Girl meets girl

I forgot, arms aren't precisely lumpy at some parts.
The 2 very reason an arms might get lumpy is whether it's fat or brawny.

When it's fit, the upper arm lump centers in the middle biceps. If not, the arm will go flat. But when you flex the arm you'll notice it'll slightly bump at the middle bicep.

The lower arm bump is placed right after the elbow joint at the brachioradialis. There's a nice visual effect if you manage to get the brach and the pronator elevation difference.

Now, if the arm's is fat, things go totally different. The first of all the arms will thicken overall, even hands gets wider; also they get a conical shape with wider begining just below the deltoids . The fat will bump at the upper part of the brachialis, right below the deltoids. In the lower arm you follow the dimension given by the upper arm and thin it until you get to the wrist.

References:

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And I want to see that chesty girl next time.