The Rhomboids (middle back) are the unsung hero of the back. They add thickness to a back, and rarely do you ever hear of anyone working them out! However everyone does work them in the process of working out their traps and the lats. Wide grip pull ups to the front work this muscle very well, so we recommend that you do Bent Over Rows and Seated Cable Rows. There are only a few exercises that target this muscle primarily.
Bare in mind that they all work efficiently, but progress would be more noticeable if you used a supplement like Testo-Max. You should see noticeable gains in about two weeks! Here are the exercises we would recommend that you perform in order to target the Rhomboids.
Bent Over Barbell Rows
Do these the same way as Bent Over Barbell Rows for upper back, the only difference being a slightly wider grip. Your really have to try to feel your shoulder blades coming together due to rhomboid contraction. Doing this exercise for upper back though, should be good enough.
Seated Cable Rows
Execute this exercise exactly the same way as Seated Cable Rows done for lats with one major difference. Instead of pulling the weight to your stomach area, pull it to where your abs meet your chest.
Rhomboid Shrugs
Rhomboid Shrugs are done either with a low pulley or on one of the Cybex or Stairmaster type Rowing Machines (this said in full recognition of your preference for free weights!)
Begin as if you are going to do regular rows but rather than bending the arms, keep them (relatively) straight and try to pull the shoulders back. You can use a rotating motion but recall your caution about rolling the shoulders while doing shrugs. This is rather like a seated, horizontal shrug.
It really gets to the middle of the back and involves the trapezius also. It could be done with a barbell or dumbells in the bent over rowing position, but you might not be able to focus on the proper movement.
Links for Reference
We recommend that if you still need more information on how to target your rhomboids, you can consult this guide here. It takes you through some more exercises that will be useful.