B. MOTOR CONTROL & MOTOR SKILL DEVELOPMENT

The significance of motor control and motor skill development for continual enhancement of sport specific performance has been emphasised by a number of authors (3, 6, 17, 30, 38, 43, 58, 59). The practical application of exercise and sports movement advancement is seen to be in cohesion with improvements in skill acquisition for a number of reasons.

An initial understanding of skill acquisition components is necessary. A definition of a motor skill as stated by Magill (43) is “a skill that requires voluntary body and/or limb movement to achieve its goal”. Magill (43) also states a definition of a movement as being “a behavioural characteristic of specific limbs or a combination of limbs that are component parts of an action or motor skill”.

When a number of movements are combined they comprise a particular complex motor skill. Magill (43) emphasises the fact that two individuals may have a different specific ability in a number of movements yet they are both able to perform a high level of skill in a sports specific action. An example may be a golf swing, a walking pattern or a tennis serve. Athletes will use a different combination of movement strategies based on their individual strengths and weaknesses across a range of complex physical abilities to produce an action, emphasising the requirement of a coach to understand that skills and sporting actions are highly complex and may be affected by many movement variables. Magill (43) mentioned that movement performed to throw a ball may differ between individuals but reach the same objective, where different characteristics between individuals could be attributed to physical features that limit or enhance skill performance. 

The significance of this explanation is the plethora of noted functional components, which can and will affect an individual’s movement quality. This further emphasises the fact that exercise prescription is required to be based around movement competencies, as prescription emphasising only individual muscle force is not considering a range of factors effecting a movement or sporting action (13, 17, 37, 43, 58, 59).

Individual differences may be derived from specific characteristics of the body, limb, and/or muscle activity, such as kinetic, kinematic, and electromyographic measures (43). Kreighbaum et al. (37) noted that a skilled performer develops the ability to synchronize motor unit firing at an appropriate instant to produce a well-timed forceful movement. Kreighbaum et al. (37) also stated that an unskilled performer may have unconventional erratic motor unit recruitment behavior and subsequently lose coordinated movement.

Voight et al. (59) presented a specific explanation of motor skill acquisition where it was stated that training to enhance rehabilitative protocols and sports specific abilities should initiate with a training program of simple activities and progress to more highly complex motor skills requiring a refinement of neuromuscular mechanisms including proprioceptive and kinesthetic awareness. They stated the importance of the clinician establishing an understanding of the way in which the central nervous system influences motor control. They noted that a myriad of mechanoreceptors offer a reflexive facilitation or inhibition function for motor neurons. This mechanism is responsible for antagonistic and synergistic patterns of muscle contraction.

Tompsett et al. (58) reported findings that noted movement characteristics for skills may be kinetic or kinematic variables such as velocity, displacement, joint angle, torque, or electromyographical characteristics. Tompsett et al. (58) noted that motor skills could be compartmentalised into abilities required for successful skill delivery or sporting skill such as speed of movement, manual dexterity, reaction time, control precision, and multi-limb coordination.

Cook (17) offered an explanation into a range of factors that are involved with the way that movement is produced. Cook (17) discussed mechanisms of joint function with the joint capsule, where soft tissues and surrounding muscles offer stability, and the larger muscles around the joint produce movement. Cook (17) noted there is an extremely complex communication system around the joint, where all structures work together, monitoring and adapting to tension and load, as well as speed of direction and position. An array of tissues combine to offer sensory information to allow for proper movement, where all this occurs as reflexive activity, which does not require conscious thought. The joint and muscles function automatically and in unison to make movement efficient. Cook (17) also stated that when the body does not function optimally, through tightness / weakness / instability / stiffness, the sensory information is changed where automatic reactions are distorted. This can decrease performance, increase fatigue, and expose the body to unnecessary stress.

Chek (13) provided a view following the notion of complexity of movement also presented by Cook (17). He noted that to ensure joint, tendon, and muscle safety, the body has a system of neuromuscular and neuromechanical receptors located throughout joints, tendons, and muscles. If the exercises used in the training environment neglect to properly prepare the static and dynamic stabiliser systems, faulty joint motion during standing functional exercises is almost inevitable.

