Before all testing, subjects were encouraged to maintain the testing position as long as possible. Subjects were not encouraged during testing, and the instructions were kept standardized throughout the entire testing procedure, regardless of testing method. Subjects were not informed of their scores until the entire testing was completed. Subjects were also encouraged to not change their current activity level between testing sessions, specifically in regard to trunk muscle activity.
Concurrent validity of the MOD tests with the ST tests was determined by the correlation between the time the subject could hold the extension and flexion positions, respectively. The level of validity as described by Meyer (28) is an accurate indicator of the extent of validity. The extent of validity was determined as follows: correlation coefficients greater than 0.80 indicated high validity, values between 0.60 and 0.80 indicated good validity, values between 0.40 and 0.59 indicated moderate validity, and values less than 0.40 indicated poor validity. Interrater reliability, intraclass correlation coefficient (3,2), was evaluated on 15 subjects: for extension, r = 0.97 and for flexion, r = 0.93.
In this comparison of our modified testing procedures (MOD) to previous standardized testing procedures (ST), correlation analysis revealed a Pearson r = 0.84 between the times for holding the flexion postures and r = 0.90 for holding the extension postures. The mean time subjects could hold each position is found in Table 2. Individual data are illustrated in Figures 3 and 4.
A high degree of correlation was found between the times on the ST and MOD methods when the 2 tests were concurrently applied to normal college-aged subjects. No other correlation values have been reported for the ST against any type of modification. Using such a test over a wide range of age groups and subjects has provided valuable clinical information and helped to establish normal trunk endurance ranges that can then continue to be used in these different populations of subjects with LBP.
The use of static endurance testing seems to be cost effective, easy, and quick to perform and requires no special equipment in the clinics, so clinicians could choose it for measuring trunk muscle endurance (30). Trunk flexion (12,26) and extension (4,22,26) endurance testing have proven to be highly reliable methods. The use of these methods, as originally described, does require a table that allows for looping a belt around the width of the table. Weight and athletic training rooms may not necessarily have these types of tables. Determining the reliability of an MOD testing method was necessary to allow more convenience and widespread use of ST testing procedures that have clearly shown benefit (26). The fact that the MOD testing method compared well with the ST method is of benefit not only to clinicians with these types of facilities but also to other clinicians implementing these testing strategies due to time constraints (avoiding time necessary to belt and loosen each subject) and for research purposes. However, it cannot be stated that the MOD method has the same reliability as the ST method due to the body type of the individual restraining the subject's lower extremities. Although the clinician, who is providing the stability, is not the same as a static belt, the clinician was not required to perform any specific resistive force to stabilize the subjects. The clinician simply lay over the subject's lower extremities (or sit on their feet in the case of flexion) and remained stationary throughout the testing procedure. Although it is most likely that the clinician was required to perform some type of isometric muscular contraction(s) to keep the subject's lower extremities from moving, the clinician never reported fatigue, and so on. None of the subjects felt uncomfortable during the MOD testing procedure, and they all felt that they were stabilized equally during both testing methods.
Although the MOD method seems to be an acceptable alternative to the ST testing procedure in a normal asymptomatic population, its implementation remains to be determined in a population of LBP subjects. Endurance times in trunk extensor (3,14,17,35) and trunk flexor muscles (7,16,24,29,31) in subjects with LBP are less than that in the normal healthy population. Because an apparent loss of muscle control after trunk muscle fatigue could be considered one of the important causes of LBP (32), it could be postulated that there is an importance on assessing trunk muscle endurance as a potential measure of future LBP. The ST methods have been implemented in different LBP populations (2,34,36) and post single-level microdiscectomy (13). Although the reasons for termination of the trunk extension endurance test seem to be a combination of fatigue (2,10,30,34), motivation (10,30), and LBP (34) (at least for symptomatic individuals), trunk endurance testing has become a tool of reference for evaluating muscle performance in patients with LBP, most notably before and after rehabilitation programs (10). It remains to be determined whether our MOD procedure, while comparing favorably with ST trunk endurance testing methods in normal subjects, will also compare as favorably in a symptomatic population.
