Evaluation scale and definitions of core and core stability in sports: A systematic review

Abstract

BACKGROUND:

Core stability has been reported to be important for improving performance in athletes. However, the variety of measures used to assess core stability has made it difficult to compare results across studies. In addition, there is a lack of consensus on precise definitions of core and core stability, which is a barrier to research in this field.

OBJECTIVE:

The overall purpose of this review was to summarize the definitions of core and core stability and measurements of core stability used in previous studies on athletes.

METHODS:

We searched four electronic databases (PubMed/Medline, SPORTDiscus, Web of Science and Science Direct) from their inception to October 2023. Studies evaluating core stability in athletes across all sports were included. We excluded case studies and case series, opinion pieces, letters to editors and studies not written in the English language. Two researchers independently assessed articles for inclusion and exclusion criteria and methodological quality.

RESULTS:

One hundred thirty-four studies were included, of which two were of high quality. The definitions of core and core stability varied widely, and ‘core’ was not defined in 108 studies and ‘core stability’ was not defined in 105 studies. The most used test protocol was the McGill test, which was used in 19 studies.

CONCLUSIONS:

There are multiple tests to measure core stability, and there is some confusion as to whether the measurement results represent core strength or core endurance. Future research papers should clarify the definitions of core and core stability, and consider core strength and core endurance separately.

Keywords

Trunk stability trunk strength trunk endurance

1. Introduction

Hodges et al. reported the existence of a feedforward contraction, in which trunk muscles initiate activity prior to limb activity [1]. Therefore, core activity is an important factor as the root part of physical activity. In addition, the acquisition of core stability is effective in improving low back pain. Exercises for core stability have been reported to help recovery from low back pain in the general population [2]. In fact, core stabilization exercises are considered a favorable method for treating pain in patients with non-specific low back pain. Compared to conventional exercises, core stability exercises are more effective in relieving pain and improving physical function in the short term in individuals with chronic low back pain [3].

Generation of intra-abdominal pressure (IAP) plays an important role in enhancing the stability and stiffness of the spine [4, 5, 6, 7]. Tayashiki et al. [8] reported that maximal hip extension torque correlates significantly with IAP ( $r=$ 0.504, $p=$ 0.024). However, since IAP is measured by inserting a pressure sensor through the nose or anus, it is impossible to measure it easily on the sports field. Therefore, many field tests, such as the double leg lowering test and the prone plank test, have been developed as alternatives to IAP measurements to examine core stability. However, the variety of measures used to assess core stability has made it difficult to compare results across studies. By organizing the metrics used to evaluate core stability in studies involving athletes, it might be possible to compare the results of these studies in the future. In addition, there is a lack of consensus on precise definitions of core and core stability [9], and the inability to conduct research with consistent definitions of core and core stability has been a barrier to research in this field.

Core stability has also been reported to be important for improving performance in athletes [10, 11, 12, 13]. Significant correlations have also been reported between core endurance using the McGill test, which consists of four assessments that measure all aspects of the torso via isometric muscle endurance and performance tests, such as sprint time and the one-repetition maximum (1RM) of squatting [10, 11]. Also, core stability has a strong correlation with dynamic balance stability, and greater improvements in dynamic balance stability were found with core training interventions for $\leqslant$ 6 weeks, performed after high volume and more frequent interventions, as well as in younger participants [12]. Moreover, core training might potentially improve skill performance in football, handball, basketball, swimming, dancing, karate, muay thai, volleyball, badminton and golf players, and in gymnasts [13].

The primary purpose of this study was to summarize the definitions of core and core stability and measurements of trunk stability used in studies involving athletes. The secondary purpose of this study was to assess the quality of studies on core stability.

2. Methods

2.1 Protocol and registration

We conducted and reported this systematic review according to the PRISMA guidelines [14], with the protocol defined a priori.

