Joint Articulation (Biomechanics in Terms of Injury Prevention)
Part 1: Shoulder Joint and Shoulder Girdle
Being someone with a history of shoulder injuries (not due to improper form, but from accidents as a youngster), I have to be even more cautious of my form when performing upper body exercises. However, this is not a bad thing. With proper form you are actually doing yourself a favor. With the proper form you will develop full muscles that are also symmetrical. It is important to know that each exercise you perform in the gym is designed based on your anatomy and is designed to target a specific muscle or muscle group placing that muscle or muscle group in its optimal position and range of motion to produce the maximum force. This is the beginning of an entire series of articles based on the biomechanical properties of the human body and their relation to proper form and injury prevention. As all you bodybuilders know, heavy weight in addition to lack of proper form is screaming for injury… “Cruisin’ for a bruisin’ “, as my grandfather would say. So, lets first take a look at the anatomy of the shoulder joint and the shoulder girdle and then we can analyze a few exercises that are dependent on these joints to see how your training can benefit from the analysis of the movement at this joint.
When most people think of the shoulder they thing of one joint and that’s it. However, the overall shoulder consists of technically two joints; the shoulder joint, which consists of the humeral head, the scapula and the clavicle and the shoulder girdle, which consists of the sternum, clavicle and scapula. The shoulder joint is responsible for movements such as flexion (raising the arm in the sagittal plane), extension (moving the arm backwards in the sagittal plane), abduction (moving the arm away from the body in the frontal plane) and adduction (moving the arm towards the body in the frontal plane). That is only to name the basics and a few. The shoulder girdle is responsible for movements such as elevation, depression, protraction and retraction, once again only to name a few of the basics. The muscles that act on the shoulder joint and shoulder girdle can be directly targeted by the position of the shoulder and shoulder girdle within in particular exercise. 
For example this is why there are different variations of the bench press, that is, flat bench, incline bench and decline bench. Each are intended to target different parts of the pectoralis major. Because the pec major has two insertions, one on the sternum and tone on the clavicle, it is beneficial to do different exercises to target both parts of the muscles, the sternal head and clavicular portion. The incline bench press places the shoulder joint at a degree in range of motion greater than 90. That means that the sternal portion is less active because it has a less straight line of pull in this positon. However, in the same position, that being greater than 90 degrees of flexion, the clavicular portion has a straight and better line of pull and will therefore be more active in this particular movement. The flat bench however, puts the shoulder at almost exactly 90 degrees of flexion or less. In this position the sternal portion of the pec major has a relatively straight line of pull while the clavicular portion does not. That means in this instance the sternal portion of the pec major will be stronger and more active in this particular exercise. This does not mean that incline bench isolates the clavicular portion of the pec major and the flat bench isolates the sternal portion, but it merely means that each exercise puts that particular portion in a better position with a straighter line of pull, allowing it to exert a maximal amount of force over the other portion in the particular movement.
All and all, the shoulder is a great joint, and thank god for it. However, because of all of its awesomeness, it places it at great risk for injury. The shoulder has a wide range of motion (ROM), not a lot of bony blocks and there are some relatively large muscles that are acting on the joint. This can create for some problems, but honestly, it is only problematic if you are not being smart about your form. Exercises aren’t designed to hurt you, they are designed to help you. However, they are designed with specifics, and these specifics are what keep you and your lovely joints safe when throwing around that weight.
I probably shouldn’t use the turn “throwing around weight” because it just sounds like there is simply no form involved whatsoever, but you get the point. When you lift heavy, you are putting larger amount of stress on the joint and working muscles. This is great, because this is what is going to allow you to develop larger muscles; however, if you go heavier and sacrifice your perfect form, you need to reconsider. NEVER sacrifice form when increasing weight. You will get hurt and you WILL be sorry. Anatomically, looking at the shoulder joint you can see there is little to know bony contact that is limiting any kind of movement as there is with other joints. Bony blocks are the first and foremost factor in the limitation of movement, then ligaments and then muscles. The shoulder is dependent on many ligaments and overall muscular girth for support of the joint itself. This can be seen in the rotator cuff muscles especially. The stability of the humeral head is dependent on this set of four muscles to basically “keep the ball in the socket” so to speak. Yes, there are ligaments holding the humeral head in place, but the muscles play a huge part in joint with the ligaments. This is why it is important to strengthen the rotator cuff muscles no matter how much you can shoulder press, or how much you can bench press. By strengthening these specific muscles you are creating for a more stable joint overall and possibly setting yourself up for being able to lift increased amounts of weight in alternate exercises.
Pictures from:
http://www.crossfitsouthbay.com/learn-yourself-a-quick-anatomy-reference/shoulder/
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