Deformation of the gluteal soft tissues during sitting

Background: Excessive deformation of soft tissues is considered to be one of the major contributing factors to discomfort and injury for individuals who sit for long periods of time. Soft tissue deformation in research has been measured under the assumption that tissues deform uniaxially below the ischium, with very small or negligible deformations taking place in other directions. Therefore, this study describes the deformation of the gluteus maximus muscle and surrounding fat tissues in the buttock region for seated subjects. Methods: In vivo measurements of the deformation for the gluteal soft tissues were obtained from MRI scans of six seated subjects. Each subject was scanned in weight-bearing and non-weight-bearing sitting postures using a Positional MRI scanner (Fonar 0.6 Tesla Indomitable™). Deformations were measured below the ischium and the proximal femur. Deformation of the gluteus maximus was also measured in the distal direction along the thigh for each subject. Findings: Our data suggest that soft tissues undergo three-dimensional deformation with considerable components below the ischium (mean of 21.4 mm) and in the distal direction along the thigh (mean of 20.3 mm). Differences in muscle deformation below the ischium were also observed between obese (mean of 27.4 mm) and non-obese subjects (mean of 16.5 mm). Interpretation: Findings of this study demonstrate that tissue deformations in sitting include complex three-dimensional motions that are not well approximated by two-dimensional models.