The functional importance of the trunk musculature in performing thorax and pelvis movements (flexion, extension, bending, and twisting) and controlling the stability of the spine against internal and external forces (21,32,33), and the interest of many coaches and practitioners in training core/trunk stability to prevent low-back and lower-extremity injuries in athletes (12), have given rise to the development of a variety of tests to assess the functions of these muscles in different settings inside and outside the laboratory.
The assessment of trunk stability in field setting is very complex because it requires the combination of different measures, for example, trunk muscle strength and endurance tests (3,6,20,28), lumbopelvic posture control assays (16,28,29). The use of trunk endurance field tests has become very popular, because trunk endurance has been identified as an important muscle capability for low-back health (1,17,18,20) and the protocols are simple and relatively inexpensive. Most of the trunk endurance tests evaluate the endurance of the flexor, extensor, or lateral bending muscles (3,6,20). However, we have no knowledge of field tests that measure the endurance of the trunk rotator muscles (e.g., oblique muscles).
In throwing and striking sports (tennis, handball, hockey, golf, etc.), the trunk rotator muscle endurance is important for both performance and the spine safety, because muscular fatigue can hinder coordination, postural control, and spine stability (8,19,27,31). In addition, a mechanical study that analyzed the response of the trunk to loading in different directions (32) found that participants had more problems maintaining trunk stability under twisting torque when compared with sagittal torque. Therefore, it is necessary to develop new and reliable protocols to measure the function of the trunk rotator muscles in sport, fitness, physical education, etc. Interestingly, although trunk rotation exercises are common in core training programs (e.g., cross-curl-up or cross-crunch, consisting in twisting and flexing the upper trunk simultaneously while lying in supine) (11,15,35), the protocols used to measure the endurance of the oblique abdominal muscles are generally based on trunk flexion motions without rotation, for example, timed (60–120 seconds) or cadence (20–30 repetitions per minute) curl-up tests performed in supine: Partial Curl-Up Test (7,10,22,25,26), and Bench Trunk Curl Test (13,14,35).
In view of this situation, we developed a flexion-rotation trunk test (FRT test) to measure the abdominal muscle endurance through movements that combine trunk rotation and flexion in lying supine (Figure 1). The main purpose of this study was to examine the FRT test reliability in field settings (schools, fitness centers, clinics, etc.). Because the repetition of the protocol may cause variations in the technique and cadence of the execution, which may improve the test results, the learning or training effect (9) was also analyzed throughout 4 testing sessions. In addition, the sensitivity of the FRT test to compare the abdominal endurance between men and women was also assessed, given the fact that in previous studies men have obtained better results than women have in dynamic abdominal endurance tests (7,26,34).
Experimental Approach to the Problem
As commented before, trunk muscle endurance measurements are usually part of the evaluation of the trunk stability (2,28), and their results could be used to establish risk factors related to low-back health (1,17,18,20). Although there are different flexor, extensor, and lateral bending endurance tests (3,6,20), the FRT field test allows us to evaluate the endurance of the rotator muscles using a simple and fast protocol that does not need expensive equipment and is easy to use in sport, fitness sessions, and physical education classes. Because rotation in the standing or sitting position with no external resistance generates low-moderate trunk activation levels (35), we developed a timed protocol in lying supine based on performing the maximum number of flexion-rotation movements (i.e., cross-curl-ups) possible in 90 seconds (Figure 1). The subject’s score was the number of repetitions accomplished by the subject in the 90-second test administration. This duration was established based on the study carried out by Knudson and Jhonston (14), comparing 3 different Bench Trunk Curl Test durations (60, 90, and 120 seconds) to evaluate abdominal muscle endurance. In this study, it was concluded that unlike the 60-second test, which according to the authors measures muscle power, the 90-second test showed a higher correlation with the 120-second test (r = 0.88; p = 0.01). Based on these data, in timed curl-up tests, 90- or 120-second durations seem to be more adequate to measure abdominal endurance than 60-second duration; therefore, looking for a higher time economy, we decided to use 90 seconds as the duration for the FRT test.