Aside from pure physical characteristics, it should also be noted that numerous professionals stated that other variables might affect an individual’s movement quality. Overdorf (47) listed a number of citations that stated ‘perception’ of competence as being a major factor for continual engagement in physical activity. Sporting movement success occurs with training overtime, where perception and confidence in movement ability lead to further engagement in motor skill practice. Tompsett et al. (58) noted that major contributors to physical literacy may be frustration and fear of failure with competitive environments creating avoidance strategies with children. Tompsett et al. (58) also noted “motor development is also influenced by exposure to movement demands, adequate instruction, and social and genetic factors”.

The above evidence demonstrates the complexity of sports actions and the factors that can effect ideal movement execution and FMD. Development of force, timing of multiple muscle actions at differing degrees over different joints, soft tissue components, receptive feedback mechanisms, positional awareness, and individual psychology are a range of factors which must be considered to have a potential effect on an individual’s movement quality.

HOW MOVEMENT SCREENING MAY LEAD TO DECREASED INJURY INCIDENCE / GREATER EFFICIENCY OF MOVEMENT

Numerous movement screening methods have been identified, where there is a common trend in the literature demonstrating that movement screening is of importance in the potential prediction of injury and has the capability in some instances to identify performers in sport with enhanced movement scores leading to improved sports play (11, 18, 25, 34, 35, 38, 39).

Giles (29) discussed the implementation of a physical competence screening where a 20-test battery outlined both the functional and athletic competence variables of athletes. He stated that running / jumping / kicking activities may be severely impeded if physical limitations seen in the hip and lower limb chain are allowed to influence learning. He also noted that a lack of physical competence in these screening areas would limit sports skill development.

Kritz (38) conducted a study to attempt to identify whether descriptive movement competency screening (MCS) scores could predict physical performance or injury over one year. Screening / testing data was taken from 91 New Zealand national level athletes. Physical performance measures of sprinting, counter movement jumps, standing static jumps, and clap push ups on a force platform were taken four times throughout the year. Injury status was recorded over the year via an online data collection system.  Evidence indicated in the study that a lower body MCS score may moderately predict lower body power for females, and a trunk MCS score may predict with trunk injury in all participants with moderate to high accuracy.

The larger body of recent evidence has a strong focus on the functional movement screen (FMS). Cook et al. (18) discussed research findings of the functional movement screen (FMS). They stated that research using this method determined that athletes who scored below the benchmark cut-off score of 14 or less possessed dysfunctional movement patterns that may correlate with a greater risk of injury. They also reported findings of a study examining female collegiate athletes and found that those who scored less than 14 had an approximate four-fold increased risk of lower extremity injury throughout the course of a season. Garrison et al. (25) completed a study of the FMS with 160 collegiate athletes. They identified that athletes with a composite score of 14 or less with a self-reported history of previous injury are at a 15 times increased risk of injury compared to athletes scoring higher on the FMS. This test involved a large number of athletes, both male and female, in a variety of contact and non-contact sports. This added to the value of the study as it demonstrated a broad base for data collection, where error may have increased potential, if there was a similar data pool (i.e. all male athletes / same sport / etc.). It should also be noted that there was potential for limitation in this study. As the criteria established by the authors stated each athlete must complete a minimum of three hours training time each week, there was no upper limit in the amount of training time for each athlete. This may have been a potential cause for injury in some athletes due to the monotony or strain from potentially excessive training over time. Keisel et al. (34) performed a study on 238 American professional football players. They established FMS scores at the start of the training camp prior to the season. A score of 14 or less with a combination of the presence of any asymmetries were examined. They found that when this combination was used, there was a high correlation with injury. They concluded that fundamental movement patterns and movement asymmetry are risk factors for time-loss injury in professional football players. Chapman et al. (11) used FMS scores longitudinally to attempt to predict performance outcomes in elite track and field athletes. They segregated the athletes into cohorts for comparison. Performance change between 2010 and 2011 was noted and the athletes were compared via a) high FMS score (greater than 14) to low FMS score (14 or less), b) athletes with at least one asymmetry vs. athletes with no asymmetry, c) athletes scoring 1 on the deep squat pattern vs. athletes scoring 2 or 3.  The results demonstrated that on category “a” athletes with a score of greater than 14 had a greater improvement in performance. On category “b” athletes with no asymmetries had a greater improvement in performance as opposed to athletes with one or more asymmetries. On category “c” athletes who scored 1 on the squat pattern had a significantly different change in performance. They concluded by stating that functional movement ability is related to the ability to improve longitudinal competitive performance outcomes.

