There has been much discussion about tendon stiffness in the last few years. Here is an interesting paper.
Here is a piece of the authors' hypothesis. " If tendons can be overloaded, their mechanical properties should change during exercise. "
The present study measured AT stiffness before and after a marathon.
Here is what they found in this study, "AT stiffness did not change significantly from the pre-race value of 197±62 N mm−1 (mean ± s.d.) to the post-race value of 206±59 N mm−1 (N=12, P=0.312). Oxygen consumption increased after the race by 7±10% (P<0.05) ". This was a small N study, but that leaves room for more large scale studies to see if it holds up.
What remains interesting and head scratching to us is that a tendon has its tension developed by the muscle contracting that is attached to it. So, one would think that a marathon would cause some fatigue in the calf which would change the tension in the achilles. But we are brought to the thought that perhaps stiffness and tension are not the same animals, not even close ?
However, the article mentions this, "A typical training effect, regardless of whether training is plyometric or isometric resistance training, is an increase in AT stiffness (Burgess et al., 2007), although the effect may be invariant to training background as runners and non-runners were found to have similar AT stiffness (Rosager et al., 2002). " Perhaps, what we are talking about however is a "baseline" level of stiffness, that is so fixed that even fatigue does not impact this low level ?
The big question is then, why the AT is so prone to injuries if stiffness remains the big question, and the goal post in rehab restoration?
Here is where these authors leave us, "Thus it may be that running itself does not predispose the AT to injuries. Rather, a combination of a rapid increase in stress, a quick crossover to new sporting activities without a training period, poor technique and/or improper footwear could play a role that has not yet been identified."
A reasonable thought, but leaving us all with more questions than answers it seems.
Additionally, 9 of the 12 subjects, the marathon induced a change in their foot strike technique but they postulated that this could be muscle fatigue related. After all, we cannot forget that there is a whole body attached to this achilles.
Achilles tendon stiffness is unchanged one hour after a marathon
Jussi Peltonen, Neil J. Cronin, Lauri Stenroth, Taija Finni, Janne Avela
Journal of Experimental Biology 2012 215: 3665-3671; doi: 10.1242/jeb.068874