I recently returned to strength training after years of only cardio. Today I mostly exercise with bodyweight and TRX, trying to avoid the muscle isolation method of classic free weigh exercises.
Back in the day when I did free weight training at the gym, I remember always having thin biceps, unproportioinal to a well developed triceps and upper body muscles.
Now I seem to be facing the same problem again, my biceps are not developing well and are visibly thin and unproportional to the rest of my body.
For the back and biceps I do Pull-Ups, TRX High Row and TRX Bicep Curl (without twisting motion until now).
After some research I found out that the inner, short head of the bicep is responsible for twisting the forearm clockwise (from POV). I added a twist motion to the TRX Bicep Curl, and I added Dumbbell Curls.
Are there any other methods or exercises to "thicken" your biceps?