Doing some extra band work on your off-days can be a good thing, and maybe even add some muscle mass to a weak area. The problem I see with guys who add this kind of extra work is that some do so much of it that it takes away from their main workouts.
The extra workouts should complement the main training, not take away from it. Paul Childress, when asked about extra workouts, once said, "If I can do an extra workout then I didn't train hard enough in my main workout." I'm paraphrasing the shit out of him but you get the point.
Still, they have merit. I think triceps pushdowns with a heavy band can be great if done the day after a big pressing workout. You don't have to do them every day; just 2-3 times a week for starters.
Now one thing I think most people should do is band pull-aparts and/or face pulls. These can be done every day and we all need extra upper back work. They don't have to be hard (you don't have to use a super strong band), just use a light or mini-band for these exercises.
This is great for the bench press and press as well as keeping balance in your shoulders. Fact is, most lifters are suffering from too much pressing. Do 100 reps a day of pull-aparts or face pulls and your upper back will thank you.
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