## 1 Introduction  
Although any prolonged sitting position may be responsible for postural deformities, such as abnormal spinal curvatures, trying to maintain a balanced and comfortable sitting posture may play a vital role in compensating for the negative side effects and securing several body parts during activities of daily living [1; 2]. This is due to the significance of balanced sitting, which is considered either as an assessment tool to measure the patient's coordination or as a treatment goal by increasing the sitting base of support [3]. In one's daily habits and tasks, the routine sitting position, in a way by which the functional independence might be achieved, can be considered an ideal activity in physical therapy [4]. This point should be taken into account, as sitting consumes more than a third of peoples' daily lives and more than half of their daily activities during working time or leisure time [5; 6; 7; 8]. Therefore, the improper sitting position might affect the muscular condition negatively, as it might excessively overload the work of muscles, which may lead to hip joint inactivity, thus altering the general postural alignment [9; 10]. For instance, computerised occupations that require sitting for a long time in front of technological machines might require the employee to sit forward-leaning, with some degree of neck fixation, to be more concentrated on the computer screens. This might greatly lead to some musculoskeletal problems, like disc prolapse or herniation at the cervical vertebrae, that have a large impact on the quality of daily life [11]. To achieve a good, balanced body condition, which is a basic task for locomotion and communication, it is important to keep normal spinal and pelvic alignments in the sitting position. For instance, a sit-to-stand task can be considered a basic task for performing all activities of daily living, and it needs to be conducted in a specific arrangement of alternative balance degrees to avoid any injuries or abnormal pressure distributions. This is because it includes a combination of several body postures, like bending, lowering down, sitting, raising the hands, standing, and locomotion [12]. Therefore, although a long sitting duration can affect body balance negatively, trying to sit in a healthy posture while making sure the pelvis and trunk form the same line, without any abnormal curvatures, and continuing the sit-to-stand tasks during prolonged static positions, such as sitting, could help in avoiding or at least decreasing the negative side effects [5]. The crossed-leg sitting (CLS) posture is habitually used, particularly in Arabic, Middle Eastern, and Asian societies. However, it is not clear whether CLS for a long duration would affect the lower limb muscles and the balance ability of the body. There is no previous study on this topic. Therefore, this study aimed to investigate the effect of CLS on the lower extremities' muscles according to electrical activity and muscular strength, as well as assessing its impact on static and dynamic body balance. The research hypotheses were that CLS could overload the lower limb muscles and alter body balance status by comparing the parameters in the following two situations:  
* before CLS;
* after 20 min of CLS. We hope this study will contribute to the knowledge and add a new understanding of sitting posture, as well as give a clear insight into sitting posture research. While this posture is commonly used as a daily habit in many societies, as there is little research analysing the impact of CLS on body balance and muscular status, the results of this study can be considered as a supplement to this new field. After the Section 1, this article will be structured as follows:  
* Section 2 presents the Materials and Methods, including participants, inclusion and exclusion criteria, the equipment used in data collection, how the data were collected, and what statistical method was used to compare two sets of data before and after CLS.