INTRODUCTION: The terms stiffness and flexibility, which refer to muscle properties, are frequently used as synonymous in the literature. However, these two muscle properties have distinct definitions. OBJECTIVE: To investigate the contribution of the measure of flexibility to the passive stiffness of the hamstrings. METHODS: Stiffness and flexibility were measured in 33 healthy individuals. An isokinetic dynamometer registered the resistance torque offered by the hamstrings during knee passive extension at 5º/s. Muscle activity was monitored during the tests to guarantee electromyographic silence. The slope of the torque-angle curve was used to determine the hamstrings passive stiffness. The range of the test was divided into three portions to calculate the stiffness of the 1st, 2nd and 3rd thirds of knee movement. Flexibility was assessed by having the examiner move the lever of the dynamometer in the direction of knee extension. The final measure was determined as the angle at which movement was interrupted because the examiner perceived a resistance to further movement and the volunteer reported a sensation of discomfort. Simple regressions were used for statistical analysis. RESULTS: The regression analysis demonstrated correlation values of r = -0.48 (R² = 0.23; p = 0.005), r = -0.54 (R² = 0.29; p = 0.001), r = -0.46 (R² = 0.21; p = 0.007) and r = -0.45 (R² = 0.20; p = 0.008) between flexibility and total, 1st, 2nd and 3rd third stiffness, respectively. CONCLUSION: Despite significant associations between the analyzed variables, flexibility explains only a low percentage of the variability of the stiffness measure. Stiffness and flexibility are not equivalent and should be analyzed independently.
Muscle; Energy; Injury
