the body (45).

Diagram 4. Front Functional Line in Tennis Serve. Myers (45)

Efficient movement development must consider the concepts of core stability. Research has shown that the topic itself can be highly controversial and situation specific. The strength and conditioning coach must adjust the training application to suit the needs of the individual athlete, to apply the differing theories of core stability models.