Extraction of effective Young's modulus of skin and subcutaneous tissues from manual indentation data

An ultrasound indentation system with a pen-size hand-held probe was used to obtain the indentation responses of forearm soft tissues. Three normal forearms were each tested with two muscular contraction states. Six different indentation rates ranging from 0.75 mm/s to 7.5 mm/s were imposed. A linear elastic indentation solution was used to extract the effective Young's modulus. The extracted material parameter was demonstrated to be repeatable and roughly rate-insensitive for the rates imposed in this study. It varied significantly due to the change of the contraction state of underlying muscles. The effective Young's modulus significantly increased from 14.0 ± 5.0 kPa to 58.8 ± 1.7 kPa as a result of contraction of the underlying muscles when the hand exerted a gripping force of 5 kg. The increase of original thickness by a factor up to 26% was also noted.