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.