As there have been positive indications for the FMS being able to potentially predict injury and improvements in performance, there have also been strong limitations in the research, and studies where minimal indication was seen. Lockie et al. (40) studied the relationship between functional movement screen scores and female athletic performance. They suggested that the FMS was limited in its ability to detect movement compensations that could impact athletic performance in female athletes. Their results showed that greater flexibility as measured by numerous FMS scores related to slower change of direction speed and poorer unilateral jump performance. They also made comment that the positioning and speed of particular individual screens is atypical to team sports. They also suggested limitations of their own study however, where they only had data collection from nine athletes. Dossa et al. (20) performed research on ice hockey players and injury prediction with the FMS. There results did not support their hypothesis that low scores (14 or less) could predict injuries over the course of an ice hockey season. They stated limitations where research with contact sports where stick and puck injuries are high may have lower capability in predicting risk, also due to the altered definition of the classification of an “injury”. They stated further limitations where they did not factor the volume of training, the playing position, or the physical maturity of the athletes being studied, which all have the potential for disproportionate results with individual athlete injury. 

This review explains a range of results and opinions on movement screening and how it may lead to improved performance. Due to the differing nature of the multiple studies, there is no strong conclusive evidence to date that movement screening is capable of accurately predicting injury and that athletes with high movement screen scores will have greater performance improvement. It is apparent however on numerous occasions that movement screening models, particularly that of the FMS with the most repetition in the literature, that there is reason for implementing a movement screening model, as it may ascertain a bench mark for functional capability with which a strength and conditioning coach can then provide further exercise prescription over time and improve the likelihood of minimizing athletic injury and enhancing performance status longitudinally. More ‘specific’ examples are required in the research in the future to form a stronger argument for higher success of implementation of movement screening.

C. MOVEMENTS VS. MUSCLES

As discussed above there is a plethora of variables, which effect FMD. There are numerous current literature sources emphasising that the capability required for efficient movement is done so through integration of numerous muscles working in combination over a number of joints, rather than isolated muscle strength (7, 13, 17, 21).

Cook (17) explained simply that when analysing movement our brain does not recognise individual muscle activity, instead the brain recognises movement patterns and creates coordination between all the muscles needed, a process that is referred to as a ‘motor program’. Boyle (7) also emphasised the philosophy of FMD by stating, “experts emphasise that functional training trains movements, not muscles”. Boyle (7) noted that research and development demonstrates that exercise prescription should not follow anatomical descriptions for movements based on patterns such as extension / flexion / adduction only to produce force, but rather follow patterns of kinetic chains, which is how the body moves in actual locomotion. This concept describes groups of muscles and joints working together to perform movements.

Epley (21) concurred with this philosophy and noted that for an exercise to improve athletic performance multiple joint actions must be timed in the proper neuromuscular recruitment patterns. The training of multiple joints in one exercise will aid in the development of coordination and improve the ability to generate force for sports movements. He also stated that single joint exercises such as bicep curls, leg curls, and leg extensions contribute little to improve performance and are only used broadly to enhance an aesthetic quality, where multi-joint actions are a much better option for providing a transfer from training to performance. He concluded by stating, “sports skills require multiple joint actions timed in the proper neuromuscular patterns, otherwise you have no coordination or ability to generate explosive force”.

