Dynamic trunk muscle endurance profile in adolescents aged 14–18: Normative values for age and gender differences

1. Introduction

Trunk field tests are common elements in educational, clinical and sport settings, as trunk muscle function has been related to low-back pain prevention/rehabilitation [1, 2, 3, 4, 5, 6, 7] and sport performance [8, 9, 10]. In general, these protocols require minimal and inexpensive equipment, do not need sophisticated data processing, and are simple to employ in groups of subjects.

Due to the fact that trunk muscle strength/power protocols normally require the use of a dynamometer (e.g., isokinetic dynamometer) [11, 12, 13, 14, 15], which is not common in field settings, most trunk field tests have been developed to measure trunk muscle endurance [1, 2, 5, 16, 17, 18, 19]. There are three main kinds of trunk endurance field tests: a) “timed” protocols, which consist in performing the maximum number of trunk motions possible (e.g., curl-ups, cross-curl-ups, or sit-ups) in a given time (30–120 s) [16, 18, 19, 20, 21]; b) “cadence” protocols, consisting in the maintenance of a certain cadence while executing trunk flexion or extension motions for as long as possible [20, 22]; and c) “isometric” protocols, which basically consist of maintaining a prone, supine, or lateral posture against gravity until exhaustion [5, 23, 24].

Based on the relationship between trunk muscle endurance and low back health [5, 25], trunk curl-up or sit-up tests are common components of most international batteries of youth fitness tests [26, 27, 28, 29, 30, 31]. These batteries show normative data stratified by age and percentiles for both genders [30, 32, 33, 34, 35], which are normally used to assess and compare the health-related trunk muscle performance of young people according to test scores and ages. Although this may provide important information to coaches, physical educators, and health professionals for prescribing abdominal exercise programs according to the participants’ ages, scientific evidence supporting the relationship between trunk test scores, health status, and age is lacking [36]. In this sense, Dejanovic et al. compared the trunk flexion endurance of a sample of adolescents between ages and genders [37] and interestingly, they found no significant differences by age or gender, nor significant age*gender interaction. This does not seem to support the stratification of the trunk muscle endurance test scores by age and gender used in the international batteries of youth fitness tests.

In this study, a timed curl-up test, i.e., the Bench Trunk Curl-up Test (BTC test) [16, 18] was used to assess the abdominal dynamic endurance of male and female adolescents between 14 and 18 years of age. The main purpose of this study was to compare the BTC test scores between ages and genders, in order to clarify if abdominal muscle endurance changes throughout adolescence in males and/or females. Additionally, as only relative reliability of the BTC test has been previously analyzed [intraclass correlation coefficient (ICC) ⩾ 0.88] [18], the relative and absolute consistency of this protocol was also assessed, which will provide valuable information about the characteristics of this test, such as typical error, minimum detectable change and learning effect.

Regarding the participants’ anthropometry (Table 1), males showed higher mass ( 0.001; F = 84.288) and height ( 0.001; F = 90.969) than females. Moreover, there were significant differences in mass between age groups ( p = 0.012; F = 3.299), but Bonferroni post hoc analyses only found differences between the 14-year-old group and the 18-year-old group in males.

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The ANCOVA showed no differences in the mean BTC test scores between age groups ( p = 0.219; F = 1.451) neither in age * gender interaction ( p = 0.848; F = 0.345). However, it showed higher abdominal endurance in males than in females (males: 90.07 ± 32.65 repetitions; females: 73.43 ± 27.74 repetitions; p = 0.001; F = 12.396) being the difference very likely positive (95–99%). Bonferroni’s post hoc analysis showed significant differences between males and females in the 17-year-old group (Fig. 2).

As there were no significant differences by age groups, mean and standard deviation test scores, percentile reference values (Table 2), and absolute and relative reliability data (Table 3) for the BTC test are only displayed by gender.

Concerning the reliability of the protocol (Table 3), the ANCOVA found significant differences for the mean BTC test scores between sessions (T1 = 72.06 ± 26.28 repetitions, T2 = 81.44 ± 31.27 repetitions; 0.001; F = 51.889) showing a difference in abdominal endurance very likely positive (95–99%). Absolute consistency represented by the typical percentage error was 18.2% for males and 16.2% for females, with a minimum detectable change of 27.3% for males and 24.3% for females. Relative consistency represented by the ICC was 0.80 for males and 0.83 for females.

Regarding the between gender comparison, males showed higher BTC test scores than females ( 0.001), being very likely positive (95–99%), which supports the results of previous studies using curl-up or sit-up tests [19, 20, 44, 46]. Due to differences in muscular mass percentage, muscular fiber size, basal blood level of testosterone, etc., males normally have higher relative strength than females [47], which would imply that males would make less effort in relative terms during each BTC test repetition and therefore could obtain better scores. In addition, anthropometric differences in height and weight between males and females in our study (Table 1) could also explain the gender differences in the BTC test scores [37, 47]. Finally, one might speculate that differences in goal orientation between males and females may influence the comparison between test scores [19]. In this sense, males are generally more competitive (performance goal) than females, who are more concerned with the correct test execution (task goal) [48, 49]. Nevertheless, future studies should confirm this hypothesis.

