Why do 40 year old men (in particular) rupture their Achilles tendons? What are the biological and mechanical steps leading to tissue failure?
40+ year olds reside in that strange place of a) having accumulated plenty of tissue damage over the years (walking 1 mile a day for 45 years is almost 100,000,000 steps) yet b) have not quite caught on that they are old and decrepit. So they try to do too much. The Achilles tendon not only gets beaten up, it is about as far from the heart as a tendon can get, impeding the blood supply, so wear and tear damage is imperfectly repaired.
So why does the tendon fail? Well, first consider: what does it do? The Achilles powers plantar flexion (left) but also resists dorsiflexion (right) (the light blue signifies the STARTING position) and the blue arrow shows the motion of the foot
The motion of the muscle on the right is called an eccentric contraction---the muscle actually lengthens as it works ("negative" work is how the weight lifters call it) It turns out that ripping the tendon almost always happens with resisting dorsiflexion, eg, when you land.
We can't avoid eccentric contraction, because it happens with every step of gait. You land on the ball of your feet, and the gastroc/soleus decelerates your heel as it "lands" on the ground.
The importance of this is that if you wanted to prevent Achilles injuries in middle aged men, you could do this with an extension block on the shoe or with a soft heel cup: the extension block on the TOP of the shoe prevents excessive dorsi-flexion; the heel cup takes it easy on the gastroc.