Chek (13) discussed generalised motor program compatibility, when referring to the concept that the brain stores “generalised motor programs”. He discussed that each motor program can be used for groups of movements that have the same relative timing. He gave an example when discussing a squat movement pattern, where he noted research demonstrated that when performing isolation exercises such as a leg curl / leg extension, there is a very poor transferal effect to improving vertical jump (VJ) performance. However there was a significant improvement in VJ performance when training the resisted squat pattern. This VJ performance concept was furthered when Dalen et al. (19) provided evidence of single joint (SJ) vs. multi joint (MJ) training with VJ performance. Their study was performed analysing proximal to distal coordination between the knee and ankle upon VJ performance. The study looked at two separate groups, where group ‘A’ completed MJ ballistic squat training with plantar-flexion in one complete movement, and group ‘B’ completed SJ plantar-flexion and ballistic squat resistance movement training on separate days. The study found that only the group ‘A’ MJ training group had a significant increase in their maximal VJ performance.

Boyle (6) described SJ or isolation exercises as non-functional for injury prevention and sports performance.  He gave alternative exercise prescriptions such as instead of leg extensions the use of split squats or other single leg variations, instead of leg curls using a single-leg-straight-leg deadlift prescription. He noted that over recent decades training prescription has progressed from training by body part to a more intelligent approach to training via movement pattern.

Gentil et al. (27) studied the effect of additional SJ exercises to a MJ resistance program, where the goal was to enhance muscular hypertrophy and muscle strength. The study randomly divided untrained men into a group ‘A’ training MJ exercises only with bench press / lat. pull down, and group ‘B’ training MJ and SJ exercises together with bench press / lat. Pull down / elbow flexion / elbow extension. The study concluded no additional muscular size or strength benefit was seen with the inclusion of SJ exercises, when elbow flexion was measured with isokinetic strength testing, and hypertrophy was measured with ultrasound.

The approach taken by the strength and conditioning professional should follow the literature which now overwhelmingly demonstrates that exercise prescriptions for sports performance should follow a method of enhancing motor development via movement based exercise prescription models.  The enhanced complexity of sports performance can only be further improved by incorporating an integrated functional approach to training prescription.

G. KINETIC LINK PRINCIPLE

The kinetic link principle (KLP) refers to the method in which the body produces momentous force. A summation of forces is produced through the body via a  ‘linking’ process of a number of segments together (37). Diagram 5 produced by Kreighbaum et al. (37) describes a sequential process by which a force is produced. It describes force starting from the proximal segments of the body and progressively transferring momentum to the distal segments. This is possible via the proximal segments being larger in mass and having a greater inertia. An upper body throwing action may be used as an example. The initial segments (i.e. trunk rotators) accelerate eliciting a force, where the next body segment (i.e. shoulder horizontal adductors / internal rotators) then initiates an accelerating force, which then refers on to the upper arm (i.e. elbow extensors), and the end point (wrist and finger flexors). The key to this process is that each segment is smaller than the previous and contains less inertia. As one segment accelerates it allows the following segment to initiate movement at its top speed as it passes on momentum to the next segment before it decelerates.  The speed of the consecutive segments gradually increases, with the result allowing the end point velocity of the final segment to be much greater than the initial. This biomechanical process occurs through the “serape / functional line effect” as mentioned previously, and can be applicable to not only a throwing action, but a range of different functional skills.

Subijana et al. (57) performed a biomechanical study on kinetic energy transfer during the tennis serve. They analysed two tennis player’s service actions via a system referred to as 3D photogrammetry, where a proximal to distal segment kinetic energy transfer was discovered by a mean correlation analysis. The analysis found that for one of the players it could successfully predict the quality of the serve with 100% accuracy and the other player 76%, based on the kinetic transfer properties seen during the action.   

Roetert et al. (52) referred to a tennis serving action. He reported the following findings in the literature:

• The largest portions of kinetic energy or force generated in the serving stroke are developed in the legs and trunk, where 51% of the kinetic energy and 54-60% of the total force are produced.

