No offense guys, but I don't think either of you have the expertise to say exactly how life-long isometric contractions will effect tendons and muscles.
And by the way, I forgot to mention that I got the link from one of the guidelines in "The super secret 'instant vertical' exercise" in The Vertical Project. Specifically, the guideline says that standing and walking on your toes quickly increases the strength of the achilles tendon and to a lesser extent the calfs.