I have a bicep that is thick at the insertion point near the elbow and thin closer to the shoulder.
When I did various dumbbell and barbell curls and experienced soreness, the sore area was the thick part of my biceps closer to the elbow.
I recently tried an exercise which I am not sure how it's called. The starting point resembles the German muscle up (or Hefesto) and Korean dips, set the bar to waist heights, you hand from the bar with hands stretched behind your back, holding the bar with a false grip (overhand), and using the biceps pull yourself up untill your back touches the bar. Chris Heiro from Thenex refers to it as a "reverse Australian pull up".
The hand movement somewhat reminds the drag curl.
The soreness I felt afterwards was in the thin part of my biceps close to the shoulder.
Notice that during conventional curls the load on the muscle decreases towards the end of the repetition as your palms are getting closer to your shoulders. To the contrary, During the drag curl the load increases towards the end of the repetition.
This raises the following question:
Does the load distribution during an exercise affects the loaded portion of the muscle lengthwise?