AXIAL STRAIN-MEASUREMENTS IN SKELETAL-MUSCLE AT VARIOUS STRAIN RATES

A noncontact optical system using high speed image analysis to measure local tissue deformations and axial strains along skeletal muscle is described. The spatial resolution of the system was 20 pixels/cm and t he accuracy was +/- 0.125 mm. In order to minimize the error associate d with discrete data used to characterize a continuous strain field, t he displacement data were fitted with a third order polynomial and the fitted data differentiated to measure surface strains using a Lagrang ian finite strain formulation. The distribution of axial strain along the muscle-tendon unit was nonuniform and rate dependent. Despite a va riation in local strain distribution with strain rate, the maximum axi al strain, E(xx) = 0.614 +/- 0.045 mm/mm, was rate insensitive and occ urred at the failure sire for all tests The frequency response of the video system (1000 Hz) and the measurement of a continuous strain fiel d along the entire length of the structure improve upon previous nonco ntact optical systems for measurement of surface strains in soft tissu es.