Although most international batteries of youth fitness tests show normative data of trunk flexor endurance by age and gender [30, 32, 33, 34, 35], no significant differences were found for the BTC test scores between age groups, nor gender * age interaction ( $p>$ 0.05). Similarly, Dejanovic et al. analyzed the 60 ∘ flexor endurance test scores (i.e., isometric flexor endurance) in adolescents aged 15–18 years, and they also found no significant differences between age groups [37]. On the contrary, when 60 ∘ flexor endurance test scores and hook-lying sit-up test scores were compared in children aged 7–14 years [50] and 3–7 years [51], respectively, significant differences were found between age groups. These trunk endurance differences in children might be influenced by physical development at these ages. Dejanovic et al. found a systematic increase in weight and height in children between 7 to 14 years and a significant low correlation of weight and height with isometric trunk flexor endurance in girls [50]. However, based on the results of Dejanovic et al. [37] and ours (Table 1) these anthropometric characteristics seem to be more homogenous in adolescents between 14 and 18 years. Therefore, it seems that trunk flexor endurance is age-dependent in children, but not in adolescents who show similar anthropometric characteristics.

Consistency analyses were performed to determine whether the lack of differences between age groups could be due to low levels of BTC test reliability and also to improve the knowledge about the characteristics of this protocol. In this sense, the ICC values calculated between both assessment sessions were high (ICC = 0.82; n = 216) and similar to those obtained in other studies on BTC test reliability (ICC $>$ 0.79) [18, 52]. In addition, other dynamic abdominal endurance tests have shown similar results, for example the Partial Curl-Up Test with ICC higher than 0.88 [21, 22, 53] and the Flexion-rotation trunk test with an ICC of 0.83 [19]. Then, based on the high ICC values, the BTC test is able to rank trunk muscle performance level properly in adolescents with similar characteristics [39], therefore supporting the findings of this study about age group and gender effect.

On the other hand, the absolute reliability analysis reported a typical error test-retest of 17.21% for the whole sample (males: 18.23%; females: 16.21%). This value is similar to those found in most absolute reliability studies of abdominal endurance field tests [19, 21, 22, 24, 52]. According to the minimum detectable change (25.32% for all participants; males: 27.34%; females: 24.31%), relatively large changes would be necessary in the BTC test scores to ensure with certainty that a real change in trunk flexor endurance has occurred as a result of an intervention rather than a measurement error. This is a common limitation of most field tests in sport science [19, 21, 22, 24, 52], which might question the absolute consistency of these protocols. Therefore, the BTC test does not seem appropriate to monitor the trunk muscle endurance progress in high performance athletes, whose fitness increases are normally low [54]. However, analyzing the moderate worthwhile effect (0.6 * SD between-subject), the BTC test could be suitable to identify moderate abdominal endurance changes in educational, health, and fitness settings, in which participants normally have a high adaptive potential [54]. In this sense, Juan-Recio et al. analyzed the short-term effect of six weeks of trunk curl-up training in high school students without experience in trunk exercise programs, and found BTC test score increases similar to the minimum detectable changes obtained in this study [44].

Timed curl-up tests generally require a long familiarization period to learn the technique, mainly the optimal cadence [19]. Although a familiarization session was performed one week before testing sessions, the change in mean BTC test scores between T1 and T2 was 12.66% (males: 14.64%; females: 10.86%), being significant in all cases and thus, showing a small learning effect in test-retest execution. Consequently, a familiarization session seems not to be sufficient to obtain stable BTC test results that allow decisions to be made on the basis of the participants’ real fitness status. On the contrary, a longer familiarization period and perhaps, a shorter time between familiarization and testing session appears to be necessary to allow participants to learn the correct BTC test technique as well as their optimal execution cadence before the testing session. In this sense, in a previous study on a similar abdominal endurance timed test (Flexion-rotation trunk test), participants needed three test executions to render the learning effect negligible [19].

Several limitations exist as to the interpretation of the data in this study. Differences in number of participants between age groups (Table 1) and the great between-subject variability in the BTC test scores (see Fig. 2), could have affected between-group comparison. In addition, as mentioned in the previous paragraph, a familiarization session was not enough to control the learning effect, therefore deficiencies in the execution technique and/or cadence of some participants might have affected the comparison between age groups. Finally, during the execution of the BTC test, it was observed that some participants had to flex the hip and raise the lumbar area from the mat in order to touch their thighs with their forearms, while others only required a trunk flexion. These differences between participants could have affected the comparison between age groups or sex because the subjects who had to flex the hip required a greater effort to perform each repetition. Therefore, caution should be taken when interpreting the results of this study.

5. Conclusions

The BTC test is a reliable test for assessing dynamic abdominal endurance and safer than sit-up tests [21, 25, 55] because it allows the isolation of the abdominal muscles minimizing iliopsoas activity, although a slight adaptation of the test to the participants’ anthropometrics characteristics would be recommended. This study provides a normative database of trunk flexor endurance in male and female adolescents aged 14–18 years. We found significant differences in BTC test scores between genders, but not between age groups, which does not support the stratification of the trunk flexor endurance test scores by age used in the international batteries of youth fitness tests. Further research with a larger population is needed to understand the relationship between trunk muscle fitness and health better and to establish age- and gender-specific criterion-referenced cut-points for trunk flexion endurance measures.

Regarding the consistency of the BTC test, the relative reliability of this protocol was high, which allows us to rank the trunk muscle performance level of each participant relative to those of others with similar characteristics [39]. However, as in most trunk field tests [17, 24], the lack of absolute reliability of the BTC test does not support the use of this protocol in high-performance sport, in which it would be difficult to distinguish between real changes and measurement errors. Due to the increase in the BTC test score from T1 to T2, it seems necessary to perform an extensive familiarization period to reduce the learning effect.