Participants
Consecutive recruitment of healthy volunteers was conducted by using local flyer announcements between October-November 2020 in Madrid (Spain). To be eligible to participate, they had to be between 18 and 65 years old. Participants were excluded if they reported thoracic pain the previous year; have been under pharmacological treatment affecting muscle tone (e.g., muscle relaxants or analgesics); have prior history of thoracic surgery, respiratory complications, or pneumothorax; and any other medical condition such as tumor, fracture or other neuromuscular condition compromising a normal skin-to-pleura distance (e.g., sarcopenia, myasthenia gravis, amyotrophic lateral sclerosis, or multiple sclerosis). This study was approved by the Institutional Ethics Committee of Rey Juan Carlos University. Prior to their inclusion in the study, all participants read and signed a written informed consent.
Based on a previous study conducting a prediction model based on anthropometric features15, a sample size of at least 96 measurements were considered as appropriate. Considering this as a prognostic study, a range from 10 to 15 subjects per potential predictor, with no more than five predictor variables, is recommended to obtain proper sample size for prediction models and for avoiding overestimation of the results16. Based on this sample size estimation, a sample size of 50 subjects would be required given the maximum cut-off of five predictors included in the final model. Since we originally included seven potential predictors in the model (age, gender, height, weight, BMI, thorax circumference and respiratory moment), we determined a sample size of at least 70 measurements, giving a total of 35 participants.