Depending on your genetics, body type and/or build, the Latissimus dorsi, also known as "Lats," may be a very stubborn muscle group for you - when it comes to building it up. There are several exercises you can perform that are aimed at working the Lats, but to me, none of them seem to be more effective than the basic pull-ups and chin-ups - that is, if you're in decent enough shape to even pull your own body weight, but we'll get to that in a moment.
The Latissimus dorsi is the widest muscle of the back. If you'd like to see a quick diagram for a visual refreshment, look below:
If you're able to build your Lats up and proper development takes place, this is the muscle that will give you that V-Shape or V-like taper to your upper torso, and especially when viewing from the back. It sort of gives you that explosive look, and makes you appear broader than what you really are. If you combine expanded Lats with bulky shoulders, even a smaller framed person can start looking wide. Throw in some well-developed trapezius muscles, and you really will be ready to sport that "tank top" this year...
Okay, now back to the main subject: If you are a beginner or have trouble lifting your body weight, pull-ups may not be for you - at least not at the moment. If you'd still like to work your Lats, you can always use the Cable Lat Pulldown machine and the pulleys used for arm rows... that is, if you own an exercise machine or have a gym membership, etc. And if you don't have access to any of this bulky equipment, there is still hope, as you can still use dumbbells for bent-over rows and so on.
I'm just not a fan of bent-over rows or the rowing motion in general. I mean really, if you want to perform this particular motion to build up the Latissimus dorsi muscle group (in hopes of widening your back), none of these cable pulleys or dumbbell rows would compare to getting in a small boat with paddles or a canoe weighted down with passengers, and rowing the thing around a lake for hours and hours, several days a month. Ha-ha! But seriously, rows work well for some people, I'd just prefer the paddle boat if I resorted to using the rowing motion in general, that's all. I must say, though, rows seem to be better at thickening the back muscles as opposed to widening them, although they can do both. BUT, the Lat Pulldown exercise and the Pull-ups using a weighted vest, seem to work better at widening this particular area of the back.
As for the Cable Lat Pulldown exercise, since you can manually adjust the weight, it is great for the beginners who can't do enough pull-ups to amount to a workout and also for the advanced bodybuilders who find the pull-ups to be less than challenging. However, I can remedy that minor problem for the advanced folks, by recommending that you execute your pull-ups & chin-ups while training with a weighted vest.
...Unless you have something out in the yard that resembles a pull-up bar or you have built one yourself, you may also be interested in buying a Power Tower, as I mentioned in my post: "Gain Muscle Mass & Power without weights." Of course, there are other cheaper chin-up/pull-up alternatives on the market, but the Power Tower also provides you with the opportunity to do other bodybuilding exercises in addition to pull-ups, such as: dips, leg lifts and push-ups on slightly inclined bars.
Anyway, good luck building this, what I call, "stubborn muscle group," no matter what method you decide to use. And if you are successful at bulking up your Latissimus dorsi area, you'll be proud of that V-shape figure that comes along with it. Now, go build those damn Lats...
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