## 4 Discussion  
This study has defined a battery of simple tests that are easy to perform in everyday practice and provide a comprehensive overview of essential components of trunk function (strength and muscular endurance). Strength-testing procedures included measurement of isometric lower back extensor strength using HHD and measurement of respiratory muscle strength. Muscular endurance of the trunk muscles was evaluated by the Biering-Sorensen, prone-plank, and side-bridge tests. Isokinetic dynamometry is often referred to as the \"gold standard\" for the measurement of trunk muscle strength [12; 15; 16; 18]. A certain number of studies have already been devoted to finding an alternative, practical, cheaper, and more user-friendly clinical tool that might provide reliable information about back extensor strength using HHD. Similar to the current study, the above studies established excellent validity and reliability for the new method [17; 18; 19]. The present study confirmed the repeatability and the short-term and long-term test-retest reliability of techniques measuring maximal isometric lower back extensor strength by HHD as excellent in all three positions. Unifying the methodology of the three existing protocols [17; 18; 19] and proving the repeatability of measurements using one dynamometer device allows us to compare obtained data measured in different postural positions and to discuss which of the positions is the most reasonable to use in terms of the biomechanics of the motion and in terms of usefulness in everyday practice. The highest values of isometric lower back extensor strength were measured in the sitting position, followed by those measured standing, while the lowest values were measured in the prone position. No other studies have compared lower back extensor strength in different postural positions. Some studies have suggested that it is reasonable  
Figure 2: Correlation between strength testing procedures, muscular endurance tests, and IPAQ. Significance of correlations is expressed through asterisks at levels as follows: * \(p\) < 0.05, ** \(p\) < 0.01, and *** \(p\) < 0.001. 1 Maximal isometric lower back extensor strength in different positions. to use pelvic fixation [12] and, at the same time, a semi-sitting position with a hip flexion angle of 40-60\({}^{\circ}\), since this position appears to increase the specificity of back extensor action (contraction) and reduce hip extensor activity [12; 23]. In sitting and standing positions, this study employed pelvic fixation to specify the movement of the back extensor muscles, as suggested above [17; 18; 23]. However, the current study did not reach the semi-sitting position (40-60\({}^{\circ}\)); it prioritized simplicity, requiring less time- and space-consuming procedures; creating a chair that could achieve the necessary angles for every subject would have added to study difficulty. On the other hand, the chair deployed herein provided similar circumstances for all patients and measurements. The contraction of lumbar extensors in this position still appeared to be the strongest out of all three positions. The anchor rails required for measurements in the standing position are even cheaper than the specially designed chair and less space-consuming. However, they are not easy to move, so the measures always need to be taken in the same room. The technique for the prone position requires no special equipment (except the HHD itself). While using an HHD, it may be difficult for the assessor to provide a counter-pressure that corresponds to the effort exerted by the patient, especially when measuring larger muscle groups [24]. This raises a question regarding the assessment of isometric lower back extensor strength using an HHD in the prone position, as the maximal strength of the examiner may limit it, and a ceiling effect may occur when testing strong individuals [15; 19; 20]. [PERSON] et al. suggest an alternative technique for prone position measurement, using a tripod fixed by a non-elastic belt to the table and the patient.