Procedure
Anthropometric data included age, gender, height, weight and BMI. Further, thorax circumference (measured horizontally at the xiphoid process) was also calculated at maximum inspiration and expiration in standing position with 90º of arms abduction17.
For ultrasound imaging, participants were in prone with their arms in abduction of 90º and elbows flexed. The assessment point was selected following the procedure as described by Folli et al.12which has been found to be highly reliable. Firstly, the spinous process and the inferior angle of the scapula was identified by manual palpation, and the half distance between these points was calculated. Second, a horizontal projection was traced medially to the spinous process of the thoracic spine. The transducer was therefore placed in the middle-distance between these points (Fig. 1 ).
The following distances skin to pleura, skin to the lowest limit of major rhomboid, and lowest limit of major rhomboid to pleura were calculated by using an ultrasound equipment Alpinion Ecube 8 (Gyeonggi-do, Korea) with a linear transducer E8-PB-L3-12T 3-12 MHz (Fig. 2 ). The mean of three repeated trials performed by the same clinician with more than 10 years of experience using ultrasound imaging was calculated for the main analysis at two moments, maximum inspiration and maximum expiration.