Abstract
It has been postulated that human tendons are viscoelastic and their mechanical properties time-dependent. Although Achilles tendon (AT) mechanics are widely reported, there is no consensus about AT viscoelastic properties such as loading rate dependency or hysteresis, in vivo. AT force-elongation characteristics were determined from 14 subjects in an ankle dynamometer at different loading rates using motion capture assisted ultrasonography. AT stiffness and elongation were determined between 10 – 80% of maximum voluntary contraction (MVC) force at fast and slow loading rates. As subjects were unable to consistently match the target unloading rate in the slow condition, AT hysteresis was only calculated for the fast rate. There was a significant difference between the fast and the slow loading rates: 120 ± 6 vs. 21 ± 1% of MVC s-1 (mean ± standard error), respectively. However, neither stiffness (193 ± 18 N mm-1 vs. 207 ± 22 N mm-1) nor elongation at any force level (13.0 ± 1.2 mm vs. 14.3 ± 0.9 mm at 80% of MVC) were significantly different between the fast and slow loading rates. Tendon hysteresis at the fast rate was 5 ± 2%. As stiffness was not sensitive to loading rate and hysteresis was small, it was concluded that elastic properties prevail over viscous properties in the human AT. The current results support the idea that AT stiffness is independent of loading rate.Electronic supplementary materialThe online version of this article (doi:10.1186/2193-1801-2-212) contains supplementary material, which is available to authorized users.
Highlights
Tendon stiffness is the most frequently used parameter to study tendon adaptation due to maturation (Waugh et al 2011), aging (Narici and Maganaris. 2006), training (Foure et al 2010; Westh et al 2008), inactivity (Reeves et al 2005) and disease (Zhao et al 2009)
Tendon hysteresis was determined for all subjects at the fast rate (Table 1)
We have recently suggested that variation in hysteresis can be attributed to signal desynchronisation and differentiating synergist muscle activation (Finni et al 2012)
Summary
Tendon stiffness is the most frequently used parameter to study tendon adaptation due to maturation (Waugh et al 2011), aging (Narici and Maganaris. 2006), training (Foure et al 2010; Westh et al 2008), inactivity (Reeves et al 2005) and disease (Zhao et al 2009). It has been postulated that human tendons are viscoelastic and that their mechanical properties, like stiffness, depend on the rate at which the load is applied This study was motivated by the observation that in the published literature, there is currently no consensus about the loading rate dependency or hysteresis of human tendons in vivo. Viscoelastic behaviour is often disregarded in tendon adaptation studies and mechanical properties are determined at a variety of different loading rates. Tendon hysteresis was only shown to be small (7%) in the portion of the AT that attaches to soleus muscle (Zhao et al 2009), but much higher (24%) in the medial gastrocnemius (MG) (Farris et al.2011b; Wang et al 2012) and lateral gastrocnemius Tendon hysteresis was only shown to be small (7%) in the portion of the AT that attaches to soleus muscle (Zhao et al 2009), but much higher (24%) in the medial gastrocnemius (MG) (Farris et al.2011b; Wang et al 2012) and lateral gastrocnemius (Lichtwark and Wilson. 2005) tendon
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