2.2 Study eligibility criteria

We included studies that assessed core stability in athletes. There was no limit in terms of how core and core stability were defined, or publication date. We excluded case studies and case series, opinion pieces, letters to editors and studies not written in the English language.

2.3 Sources and study selection

We searched four electronic databases (PubMed/ Medline, SPORTDiscus, Web of Science and Science Direct) from their inception to October 2023. All search strategies were created by an experienced librarian. The last search was run on 24 October 2023. The following keywords, combined with Boolean operators (AND, OR) were used: (“core strength” OR “core stability” OR “core stabilization” OR “core instability” OR “trunk strength” OR “trunk stability” OR “trunk stabilization” OR “trunk instability”) AND (“sports” OR “athletes”) AND (“injury” OR “performance”). Two authors (S.E. and T.H.) independently screened titles and abstracts for eligibility. Subsequently, the three reviewers (S.E., T.H. and T.S.) independently screened full-text articles as required to determine eligibility. All screening was performed independently and, at each stage, disagreement was resolved by discussion until a consensus was reached.

Figure 1.

Study selection process.

2.4 Data extraction and management

After elimination of duplicates, studies were exported to Endnote software and the data were extracted by the reviewers (S.E., T.H. and T.S.) using a customized data extraction form. Discrepancies were resolved by discussion and agreement. The data items extracted were: author(s), year, sports discipline, final sample size, sex distributions, study design, scale used to evaluate core stability, definition of core and core stability, and duration and method of intervention. There was no blinding of the study author, institution or journal throughout the data extraction process.

2.5 Assessment of methodological quality

Each full-text article was assessed by three independent reviewers (S.E., T.H. and T.S.) and scored using the PEDro scale [15]. Any discrepancies were resolved by discussion and consensus. The PEDro scale has been developed to measure methodological quality and internal validity of randomized studies [16]. Each of the 10 items is scored as being either present (1) or absent (0), and the total score out of 10 is then calculated, with larger numbers indicating better quality. The scores were categorized into three categories to define the overall quality of each article, as follows: $\leqslant$ 4: poor, 5–6: moderate, and $\geqslant$ 7: high quality.

3. Results

3.1 Evidence synthesis

The initial search identified 864 articles for screening (after eliminating duplicates). Figure 1 summarizes the study selection process and reasons for excluding studies. The full texts of 155 articles were assessed for eligibility; among them, data from 134 studies were extracted after excluding six non-eligible studies.

3.2 Characteristics of the included studies

We included 134 studies with a total of 9762 (range 7–634) participants (https://dx-doi-org.web.bisu.edu.cn/10.3233/IES-230177online Supplemental Table 1). Of the 134 studies, 66 were cross-sectional studies, one was a retrospective study, 14 were prospective studies, 20 were intervention studies, and 33 were randomized controlled trials (RCTs).

3.3 Methodological assessment

The results of methodological quality assessment of the 134 studies are shown in https://dx-doi-org.web.bisu.edu.cn/10.3233/IES-230177online Supplemental Table 2. A score of over 7 (high quality) was reached by two studies [17, 18], scores of 5 to 6 (moderate quality) were reached by 13 studies [19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31], and a score of below 4 (low quality) was reached by 119 studies [10,11,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148]. Only one study was blinded for all subjects and all therapists who administered the therapy.