Our main purposes in this investigation were to assess the relative and absolute reliability (36) of the FRT test and to know the number of trials needed to make learning effect negligible (9) before using the test in field settings. As Hopkins stated in a review of measures of reliability in sports medicine and science (9), reasonable precision for estimates of reliability requires studies with approximately 50 participants and at least 3 administrations of the test. In this study, we used a longitudinal design in which 51 volunteers performed the FRT test 4 times, separated by 7 days each. This allowed us to analyze both the consistency of the FRT test scores and the learning effect.
Seventy volunteers initially took part in the study, of which 51 (35 men and 16 women) completed the 4 recording sessions (Table 1). The subjects were informed of the experimental risks and signed an informed consent form before the investigation. Approval for the investigation was provided by the Ethic Committee of the University. People with known medical problems, histories of spinal, shoulder or hip surgery, or episodes of back pain requiring treatment 12 months before this study were excluded. All the subjects were recreational physically active, participating in aerobic, strength, and sport training with a workout frequency of 2–5 d·wk−1.
After a measurement schedule, each participant executed the FRT test in 4 different sessions (T1, T2, T3, and T4), separated by 1 week each and conducted at the same time of the day for each participant (between noon and 2:00 PM). The trials were performed in an acclimatized fitness room (18–22° C) at the University during the first 3 months of the year. The participants were encouraged to not change their regular activity level at that moment throughout the study (mainly in relation to the trunk muscles), to not perform a workout session at least 15 hours before each recording session, and to maintain a good sleep routine and to not eat and drink excessively before testing.
Seven days before the first trial, there was a familiarization session in which the subjects were informed of the test execution rules and the recording schedule. In this session, the participants did not perform the test, but they only carried out 10 repetitions to familiarize with the basic technique of the test.
As mentioned above, the FRT protocol is a timed cross-curl-up test that consists of performing the maximum number of trunk flexion and rotation movements possible in 90 seconds. To carry out the test, the subject was placed in a supine position on a semirigid mat, resting the sole of the feet on the floor, with legs together and a knee flexion of 90° (Figure 1). A manual goniometer (Comed, Strasbourg, France) was used to standardize the knee position in each subject and trial. The back and head were rested on the floor and the arms were stretched out over the trunk, with the hands resting on the thighs, overlapping, with both thumbs interlocked. An experimenter held the subject’s knees in the aforementioned position (Figure 1) and helped to avoid the modification of the lower limb position during the execution of the test. For this, the experimenter was kneeling at the feet of the subject, pressing with the fists on the outer side of subject’s knees (Figure 2).
In each repetition, the subject first carried out a trunk flexion rotation until he or she touched with his or her hands the outer side of one of the experimenter’s fists (fifth knuckle; Figure 1), and then the counteraction, this is to say, the subject returned to the initial position, until he or she touched the mat with his or her head. During the execution of the test, the subject performed twists to one side and the other consecutively. Only those repetitions that were performed correctly were counted, that is, those in which the hands touched the external side of the fist in the lifting of the trunk, and in which the head touched the mat in the lowering of the trunk (Figure 1). The subject was not encouraged during testing. In addition, he or she was neither instructed about the most efficient performance cadence nor informed about his or her FRT test scores.
All analyses were performed with SPSS version 20.0 (SPSS Inc., Chicago, IL, USA). Standard statistical methods were used to calculate the mean and the SD of the FRT test scores (abdominal endurance) for each trial and sex. The relative intrarater reliability of the measure was determined using an intraclass correlation coefficient (ICC; 2-way random effects model). In addition, the absolute intrarater reliability was analyzed by calculating the standard error of measurement (SEM). The SEM was expressed both as number of repetitions (SEM = SD of the difference scores between 2 trials/√2) and as percentage of the mean value of the measurements (SEM = mean of the difference scores between 2 trials × 100/mean of the first trial). To analyze the changes in the reliability measures over time, separate calculations of ICC and SEM were performed on consecutive pairs of trials: T1–T2, T2–T3, and T3–T4. A comprehensive review on the quantification and use of ICC and SEM to assess relative and absolute reliability has been previously presented by Hopkins (9) and Weir (36). Finally, an analysis of variance (ANOVA) with repeated measures was calculated to assess the learning effect throughout the trials (T1, T2, T3, and T4) and to explore the differences between sexes. Where applicable, post hoc analyses were performed using the Bonferroni test. An alpha level of 0.05 was considered significant for all analyses.
