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Although conditioning is a recognized creep-related phenomenon, its importance with respect to human, in vivo, conditions has, until recently, not been investigated, and mechanical testing of isolated tendon may not necessarily represent human in vivo conditions, where concerns regarding tissue storage, clamping technique, perfusion and pressure can be avoided.
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Because of the viscous, time-dependant properties of tendon, it is likely that the duration of each repeated tensile loading event will in some way influence the load–deformation properties of the tendon. The issue of history dependency in the mechanical behaviour of human tendons in vivo was addressed by Maganaris
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The loading pattern caused tendon elongation to increase by 5 mm from the first to the fifth contraction, without any significant changes thereafter. A similar pattern and magnitude of changes were found in the tendon residual deformation after relaxation. These changes clearly demonstrate a ‘conditioning’ effect.
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Adaptations of tendons to chronic regimes of loading, unloading and ageing, new findings and perspectives
In contrast to previous beliefs, it has recently been shown that human tendinous tissue is metabolically rather active in response to activity. In fact, using the microdialysis technique it has been shown that an acute bout of exercise immediately reduces human tendinous collagen synthesis followed by a dramatic increase in the subsequent days (Langberg et al. 1999b). Chronic loading appears to increase synthesis and degradation, although the latter occurs primarily in the initial phase of a period in persons subjected to increased physical activity (Langberg et al. 2001).
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However, the influence of the various components of the extracellular matrix or the collagen fibril size, number or density, on the mechanical properties of tendon has not been established. Moreover, up to recently, whether the influence of various forms of physical activity, ageing and a combination of these, would influence the mechanical properties of human tendons was mostly unknown. However, new findings, based on the above-described ultrasound techniques, show that the mechanical properties of human tendons, assessed in vivo, undergo substantial changes both with ageing and disuse. In both conditions, a decrease in tendon stiffness has been found (Fig. 5) (Reeves et al. 2003a; Karamanidis & Arampatzis, 2005; Narici et al. 2005; Reeves et al. 2005b; Maganaris et al. 2006).
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As a matter of fact, in vitro studies have shown that older tendon tissue displays: (1) an increase in collagen cross-linking; (2) a reduction in collagen fibril crimp angle; (3) an increase in elastin content; (4) a reduction in extracellular water and mucopolysacharide content; and (5) an increase in type V collagen (Kjaer, 2004). Despite the reduction in tendon stiffness with ageing, resistive loading has been shown to significantly reverse/mitigate these alterations (Reeves et al. 2003a,b, 2005a).
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Animal data show that tendon may undergo either qualitative (Buchanan & Marsh, 2001; Viidik, 1967) or hypertrophic changes (Woo et al. 1982; Birch et al. 1999), or both (Woo et al. 1982)
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Surpisingly, however, a total training stimulus of 9 months of running in previously untrained subjects did not result in tendon hypertrophy of the Achilles tendon (Hansen et al. 2003). At the same time it has been shown that resistance training for 3 months induced marked changes in the material properties of human tendon in the absence of any tendon hypertrophy (Reeves et al. 2003a,b). It is possible that these apparent discrepancies may be explained by the training mode, namely endurance versus resistance training. On the other hand, perhaps qualitative changes in the extracellular components preceed any hypertrophic response since metabolic activity increases with an acute bout of loading (Langberg et al. 1999b; Kalliokoski et al. 2005; Miller et al. 2005; Bojsen-Moller et al. 2006), and there is a lack of hypertrophy with loading for months, while the material changes (Young\’s modulus) are apparent fairly soon (Reeves et al. 2003a,b). Furthermore, what remains to be established is whether the tendon hypertrophy induced by chronic training represents a physiological response to reduce tissue stress or a tissue repair response to damage induced by repeated loading (Rosager et al. 2002).
S. Peter Magnusson, Marco V. Narici, Constantinos N. Maganaris, Michael Kjaer (2008) Human tendon behaviour and adaptation, in vivo
The Journal of Physiology 586 (1) , 71–81 doi:10.1113/jphysiol.2007.139105
(Published)
I find this article extremly interesting. It suggest that repairing and growth of connective tissue is even more complex than we figured.
In this thread was discussed if the growth of TA could happens with hypertrophy or hyperplasia:
Yet another theory of how we grow
pudendum in this thread:
Possible reason for PE induced growth
reached some evidence that hyperplasia, or cellular proliferation, was a mechanism used to make connective tissue heal and grow.
Now, the article cited suggest that there are (at least) 4 kinds of response to a stress :
1) elastic reaction;
2) viscoelastic response;
3) hypertrophy;
4) hyperplasia.
Any of this adaptive response could be triggered by a given stimulus (either acute, repeated or chronic).
The preceding studies reported in this thread suggest that there isn’t a distinct plastic deformation phase and micro-tears repairing phase, if I’m not reading them wrong: micro-tears are produced at the same time plastic deformation occurs.
Further: after the very few stretches, plastic deformation is less pronounced or un-existent, while micro-tears are increasing.
Finally, we have that high loads, repeated over time, could lead to a delayed hypertrophy adaptation in connective tissue.