• As seen in diagram 5 and 6, each segment has a cocking or stabilization phase, and an acceleration phase.

• Segment ‘drop out’ or kinetic chain breakage decreases the ultimate force and energy available to produce a forceful movement, and also puts excessive unnecessary strain on the surrounding segments. 

• A 10% reduction in energy transferal from the hip or trunk requires a 14% increase in shoulder rotation velocity or a 22% increase in shoulder mass to create the same amount of kinetic energy for the movement.

The KLP is a very trainable aspect of functional development where analysis of the process one uses to efficiently accelerate at different joints can be identified, and exercise prescription can be applied to enhance the method of force transferal through the body. This process is common in a number of sports such as baseball, golf and tennis (32, 57), where biomechanical feedback is given to the individual athlete so alterations can be made to decrease the potential for injury occurrence and improve the power profile through one or more specific sporting actions.

Diagram 5. The Kinetic Link Principle. Kreighbaum et al. (37)

Diagram 6. The Kinetic Link Principle In Action. www.timhartwig.com

(Transferal of momentum / summation of forces from proximal to distal)

H. IMPORTANCE OF POSTURAL SCREENING

Prior to initiating an exercise prescription a great deal of importance must be placed on an appropriate initial screening and assessment (36, 38). Over recent years, a larger degree of emphasis has been placed on this where ‘screening’ systems are now becoming a recognised part of the exercise prescription process for particularly athletic and also other populations (17, 18, 26, 30, 38, 58). An approach adopted by numerous authors demonstrates the use of initial postural screening, and subsequent movement screening to identify potential compensatory patterns an individual may use to complete a movement skill (29, 31). These compensatory patterns are then considered with the exercise prescription and rate of progression of individual prescription.

Postural analysis has been noted as an important factor to consider as an initial step to identifying potential movement asymmetry and discrepancy (36, 38). Kendall et al. (36) noted that in the standard posture the body is in the “ideal alignment for weight bearing”, where the body is in position that favors optimal function. Kritz (38) noted that a definition of optimal static standing posture is “when the least amount of physical activity is required to maintain body position in space and that which minimises gravity stresses on body tissues”. A postural screening identifies the degree of effort or most appropriate position the body uses to maintain a static standing (or seated) position. If an athlete has faulty static posture, this will relate to additional energy being required prior to producing a movement to get into the initial most appropriate position (38). Kritz (38) states that anticipatory strategies that contribute to faulty movement patterns are less efficient, causing the athlete to expend additional energy to perform a proficient movement, where anticipatory strategies have been seen to negatively influence power production. Kritz (38) noted that a range of authors stated that postural assessment indicates the presence of muscle impairments, which are also indirectly associated with movement impairments. Cook (17) when discussing posture noted that the way the body is held has a lot to do with the way that it moves, where the starting position influences the movement that is to follow. When the body begins in a sub-optimal position, receptor influence attempts to make up for the problem by unnecessarily altering the biomechanics in an attempt to catch up or correct the movement.

The observer must identify differences compared to the norm where variations in body type, shape, size, and proportion must also be considered for each individual. An example of optimal posture is demonstrated below, with an explanation of ideal position for larger joints of the body.

Table 3. Adaptation of Ideal Postural Criteria in a Lateral Position. Kritz (38)

Diagram 7. Ideal Posture in Lateral Position. Kendall et al. (36)

E. FUNCTIONAL MOVEMENT ASYMMETRIES & DISCREPANCIES

The identification of functional movement asymmetries and discrepancies has been noted by numerous authors to be of major importance in the correct exercise prescription for athlete movement development (6, 10, 17, 24, 25, 26, 34, 35, 38). To establish a minimum benchmark of movement competence across a number of base movement skill qualities has been noted to be of high significance amongst initial training prescription for athletes (6, 7, 17). Where there is an asymmetry or discrepancy in the development of force, the athlete is contributing to a decrease in the amount of correct utilisation of momentum and energy, an increased potential of injury through improper biomechanics, and a potentially early onset of fatigue through physiological structures (6, 7, 17, 59).

