How does length of the bar affect force on the muscles? Fitness Model, Rob Riches, claims that doing bicep curls with Olympic bars is different than lifting with normal bars. 

Biceps have always been a favorite muscle group of mine to train, but
  since I’ve started using the Olympic bar to curl with, I’m noticing my
  arms start to thicken out, where as before they had good size from the
  side view, but when viewed from straight on, I felt as though they
  lacked the width I should have in them.

Source
Normal barbell curl (short)

Olympic barbell curl (long)

Does Rob Riches' tip make sense when explained with physics? Let's take two barbells. The first barbell is an Olympic barbell loaded to a total of 60 lbs. The second barbell is a normal length barbell at 60 lbs as well. How would the force on the biceps differ when lifting these two different length but identical weight barbells? I don't see how the biceps would feel any difference. 
 A: Provided your hands remain the same distance from the centre of each bar then in a simple physical model the weights can be any distance further out (assuming they are at the same distance and the bars remain the same weight).
Differences in arm strength (right v left) and not exactly holding the bar in symmetric positions can add lots of complicated rotations that might lead to less efficient lifting but more exercise for the muscle.
Though it is hard to be sure with physiological models, the human body is a long way from a simple mechanical model!
A: Ok, I am a layperson when it comes to human physiology (and so are the vast majority - if not all - members of Physics SE), but with that disclaimer, I am going to attempt answering your question. 
What is the difference in load on the biceps when doing a biceps curl with a long (Olympic) versus a sort (normal fitness) barbell of the same total weight?
First: forget about differences in moment of inertia, etc. This could lead to small differences in the allowed asymmetric load on both arms, but would not lead to a different total load on both biceps combined.
My hunch is that the longer bar will allow for significantly more temporary storage in the form of elastic energy. This effect (due to bending deformations of the bar) would become increasingly important with heavier weights. Such elastic storage would allow for more explosive load on the biceps, compared to a shorter bar with the same total weight.
Just my 2c. Ask a physiologist if you want to get a more definite answer.
