The amount of in-depth lore that Blizzard has given to Overwatch makes it rather easy to question anything about the game that isn't specified outright, especially whether or not certain characters are human or omnic.
Especially with how similar to humans the omnics can look, things like this sometimes need specification. But luckily, you can always just ask Blizzard for any lore-related questions you may have about the game.
If you look at the concept art for Lucio, it's very easy to wonder whether or not the armor and enhancements around his legs are formed around proper human legs or whether or not they're robotic. Especially when you factor in how bulky the armor is, questioning it is easy.
However, the official Overwatch Twitter account confirmed that his legs are not robotic in the least and that he's simply got some awesome looking armor on his legs.
In the grand scheme of Overwatch lore, this isn't too important of a detail, but clarifying all questions is never a bad thing.
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