Effects of stretching velocity on passive resistance developed by the knee musculo-articular complex: contributions of frictional and viscoelastic behaviours.

It is commonly accepted that the passive musculo-articular complex (MAC) displays a viscoelastic behavior. However, the viscosity of the MAC is still not well understood when considering the relationship between the passive resistance offered by the MAC and the stretching velocity. Therefore, in order to obtain a better knowledge of the mechanical behavior of the passive MAC, nine subjects performed passive knee extension/flexion cycles with the hip angle set at 60 degrees on a Biodex dynamometer at 5 degrees, 30 degrees, 60 degrees, 90 degrees and 120 degrees s(-1) in a randomized order to 80% of their maximal range of motion. Results show significant (P<0.001) increases with the stretching velocity for the passive torque (between +17.6 and +20.8% depending on the considered knee angle), the potential elastic energy stored during the loading (E: +22.7%), and the dissipation coefficient (DC: +22.8%). These results suggest that the role of viscosity in the MAC's mechanical behavior is limited. A linear model was well-fitted on torque-velocity (0.93<R2<0.98), E-velocity (R2=0.93) and DC-velocity (R2=0.99) relationships. The linear relationship between DC and velocity indicates that the DC does not tend towards zero for the slowest velocities and that the dissipative properties of the MAC could be modeled by combining linear viscosity and friction. The present study would allow the implementation of a rheological model to simulate the behavior of the passive MAC.