THE INFLUENCE OF STRAIN-RATE ON THE PASSIVE AND STIMULATED ENGINEERING STRESS LARGE-STRAIN BEHAVIOR OF THE RABBIT TIBIALIS ANTERIOR MUSCLE

The passive and stimulated engineering stress-large strain mechanical properties of skeletal muscle were measured at the midbelly of the rab bit tibialis anterior. The purpose of these experiments was to provide previously unavailable constitutive information based on the true geo metry of the muscle and to determine the effect of strain rate on thes e responses. An apparatus including an ultrasound imager, high-speed d igital imager, and a servohydraulic linear actuator was used to apply constant velocity deformations to the tibialis anterior of an anesthet ized neurovascularly intact rabbit The average isometric tetanic stres s prior to elongation was 0.44 +/- 0.15 MPa, Daring elongation the ave rage stimulated modulus was 0.97 +/- 0.34 MPa and was insensitive to r ate of loading. The passive stress-strain responses showed a nonlinear stiffening response typical of biologic soft tissue. Both the passive and stimulated stress-strain responses were sensitive to strain rate over the range of strain rates (1 to 25 s(-1)). Smaller changes in ave rage strain rate (1 to 10, and 10 to 25 s(-1)) did not produce statist ically significant changes in these responses, particularly in the sti mulated responses, which were less sensitive to average strain rate th an the passive responses. This relative insensitivity to strain rate s uggests that pseudoelastic functions generated from an appropriate str ain rate test may be suitable for the characterization of the response s of muscle over a narrow range of strain rates, particularly in stimu lated muscle.