Unusual erogenous zone?

So me and my man have been together about 10 months, and only recently started having regular sex so we're still kind of learning each other's bodies. I've covered the basic bases; like many other guys he likes his neck sucked and bitten, tummy petted, back scratched, etc. But he absolutely LOSES it when I massage the tendons in his legs! I know inner thighs are a hot spot but I don't get much reaction from lightly touching the skin, just when I press firmly on the tendons just below the groin. Like he goes nuts, full on squirming, head-tossing, eyes rolling back, moaning, the whole nine. I wondered if anyone else has found this and if it's a lot more common than I think? I've never heard anything about it before and it rocks :)