3.4 Definition of core and core stability

Overall, there was no consensus regarding the definitions of core and core stability (https://dx-doi-org.web.bisu.edu.cn/10.3233/IES-230177online Supplemental Table 3). Some studies that used the word ‘trunk’ failed to specify that it meant ‘core’. One hundred eight studies [10,11,17,21,23,24,25,26,28,29,30,34,35,36,37,38,39,40,41,42,43,45,46,48,49,50,51,52,53,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,86,88,89,90,91,93,95,96,97,98,99,100,101,102,103,104,105,107,110,111,112,113,114,115,119,120,121,122,123,125,127,128,129] did not provide a description of the definition of core, and 105 studies [10,11,17,19,20,21,23,24,25,26,27,29,30,32,33,34,35,36,38,39,40,41,43,46,48,49,51,52,54,55,58,59,60,62,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,85,86,87,88,89,90,91,93,94,95,96,97,98,100,101,102,103,104,106,109,110,112,113,114,115,116,117,118,119,120,121,123,125,127,128,129]did not provide a description of the definition of core stability. Each of the definitions of core described in the five studies [149, 150, 151, 152, 153] was cited in three studies; a box with the abdominals in the front, paraspinals and gluteals in the back, the diaphragm as the roof, and the pelvic floor and hip girdle musculature as the bottom [149, 151]; a box or a double-walled cylinder with the abdominals as the front, paraspinals and gluteals as the back, the diaphragm as the roof and the pelvic floor and hip girdle musculature as the bottom [151, 153]; the musculoskeletal core of the body includes the spine, hips and pelvis, proximal lower limbs and abdominal structures [150]; the lumbar vertebrae, the pelvis, the hip joints, and the active and passive structures that either produce or restrict movement of these segments [152]. The fifth definition, Kibler et al.’s definition of core stability, which defines 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” was used most often [150].

3.5 Scale used to evaluate core stability

Core stability was represented by three words: stability, strength and endurance (https://dx-doi-org.web.bisu.edu.cn/10.3233/IES-230177online Supplemental Table 4). The Sorensen test [54, 70, 71, 74, 83, 94, 107] and double leg lowering test [65, 70, 83, 94, 96, 108, 148] were the most commonly used indices to assess core stability (seven studies); isometric trunk extension strength was the most commonly used index to assess core strength (16 studies) [21, 27, 29, 38, 48, 51, 52, 66, 117, 119, 120, 140, 143, 144, 145, 146]; and the prone plank test was the most commonly used index to assess core endurance (13 studies) [20, 36, 54, 55, 61, 64, 73, 76, 78, 91, 99, 106, 130]. The most frequently used test protocol was the McGill test (which includes the abdominal fatigue test, Sorensen test, prone plank test and side plank test), being used in 19 studies [11, 18, 35, 38, 46, 51, 87, 88, 104, 116, 117, 118, 121, 126, 133, 138, 139, 142, 148].

4. Discussion

The primary purpose of this systematic review was to summarize the definitions of core and core stability, and the measures of core stability used in studies on athletes. The secondary purpose of this study was to assess the quality of studies on core stability. Of the 134 previous articles included in this study, a score of over 7 (high quality) for methodological quality assessment using the PEDro scale was reached by only two studies. There was no consensus regarding the definitions of core and core stability. One hundred and eight studies did not provide a description of the definition of core, and 105 studies did not provide a description of the definition of core stability. The most used test protocol for measuring core stability was the McGill test, which was used in 19 studies.

4.1 Core and core stability definition

Despite the fact that many of these studies investigated the core and core stability, these studies did not specify definitions of core and core stability. The definitions of core provided by each of the five studies [149, 150, 151, 152, 153] were referenced by three studies each, and were the most referenced definitions. Although the previous studies that were cited as the source of the definitions differed between studies, “a box with the abdominals in the front, paraspinals and gluteals in the back, the diaphragm as the roof, and the pelvic floor and hip as the bottom” [149, 151] was the most cited definition of the core in the six studies [22, 33, 92, 94, 108, 109]. Many of the other definitions used were adaptations of this definition. Some definitions did not include the hip joint or hip muscles (e.g. gluteals), which might be related to the interchangeable use of the words “core” and “trunk” [154, 155, 156]. Some of the definitions did not describe a specific muscle, such as “the core musculature is composed of 29 pairs of muscles that support the lumbopelvic-hip complex” [157] or “the area between the sternum and the knees, with a focus on the abdominal region, lower back, and hips” [158]. The most used and referenced definition of core stability was “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” by Kibler et al. [150]. The next most frequently used definition of core stability was “the body’s ability to maintain or resume an equilibrium position of the trunk after perturbation”, and a similar definition was “the body’s capacity to maintain or resume a relative position of the trunk after perturbation”. There were no other definitions that were cited in more than one article. A commonality among many definitions of core stability was “the ability to control the core”.

