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