Abstract
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.
Highlights
IntroductionThe passive mechanical properties of a musculo-articular complex (MAC), including structures spanning the joint (Riemann et al 2001), can be determined by using the relationship between the articular angle and the passive torque developed by the MAC in resistance to motion (Gajdosik 2001; Gajdosik et al 2004, 2005; Magnusson 1998; Magnusson et al 1995, 1996a, 1998, 2000a, b; McNair et al 2001, 2002; McNair and Portero 2005; Nordez et al 2006, 2008b)
Two subjects were excluded from the study because their sEMG RMS levels during the passive stretching protocol were higher than 1% of the maximal level reached during isometric contractions
Increases in passive torque, E and dissipation coefficient (DC), are only approximately 20%, while the stretching angular velocity was multiplied by more than 20. These results suggest that the passive torque has relatively little velocity dependence, and that viscosity does not play a major role in the passive mechanical behavior of the musculo-articular complex (MAC)
Summary
The passive mechanical properties of a musculo-articular complex (MAC), including structures spanning the joint (Riemann et al 2001), can be determined by using the relationship between the articular angle and the passive torque developed by the MAC in resistance to motion (Gajdosik 2001; Gajdosik et al 2004, 2005; Magnusson 1998; Magnusson et al 1995, 1996a, 1998, 2000a, b; McNair et al 2001, 2002; McNair and Portero 2005; Nordez et al 2006, 2008b). The dissipation of energy during a loading/unloading cycle could partially be due to friction (Esteki and Mansour 1996)
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