Validity and inter-day reliability of a free-oscillation test to measure knee extensor and knee flexor musculo-articular stiffness

The purpose of this study was to assess the validity and inter-day reliability of musculo-articular stiffness (MAS) in the knee-extensor (KE) and knee-flexor (KF) muscle groups, measured with a free-oscillation technique. Fourteen participants were measured, on two occasions, for KE and KF maximal isometric voluntary contraction, rate of torque development (RTD) and electromechanical delay (EMD), along with MAS using multiple sub-maximal loads relative to the individual's maximal voluntary contraction (MAS%MVC). Furthermore, 18 participants were tested for MAS using one fixed assessment load for each muscle group (MASFL) during a separate series of tests on three occasions. MAS%MVC was significantly increased as load increased both in KE and in KF (p < 0.01) fitting a curvilinear relationship as depicted in similar studies. Validity was demonstrated relating MAS%MVC to RTD (r = 0.51-0.71, p < 0.05) and to EMD (r = −0.56 to −0.67, p < 0.05). While MAS%MVC reliability (ICC = 0.62-0.89; CV = 8.1-13.1%) was questionable to acceptable, MASFL exhibited good to excellent reliability (ICC = 0.81-0.94; CV = 3.7-6.5%). No significant systematic bias was detected for any of the variables considered. The assessment of KE and KF MAS using the free-oscillation technique appears to be valid and reliable, with the use of MASFL yielding higher reliability than the use of MAS%MVC.