Hysteresis measurements in intact human tendon

Mechanical hysteresis in tendons has traditionally been quantified from ten sile testing of isolated specimens. Limitations associated with tendon disp lacement measurement and clamping, and uncertainties as to whether in vitro material represents intact tendon function necessitate measuring hysteresi s under in vivo conditions. In the present study such measurements were tak en in the human tibialis anterior (TA) tendon. Having the foot fixed on a d ynamometer footplate, the displacement of the TA tendon during stimulation and relaxation of the TA muscle was recorded by means of ultrasonography in six men. Combining moment data corresponding to 0, 20, 40, 60, 80 and 100% of maximum voltage moment and the respective tendon-displacement data, a h ysteresis loop was obtained between the load-displacement curves during con traction and relaxation. Measurement of the hysteresis loop area yielded a value of 19%. This value agrees with results from in vitro tensile tests of low-stress tendons, suitable for tensile force transmission and joint disp lacement control. In fact, the human TA tendon has such functional characte ristics. The methodology presented allows design of longitudinal and cross- sectional experimental protocols, and in vivo assessment of tendon function and propensity to overheat. (C) 2000 Elsevier Science Ltd. All rights rese rved.