Acute effects of static stretching on passive stiffness of the hamstring muscles calculated using different mathematical models

Background. The aim of this study was to assess the effects of static stretching on hamstring passive stiffness calculated using different data reduction methods. Methods. Subjects performed a maximal range of motion test, five cyclic stretching repetitions and a static stretching intervention that involved five 30-s static stretches. A computerised dynamometer allowed the measurement of torque and range of motion during passive knee extension. Stiffness was then calculated as the slope of the torque-angle relationship fitted using a second-order polynomial, a fourth-order polynomial, and an exponential model. The second-order polynomial and exponential models allowed the calculation of stiffness indices normalized to knee angle and passive torque, respectively. Findings. Prior to static stretching, stiffness levels were significantly different across the models. After stretching, while knee maximal joint range of motion increased, stiffness was shown to decrease. Stiffness decreased more at the extended knee joint angle, and the magnitude of change depended upon the model used. After stretching, the stiffness indices also varied according to the model used to fit data. Thus, the stiffness index normalized to knee angle was found to decrease whereas the stiffness index normalized to passive torque increased after static stretching. Interpretation. Stretching has significant effects on stiffness, but the findings highlight the need to carefully assess the effect of different models when analyzing such data. (c) 2006 Elsevier Ltd. All rights reserved.