The main purpose of this study was to validate equations of predictions in a healthy population according to their anthropometric data for eight muscle isometric endurance field tests. The main interest of the creation of these normative data is, in future clinical trials, to evaluate at best people with low back pain, for example, and to propose individual care based on these evaluations during rehabilitation. But to validate these equations, it was necessary to ensure the reproducibility of the muscle endurance tests because original positions from a few tests were altered. Indeed, some positions induce pain or premature fatigue in patients with low back pain, thereby inducing a false evaluation. Our results demonstrated an excellent test-retest reproducibility with ICCs between 0.915 and 0.996 for all tests.
In view of the excellent reproducibility of each test, we have attempted to elaborate normative data for each test according to the anthropometric data of each subject. Our results demonstrated that age, height and mass, but not gender, influence the performance. In view of the literature, this matter has already been widely discussed in recent years but with varied results [25, 26, 27, 28]. The study of McIntosh et al. [27], with a distribution close to our population, showed a similar finding to our study concerning the influence of age but not gender on the performance in five isometric field tests. In contrast, Strand et al. [28] showed a statistical difference between men and women with a test duration 49% higher in men than in women. Before muscle evaluations, all subjects [28] described their habitual physical practice, with 1% of participants declaring that they never practised physical activity, 13% rarely, 26% once or twice a week, 41% three to five times a week and 19% more than five times a week. But the authors did not specify the sex distribution of the population practising a physical activity. Moreover, the authors did not consider the level of physical activity in determining their groups. Indeed, in their study, the authors separated their population into four groups according to their sex and their athletic status: male non-varsity athletes (n = 134), female non-varsity athletes (n = 227), male varsity athletes (n = 59) and female varsity athletes (n = 50). The 109 varsity athletes were all members of the National Collegiate Athletic Association (NCAA). Nevertheless, Strand et al. [28] highlighted that plank test hold time was significantly higher in subjects who practised physical activity at least three times a week. But it is impossible to determine whether the results found were due to a gender or a training effect. And so, the usual level of physical activity according to gender and age is an essential factor to interpret the results of the measurement, as is done in our study.
In our study, we identified the agonist/antagonist ratios for different muscle belts because of their utility before the beginning of the training programme proposed during rehabilitation in low back pain, for example. Indeed, the evidence of a muscle imbalance will direct the clinical decisions regarding the construction of the physical training programme. Imbalance commonly refers to a modification of the strength balance between agonist and antagonist muscles that could accompany a proven pathology, articular conflict or functional impairment. Although no prospective study has been conducted to determine what proportion of a muscle imbalance can cause an injury in an athlete, these ratios are also used in injury prevention in sport because for many authors they constitute an element of functional specificity of joints [29]. However, the primary means to determine the ratios between agonist and antagonist muscles is the use of isokinetic dynamometers, which are very expensive and associated with difficult manipulation. So, the evaluation of postural ratios is most often done through field tests. But there are no known norm ratios in field tests, which are often limited to situating the subject by age [25] or gender [28], thereby limiting the interpretation of results. That is why the prediction equations defined in our study allow the subject to be situated according to their anthropometric criteria, i.e. gender, age, mass and size, while also identifying agonist/antagonist muscle imbalances, which will allow the individualization of rehabilitation programmes.
Limitations of this study
The isometric assessments are associated with a number of disadvantages, such as the impossibility of appreciating the work developed due to the lack of movement during the exercise, a punctual and restrictive definition of the relationship between the muscle tension and length, and technical difficulties in simultaneously evaluating agonist and antagonist muscle groups [30]. But isometric assessment is a reliable method when the standardization of measure is respected [27]; it’s a method that does not require expensive material and a specific room for the evaluation as with isokinetic dynamometers.
The evaluation of muscle endurance in humans often depends on motivation. Moreau et al. [31] explained that psychological issues in regard to motivation and fatigue would be better studied before the measurement of muscle function itself. Indeed, if the subject does not cooperate by giving maximum effort to the point of fatigue, the endurance time can be compromised by a premature termination due to a mental factor but not because of a physical limitation [31]. In our study, the subjects were voluntary participants who were fully informed about the nature of the tests. So, we can conclude from that, that the motivation was equal for all subjects and thus this did not negatively influence their performance.
Lastly, our study was carried out in a French population; thus, as explained by Hogrel et al. [32], our results could present a few variations depending on the races involved (Africa, Asia), because of morphologic, anatomic and cultural differences.