The closer the hands, the more you recruit the biceps. Underhand grip uses biceps more than overhand. Use them all, find what works. Back is probably the hardest for people to learn how to work properly. Don't find the exercise that you can do the most reps, find the one you feel the most in your lats. learn to squeeze at the top, like your trying to squeeze a tennis ball between your shoulder blades. Just try it sitting in your chair, put your arms straight out to the sides, then draw your elbows in to your ribs or hips, and squeeze your back muscles together like your trying to pinch the ball and hold it. That is the feeling you need to find when working out, but it takes time. You will know more in a day or two after your workout.