Cable rows are a great exercise to work your lats and middle back. Form is of the utmost importance when doing this exercise as your lower back can be easily injured if proper technique is not followed.
You can use either a bar, rope or a V handle. I prefer the latter. Position yourself on the cable row machine and grasp the handle with both hands. Keep your knees slightly bent and you back arched. Bend forward and when you are at peak extension slowly pull the handle into your upper stomach area and hold it there for a one count. This will ensure peak back muscle contraction. Ease the weight back to the starting position slowly so you are working the muscle on the return portion of the rep as well. Warm up thoroughly and use a weight that you have to fight to get 8 - 10 hard fought reps. 3 - 4 sets if done properly should leave your back muscles pumped and exhausted.