4.2 Scale used to evaluate core stability

The three terms (core stability, core strength and core endurance) were not clearly differentiated and were used interchangeably. In addition, a single test was used as a method to measure all three abilities (core stability, core strength and core endurance). Thus, there might be differences in perceptions about what the measurement results represent in some tests. Core strength was used primarily in tests measuring isometric and isokinetic trunk and hip muscle strength. The most used test protocol was the McGill test (which includes the abdominal fatigue test, Sorensen test, prone plank test and side plank test). Previous studies reported that the Sorensen test [159] showed an average intra-tester and inter-session reliability, with an intraclass correlation coefficient (ICC) of 0.88 (95% confidence interval $=$ 0.80–0.92), and the plank test [160] performance across three trials had an ICC of 0.97 (95% confidence interval $=$ 0.94–0.99). The McGill test measures the amount of time that the participant can maintain a particular posture and is considered to measure core muscular endurance. Therefore, core endurance was primarily assessed in the four tests included in the McGill test, and also in the sit-up and push-up tests. Some tests (e.g. sudden force release test) were also performed to measure the response of trunk muscles to external disturbances. Balance tests, such as the upper quarter Y balance test [43] and single-leg stance balance test [94], and performance tests such as drop vertical jump [99], single-leg squat [85, 99], and medicine ball throw [109, 117] were also used for evaluation of core stability. Thus, core stability might be a word that encompasses core endurance and core strength.

4.3 Future research

Due to the absence of high-quality research on core stability in athletes, further research is needed before robust conclusions can be drawn. Research should focus on a standard, sport-specific definition of core and core stability, survey instruments should be adapted and validated in an athlete population, and studies should include athlete-specific outcomes. Although there are a variety of methods for evaluating trunk stability, in the future, it will be necessary to organize them from the perspective of which methods are (1) related to athletic performance, and (2) which are related to injury prevention.

4.4 Limitations of this review

Our search terms were limited to ‘core’ and ‘trunk’, and we only included studies that were in the English language. Moreover, the studies included in this review were limited to those of athletes and thus studies of other populations were excluded. There was also no blinding of the reviewers to the study authors at any stage.

5. Conclusions

The overall purpose of this systematic review was to summarize the definitions of core and core stability, and the measures of core stability used in studies of athletes. There are multiple definitions of core and core stability, with no consensus between them. The most referenced definition of the core was “box with the abdominal muscles in the front, the paraspinal and gluteal muscles in the back, the diaphragm in the roof, and the pelvic floor and hip joints in the base” and the most referenced definition of core stability was “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”, and should be used in future studies. In addition, there are multiple tests to measure core stability, and there is some confusion as to whether the measurement results represent core strength or core endurance. Future research papers should clarify the definitions of core and core stability, and consider core strength and core endurance separately.

Author contributions

CONCEPTION: Shota Enoki.

PERFORMANCE OF WORK: Shota Enoki, Taisei Hakozaki, Takuya Shimizu.

INTERPRETATION OR ANALYSIS OF DATA: Shota Enoki, Taisei Hakozaki, Takuya Shimizu.

PREPARATION OF THE MANUSCRIPT: Shota Enoki.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Shota Enoki, Taisei Hakozaki, Takuya Shimizu.

SUPERVISION: Shota Enoki, Taisei Hakozaki, Takuya Shimizu.

Supplementary data

The supplementary files are available to download from https://dx-doi-org.web.bisu.edu.cn/10.3233/IES-230177.

Footnotes

Acknowledgments

The authors have no acknowledgments.

Conflict of interest

None.

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