Table 2 shows the absolute and relative reliability analysis results, along with the changes in the mean scores (Bonferroni post hoc) of the FRT test across the trials. The ICCs obtained between trials increased with the repetition of the test, in both the total sample and the men and women groups. In the same way, SEMs tended to reduce throughout the study, reaching values of 7.27 repetitions for men and 4.24 repetitions for women between T3 and T4.
The ANOVA found significant differences for the FRT test mean scores between trials (p < 0.001; η2 = 0.48): T1 = 70.8 ± 15.4 repetitions; T2 = 79.9 ± 20.1 repetitions; T3 = 88.1 ± 24.0 repetitions; T4 = 91.8 ± 26.1 repetitions (Figure 3). The number of repetitions obtained in the FRT test increased 12.87% from T1 to T2 (p < 0.001) and 10.21% from T2 to T3 (p < 0.001). On the contrary, the increase between T3 and T4 was low (4.25%) and not significant (p = 0.108).
In Figure 4, we can see the mean and SD of the results obtained in the test for both sexes. The ANOVA showed a higher abdominal endurance in men than in women (p = 0.003; η2 = 0.17). Results in men increased between all the trials, although the increase in the number of repetitions from T3 to T4 was only 4.60% (p = 0.044). On the other hand, the increase in women was significant from T2 to T3 (p = 0.012), but not between T1 and T2 or between T3 and T4 (Table 2).
Although trunk muscle function should be assessed and trained in all planes of motion for multidirectional competence (12), field tests that measure the trunk rotator endurance in the horizontal plane are lacking. The objective of this study was to analyze the reliability of a new field test, the FRT test, based on the repetition of trunk flexion-rotation movements in lying supine, and also to examine the effect of repetition and sex on test results. The data obtained in this study indicate that the reliability of the test is good, but this reliability depends on the number of times the test is repeated, because throughout the recording sessions, absolute and relative consistency of measurements increased and learning effect of the test (i.e., difference in mean scores between trials) reduced considerably. In addition, the comparison between sexes showed a higher abdominal endurance in men when compared with that in women and also a higher learning effect in men, especially at the beginning of the study.
In relation to the reliability analyses (Table 2), the high ICCs (0.83–0.94) and the low SEMs (12.87–4.25%) obtained between the different recording sessions show the high relative and absolute consistency of the measurements. The SEMs of the FRT test were similar (7,34) or even lower (7,22) to those found in previous abdominal endurance tests. In addition, the ICCs were similar to those obtained in other studies during trunk endurance field tests reliability analysis. Most field tests in the literature show ICCs >0.75: (a) dynamic trunk flexion tests, for example, the Bench Trunk Curl Test with an ICC >0.79 (13,34) and the Partial Curl-Up Test with an ICC >0.88 (10,22,25); (b) isometric trunk flexion tests, such as the Flexor Endurance Test with an ICC >0.93 (3,6,20); (c) isometric trunk extension tests, such as the Biering-Sorensen Test with an ICC >0.75 (3,5,6,20,30); and (d) isometric lateral flexion tests, such as the Side Bridge Test with an ICC >0.76 (3,20). However, it is difficult to establish direct comparisons between studies because the ICC is sensitive to the between-subject variability (9,36).