Movement asymmetries and discrepancies can be classified as a process of poor biomechanics (17). This refers to movement ‘mistakes’ in which the body compensates and uses a process of sub-optimal joint alignment, postures, and coordinative applications (17). Tompsett et al. (58) discussed the common previous trend in assessment noting that coaches often focused on performance indicators such as speed and distance, where current trends are leading more towards the pre-season analysis of movements, identifying which athletes possess or lack the movement capability to perform essential movements required for sports performance. Progression of sports specific skills may be restricted by the poor initial development of basic movement competencies, where individuals will progress to a point and then plateau in performance where they are then limited by their own movement inabilities.

Burton et al. (10) discussed the use of functional movement testing, which is to identify abnormal movement patterns, where when identified exercise interventions can be applied to normalise the dysfunctional pattern. Cook et al. (18) discussed the importance of injury prevention through the use of a screening tool. Cook et al. (18) noted an important factor in prevention is to quickly identify deficits in mobility, stability and symmetry because of their potential influences on creating altered motor programs throughout the kinetic chain. Keisel et al. (34) studied injury prediction following asymmetries into fundamental movement patterns. They tested American professional football players using the “Functional Movement Screen” (FMS) prior to starting the training camp. They stated that players who demonstrated a combination of asymmetry in 1 or more out of 7 tests and who had a score below the established safe “cut-off” were at a much higher risk for musculoskeletal injury. Zahalka et al. (61) studied strength asymmetry of soccer goalkeepers. Zahalka et al. (61) used three different VJ testing methods. They used the counter movement jump / counter movement jump with no arms / squat jump. Their results demonstrated that countermovement jumps produced the best VJ score, however also elicited the largest unilateral force asymmetry between legs. They stated that monitoring of power level and strength asymmetries in the preparatory phase of training enables identification of possible strength imbalances in elite soccer goalkeepers. Zahalka et al. (61) concluded that their screening was a useful tool for both future performance enhancement and injury prevention.

Cook et al. (18) explained the term “regional interdependence”, which is used to describe the relationship between regions of the body and how dysfunction in one region may contribute to dysfunction in another. Boyle (6) furthered this concept when he discussed the “joint–by–joint” approach when discussing the potential ramifications of asymmetry and discrepancy for the athlete. This theory involved conceiving the body as a stack of joints, where each joint has a specific function and is prone to predictable levels of dysfunction. A key feature to be noted is that the main purpose of each consecutive joint alternates between mobility and stability (see table / diagram 2). The concept states that injuries relate very closely to proper joint function, where a problem (discrepancy) at one joint usually presents as pain or altered function through compensation at the joint above or below. The theory suggests that if a mobile joint becomes immobile, the stable joint above or below is forced to move with compensation, becoming less stable and potentially painful. An initial example was provided in the case of the lumbar spine, where if there is loss of function of the joint below (i.e. poor hip mobility), the lumbar spine has to take over and provide increased mobility as compensatory function causing undue stress to the associated structures. The key process of this concept is to consider the state of function in the above and below joints to an area reported as having pain or discomfort. The exercise prescription is focused around incorporating increased mobility or stability of the nearby joint, which in turn restores appropriate function to the associated joint. The result effect being that each joint functions based only on its primary purpose. Boyle (6) uses a secondary example explaining the prolific nature of knee pain associated with ankle mobility issues. Many sports involve standing and running where an immobile ankle causes the stress of landing to be transferred to the joint above. The knee has to take on an increased role of mobility, where over time this causes increased stress to the structures of the knee.

Table 2. Adaptation of Joint-by-Joint Approach. Boyle (6)

Diagram 1. Anatomical Man. www.medindia.net

The literature demonstrates that appropriate FMD is associated with the identification process of asymmetry and discrepancy. Correct identification and subsequent individual exercise prescription is of importance early in the learning period for effective movement development (10, 18, 34, 58).  

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.