I can answer this... (lots of research on foot structure to help interpret footprints found in the field... a couple of GetBiggers might know what I mean by that).
A thickening of the Achilles tendon is actually a cold adaption... usually accompanied by a robustness of the lower leg.
Cold adaption in northern Europeans (everyone except the Vikings: sexual selection for stature and strength) is usually expressed by a foreshortening of the limb ratios... shortened forearms and shins, which in turn shows as a thickening of the musculature and reduction in the size of the hands and feet.
Similar trends are evident in Neanderthals... archaic Neanderthals are replete with cold-adaptive features (200,000 to 75,000 years ago) which gradually seem to have been bred out, presumably by intermixing with modern humans. The modern humans who replaced them (possibly diluted them) then in turn began to evolve along similar lines due to similar environmental pressures.
I myself have some very obvious cold-adaptions: short limbs, short stature, small hands and feet... but it does have some bodybuilding benefits.
At 5'5'' I have (proportionately) huge Dorian-like calves: 17.5'' cold with a gastroc that extends fully half way down my shin and a soleus muscle that extends two thirds the way down my shin... in fact, my lower legs are so robust that my ankle measurement (9.75'') actually exceeds my heel-to-toe foot length (9''). Similarly, my forearm muscles extend all the way to my wrist with practically no evident tendon... makes it difficult to wear a watch.
The Luke
