This study focused on the kinematic data of the hip motions of 10, common, Asian sitting positions. The significant finding was that, of the 10 positions, squatting showed the highest flexion angle (99.7º; 47.3º–122º). Our finding was similar to that of research on Indian subjects. That work investigated 5 sitting positions: squatting with the heels down; squatting with the heels up; kneeling dorsi-flexed; kneeling plantar-flexed; and cross-legged. The Indian study found that squatting with the heels down produced the maximum hip flexion angle (95.4º ± 27º). Squatting is best defined as the position where the foot contacts the ground in a way that brings the body down over the foot, requiring maximum hip flexion. Many ADLs require the squat position, such as toileting (especially with Asian-style toilets). In Western countries, the basic squat and chair squat are functional, multi-joint exercises that generate maximum hip flexion (particularly the chair squat).
In this study, the cross-legged sitting position had the highest abduction angle (28.9º; 9.9º–45.7º) and external rotation angle (62º; 37.6º–81.7º). The cross-legged position is also known as the “crossed-legged tailor” position and the “Buddha” position. In Thailand, it is commonly used for resting, eating on a floor, or leisure activities such as yoga. Moreover, the ability to sit cross-legged is one of the functional outcomes employed to evaluate patients after a total hip arthroplasty. The study on the Indian population performing the cross-legged sitting posture found a mean abduction angle of 39º (19º–57º) and a mean external rotation angle of 49º (42º–58º). That work found more hip abduction angles and fewer external rotation angles than our study. However, the Indian research used a simple manual goniometer to measure angles; those values would be less accurate than ones obtained with 3DMA equipment, especially for rotational angles. In comparison, a kinematic study of Chinese people found a median abduction angle of 12.7º (1.3º–32.7º) and a median external rotation angle of 2.3º (-11.9º–36.4º). Those values are much lower than the corresponding ones from our study. The Chinese study subjects were instructed to sit cross-legged with a foam cushion only underneath their buttocks, not at the leg and foot areas. However, that particular way of sitting cross-legged is not commonly used for the performance of Asian-style ADLs. It might also lower the hip abduction and external rotation angles.
The authors of the current investigation found that the hip ROMs were associated with both gender and being overweight. There were more flexion angles (4 out of 10) and fewer abduction angles (6 out of 9) in the female group, with a P value < 0.05. However, there was no direct relationship between gender and rotational angles. The hip rotational ROM depended on the sitting position. In the case of the overweight subjects, there were fewer hip flexion angles and more hip abduction angles, with 5 out of 9 sitting positions having a P value < 0.05. The kinematic data of the transverse plane showed the heterogeneity of the rotational angles depended on the sitting position. In a kinematic study, Huffman et al. investigated the effects of higher BMI on hip ROM. They demonstrated that, during the sitting and sit-to-stand postures, there was a greater increase in the peak abduction angle for their high-BMI group than for their normal subjects. We believe that the lower hip flexion ROM of the overweight group in the current research may have been connected with a higher level of posterior thigh tissue (indicated by the greater thigh circumference of those subjects) as well as movement while changing position. To our knowledge, no study has previously focused on the associations between either gender or weight group and the hip ROMs during the sitting positions used for the ADLs. However, one kinematic study compared the ROMs of young adults and the elderly while kneeling. That work found no apparent differences in the knee and ankle joints of the 2 groups. Nevertheless, the research did find a higher maximum hip flexion angle for the elderly than the young adults (100.5º and 67.5º, respectively). In the current study, the authors recruited healthy adults aged under 35 years. The age factor would therefore have been most unlikely to affect the main findings of our study. The authors suggest that all factors–gender, weight, and age–should be considered while determining the hip ROM for each sitting position used for the ADLs.
The increase in knowledge gained through kinematic data studies benefits various professional fields. For physiotherapists and prosthetists, gaining a better understanding of these functional motions might aid the development of prosthetic designs that meet the functional needs of patients (especially those in non-Western countries). Orthopedists can adapt the knowledge to treat patients both conservatively and surgically. Activity modification and the avoidance of aggravating activities are key to the conservative treatment of hip disorders. Labral tears and intra-articular hip pathologies are often associated with groin pain that is exacerbated by flexion and rotatory hip movements. The symptoms experienced by femoroacetabular impingement syndrome patients are aggravated in the flexed, adducted, and internal rotated positions. Patients diagnosed with subspine hip impingement suffer severe pain with hip flexion angles greater than 90 degrees. Being aware of the reference values of the hip motions of each common sitting position will help with the education of patients who have hip disorders. In turn, it will assist them to minimize or prevent symptoms.
In order to apply kinematic data to patients who had undergone a total hip arthroplasty (THR), Miki et al. investigated the anatomical hip ROMs after THR using a navigation system. They found a wide range of passive hip ROMs intraoperatively: 113º of flexion, 46º of abduction, 75º of internal rotation, and 36º of external rotation. That research team then studied the patients immediately after their THRs to determine the effects, if any, of the anesthesia, muscle relaxants, surgical techniques, and implant designs that had been utilized. However, their reference values should be cautiously applied in clinical practice. This is because 4 sitting positions in the current study (cross-legged, figure-four, left-monk, and right- monk) demonstrated external rotation angles that were higher than the reference values determined by Miki and colleagues. Hence, patients who undergo THR should be cautioned about immediately using these 4 positions. Generally, surgeons will know the safe and stable hip ROMs in the operative theater. Understanding these data could prevent dangers, such as impingement or dislocation after sitting during the ADLs. Physical medicine and rehabilitation physicians might apply these data to design patients’ postoperative protocols and prevent adverse events.
This study had several limitations. Firstly, there was a wide variety of ways in which the 10 sitting postures were executed and achieved. The authors attempted to reduce variations by demonstrating the correct posture for each sitting position and allowing the participants to practice them before the trial. The maximum angle achieved for each sitting position was measured; this is where the use of a 3DMA system is superior to a static measuring device like a goniometer. Secondly, all subjects were young and healthy. These data do not cover elderly patients nor those with lower extremity disorders. Finally, the study regarded the hip angles as the intersegment angle between the pelvis axis and the thigh. No data were collected on spinopelvic parameters that might affect the hip joint while changing position. However, as all subjects in the study were healthy volunteers, the authors believe that all would have normal spinopelvic mobility. The pelvic tilt is an interesting variable for further research as it might affect the sitting posture of patients with pathologies of the spine and/or hip.