The FRT test allows a reliable assessment of the flexor-rotator muscle endurance via a simple protocol that can be easily applied outside the laboratory. Nevertheless, as presented in Figure 3, the test scores increased throughout the study, showing a clear learning effect that must be taken into account before using it in field settings. Improvements of the FRT test scores across the longitudinal study may have occurred because of changes in the technique and the cadence of test execution during the first recording sessions. It is also probable that these improvements were related with motivation (9), as although we did not inform the individual of the test score, he or she was able to count the number of repetitions performed, in an attempt to improve his or her performance or beat his or her peers in future recordings. Interestingly, there seems to be an asymptotic function between trials and scores in the FRT test (Figures 3 and 4), because the increase in the test scores along the 4 trials reduced progressively until the differences between T3 and T4 were very small (men: 4.60%, p = 0.044; women: 3.28%, p = 1.00). According to these data, it would be necessary to perform the FRT test at least 3 times for the results to be consistent and in this way control the learning effect of the test. We cannot establish direct comparisons between our results and those of previous studies, because most researches that have analyzed the reliability of field tests measuring trunk muscle endurance only carried out 2 trials (test-retest), with the exception of studies such as those of Moreland et al. (22) or Cowley et al. (4), in which subjects performed 3 trials.
When comparing the FRT test results between sexes (Figure 4), we found higher trunk flexor-rotator endurance in men than in women (p = 0.003). Previous studies that used dynamic trunk flexion tests to measure the abdominal endurance also found these differences in favor of the men (7,26,34). But when isometric trunk flexion tests were used, no differences between sexes were found (6,20). Other studies that analyzed the effect of sex on performance in the Side Bridge Test (trunk lateral flexion isometric endurance test) also found differences favoring the men (6,20). On the other hand, the studies carried out with the Biering-Sorensen Test (trunk extensor isometric endurance test) found differences favoring women in studies of nonathletes (20) and similar results between men and women in studies with athletes (6). According to those studies, there seems to be an interaction between sex and training level that may modulate performance in this type of field tests. Future studies should analyze the results obtained by men and women in different sports and with different training levels in the FRT test, especially in sports in which the endurance of the trunk flexor-rotator muscles is important (e.g., tennis, judo).
The differences between sexes were not only reduced to the test scores but also to the increase in the results throughout the 4 trials. Especially, it is worth pointing out the differences between men and women when comparing T1 and T2, in which we can see that men improved their scores in the second trial significantly (16.28%, p < 0.001), whereas women showed a lower and nonsignificant increase (4.22%, p = 1.00). We do not have enough information to establish the origin of these results; nevertheless, even though psychological variables were not analyzed in our study, the differences between sexes could be related with differences in goal orientation between men and women. It is known that men and women differ in their goal orientations (23,24), that is, men are more motivated to compete with their peers, whereas women are more concerned with the correct execution of the training. Therefore, it is possible that during the first 2 trials, women paid attention mainly to performing the test correctly (test score being less important), whereas men may have centered their attention on increasing the number of repetitions performed previously and on obtaining better results than their peers. Nevertheless, this hypothesis must be confirmed in future studies.
Taking into account that trunk rotation endurance seems an important factor for high performance in both, throwing-striking sports (e.g., tennis, golf, baseball) and combat sports (judo, karate, etc.), and that its deficit in golfers has been related to low-back pain (17), coaches and physical trainers would do well to evaluate the trunk rotation endurance of their athletes. In this sense, isokinetic dynamometry protocols have been developed in research and clinical settings to assess the endurance and strength of the trunk rotator muscles (17); however, these evaluations are expensive and not easily accessible to coaches, fitness instructors or physical educators. On the contrary, the FRT test is a reliable field protocol that requires minimal and inexpensive equipment and is simple to employ in groups of subjects. For example, a sport team or a physical education class can be divided into pairs to administer the FRT test in 2 phases: (a) one member of each pair would perform the test first (performing partner), with the help of the other member of the pair (testing partner), who would hold the legs of his or her partner and count the repetitions performed correctly; (b) then, the roles would be inverted. Because the FRT test duration is very short (90 seconds), a whole group or team could be measured in a few minutes. Nevertheless, because of the learning effect observed in this study, mainly in the group of men, it is advisable to perform an extensive familiarization period before testing (at least 3 trials of practice) to make learning effect negligible.
The authors wish to thank the University students who took part in this study. This research was made possible by the financial support of Ministerio de Ciencia e Innovación (DEP2010-16493) and Generalitat Valenciana (ACOMP/2011/130), Spain.
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