## 4 Discussion
The portable dynamometer is held between these [19]. External fixation can reduce measurement errors compared to assessments in which the assessor fixes the HHD manually. The [PERSON] et al. study results showed that, when using an HHD to assess lower back extensor strength, an HHD fixed with a tripod was superior to one that was manually set [19]. On the other hand, an ICC of \(\geq\)0.90 is required for clinical application to ensure valid interpretation of any findings [25]; this study achieved this figure from the prone position without using tripod fixation. However, it must be borne in mind that movement in the prone position alone does not comply with the biomechanics of everyday life for most people. It may be anticipated that this movement could lead to discomfort in patients with low back pain and older people. Some of the studies even point out that several of their patients, those with hyperkyphosis due to osteoporotic fractures, were unable to perform the task [19]. The outcomes herein indicated that isometric lower back extensor strength in the prone position was mainly dependent on age (decreasing with advancing age). However, the isometric back extensor strength in sitting and standing positions seemed to be dependent not only on age, but predominantly on sex and weight; in the study population, weight was related to sex (data are not displayed); however, it can be assumed that the same pattern may also be seen in other populations of volunteers/patients. This finding accords with observations made by other authors [16], in which males exhibited higher extensor strength than females. Some authors explain this finding in terms of more excellent anthropometric dimensions and more elevated experience of maximum effort in men, which may allow them to achieve higher forces [16; 26]. [PERSON] et al. posit that absolute muscle strength is determined by the quantity of force-generating elements and their geometry, so the strength of a muscle is mainly determined by its overall cross-sectional area [4]. The difference in maximum strength between the sexes may thus be simply associated with the gross muscle-size differences between men and women, a factor present in almost all skeletal muscles. The current study disclosed a robust and significant correlation between strengthening results using HHD from the sitting and standing positions. Respiratory muscle strength proved a significant moderate correlation with isometric back extensor strength in sitting and standing positions, which could confirm validity for the technique measuring the lower back extensor muscle strength in sitting and standing positions using HHD. However, outcomes obtained from the prone position did not correlate strongly with those obtained from the other two positions. This did not hold for the technique performed in the prone position since the results did not correlate with respiratory muscle strength at all. These findings suggest to other working groups, who aim to assess lower back extensor strength using HHD in their practice, that one of the positions may be chosen to assess lower back extensor strength, preferably in a sitting or standing position, depending on the availability of equipment. In further works, the authors aim to use all these tests to observe the performance of patients with LBP and NMD; how do these results correlate and how do they differ from healthy volunteers. Only then will it be possible to exclude tests that seem to be redundant for clinical practice. Respiratory muscles are an essential component of the core system. The strong correlation between maximal inspiratory pressure and maximal expiratory pressure that appeared in the results herein may support the validity of the techniques chosen for measuring respiratory muscle strength. In this study group, even though the volunteers tested were healthy, the overall strength of respiratory muscles in different subgroups did not exceed 100% of the predicted value.