The flexion time was notably longer in our study compared with previous studies (5,26) for both testing methods. Although the parameter for termination of the flexion test was previously established as breaking of the 60° angle (26), it did not clearly define the exact criterion for test termination (whether the test was terminated if the subject broke the plane at all or if the test was terminated when the subjects contacted the prefabricated wedge that was 10 cm behind them). It was determined in a pilot testing before this study that the reliability was less favorable using breaking the 60° plane in any manner as a criterion for test termination compared with when the subject broke the plane of testing and contacted the prefabricated wedge. Although this was a different criterion for test termination than previously used (26), it was a more clearly defined criterion in our opinion as it was often difficult to ascertain if the subject broke the 60° plane throughout the entire spine as many subjects would lose lumbar lordosis yet appeared to maintain the 60° angle as measured through the midline of the spine and not to contact the wedge. Due to this potential complication in test termination criterion, we decided on the criterion of the subject contacting the prefabricated wedge. Our test-retest reliability was lower compared with the study of initial standardization (26), and although the different criterion for test termination may be a factor for this, it is our opinion that the reliability results would have compared even less favorably as our pilot reliability testing using the previously established criterion was poorer than the criterion we implemented. Contacting the wedge was a criterion used in Chan's study (5). Although our mean time scores and SD were still longer than Chan (5), our times were more comparable to this study than the study of McGill et al. (26). Chan (5) also used a more specific subject base (college-aged rowers) compared with our subjects. The variability of our subject base could have accounted for the differences of times, especially the SD range. Also, more recently, alternative testing positions for abdominal endurance have been advocated (6). Testing in the same manner as McGill et al. (26) except that the initial starting position (with a wedge) of 45° and in a trunk curl up position (with bilateral scapulae clearing the table) was more time effective and showed less variance than the 60° trunk starting position (6). The curl up exercise was considered easy, convenient, and representative of trunk flexor effort; it was considered a preferable alternative to the 60° flexor exercise for healthy women (n = 28) of mean age 23.8 ± 2.4 years (6). This would seem to concur with feedback from our subjects, many of whom felt that the limiting factor in their test termination was hip flexor muscle pain, fatigue, and so on, compared with abdominal muscles as the limitation.
The method of test-retest reliability may also be a consideration. Previously established standard reliability (26) was established both before and after testing with 5 subjects tested consecutively. Our testing involved many more subjects and less days of testing, which is more likely representative of clinical situations. The fact that all our subjects were the emphasis of reliability testing and the fact that our subject pool seemed variable in weight and testing duration could not only account for the lower level of reliability but also be more representative of the actual clinical situation.
Another factor that may warrant consideration is age range of subjects in the various studies. Although our subjects were all college students, our age range was quite variable (22-38 years) and our mean age was greater than previous studies (20.52 ± 1.16 years) for Chan (5) and (23 ± 2.9 years) for McGill et al. (26). Although the age ranges were not listed for other studies, our age range was quite large (22-38 years), which could also contribute to the variable fitness levels and test tolerance, therefore the greater variability in the flexion endurance scores and SD.
With our testing, the individual providing stabilization was consistent. This individual weighed 90.72 kg, who was heavier than every subject tested except one (who weighed the same). Pilot testing to determine the relationship between the individual providing stabilization during testing and the subjects being tested was implemented with a second individual providing stabilization. This second individual weighed 70.3 kg, which was greater than the mean weight of 68.4 kg in the subset of 15 subjects used in this pilot study but less than 6 of these individuals in this subset (range was 47.7-90.72 kg). Correlation analysis for this pilot study revealed a Pearson r = 0.84 between the times for holding the flexion postures and r = 0.61 for holding the extension postures and r = 0.85 between the times for holding the flexion postures and r = 0.62 for holding the extension postures when comparing the first and second individuals providing stabilization. Therefore, flexion compared very favorably with prior testing methods with this second tester, but correlation between the various extension methods was only good compared to high with other correlations. Additional testing should be implemented to determine this specific relationship before its implementation.
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