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Your Upper Back Muscles: What Do They Do?About Your Upper Back Muscles No matter what you do in the day, you’re constantly putting stress on your back muscles, whether it’s sitting up in bed, pulling up a chair to the dinner table or even carrying your heavy briefcase to work. All of these activities require stability provided in part by your upper back muscles. They mostly consist of the latissimus dorsi (the lats), the rhomboids and the trapezius (traps). Aside from providing stability to your spine, the back muscles are also prominently used during any sort of pulling motion as in rowing or the tug-o-war game at field day. Let’s start with the lats! Latissimus dorsi It’s natural to start with the lats, because it’s the largest of the upper back muscles. It’s a triangular-shaped muscle responsible for adduction (movement towards the body), internal rotation and extension of the shoulder joint (anatomically known as the glenohumeral joint). It’s also very important in stabilizing the spine in extension (playing limbo) and flexion (bending over) of the back. But how does it work exactly?
Let’s think about this. Because the lats are attached to the inside of the upper arm, it’s easy to understand that when the muscle shortens, it’ll rotate the arm inwards and pull the upper arm closer to the body (as in a row). This is why different hand grips (either vertical or horizontal) during the t-bar row has the potential of working the muscle differently (stabalization is required from other muscles). This is where that pulling motion comes from. If you’ve ever seen a swimmer’s body, you’re notice the huge lats. Swimming is what actually gives them that V-shaped trunk. Why? Well, they’re constantly pulling themselves through the water! Any motion that pulls your arms back towards your body is working your lats.
There are 2 rhomboid muscles – a rhomboid major and rhomboid minor. I’ll only get into the rhomboid major here since it’s the largest and has more prominent applications for back stability. The rhomboid major attaches right at the top of the thoracic vertebrae (about the level of your shoulders) and runs to the medial edge of the scapula. Thus, when the muscle shortens, the shoulder blades are pulled closer together. In other words, when you’ve done your t-bar row and you want to get that extra couple inches and really feel the squeeze in your back, this is where your rhomboids start to kick in. Extending an already extended shoulder joint is its function. Trapezius
The traps start off attaching to the base of the skull (called the occipital bone), runs downward to the thoracic region of the spine and also laterally to the spine of the scapula and the acromion (anatomical locations on the scapula). These different directions allow for very different movements using a single, large muscle. Some of these include shrugging your shoulders, stabilizing the shoulder joint and tilting your head back. Your upper back muscles are extremely important for a huge amount of movements along with core stability and posture. Don't take them for granted!
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When trying to understand the function of a muscle, it’s always easiest to start with it’s attachment points. In the case of the latissimus dorsi (we’ll call it lats from here on out), it arises from the lower thoracic region of the spine (about the middle of the back) and from the iliac crest (when you rest your hands on your hips, you’re placing them on the iliac crest). The muscle runs around the side of your body, intercalating with the muscle fibers of your abdominals (oblique muscles – for stability!). Moving up the back, the lats then attach to the medial side of the humerus (the part of your upper arm closest to your ribs).
Many people don’t really know what the rhomboids are, so you’re in for a treat! And no, we’re not talking about geometry here. This one’s needed for pulling your shoulder blades (scapulas) together – kind of like when you pull something very closely to your body or hyperextend your shoulder joint. Most people with poor posture also have very weak rhomboids because their back is always supported by a chair. 
The trapezius (we’ll call it traps from now on) is a diamond-shaped muscle that lies on top of the rhomboids and part of the lats. The basic actions of the traps include retracting the scapula (pulling the shoulder blades together), depressing the scapula and elevating the scapula. Although these movements are in different directions, the trapezius is mostly involved in all of them simply because it’s muscle fibers run in different directions.