The purpose of exercise is to place stress upon the body in order to create a physiological adaptation. This adaptation is specific to the type of stress used and the particular system being stressed. While this is a simple concept to understand, what is not well understood is the fact that the stress is only the catalyst for the adaptation. The key to insuring that the adaption fully takes place is rest and recovery. So one way of thinking about this process is as a reaction which would look something like the following:

system + stress + rest -> result (adaptation)

The Acromioclavicular Joint (AC) is often a problem area for many individuals, both in athletic and general populations. The AC joint is located at the top of the shoulder formed by the junction between the acromion and the clavicle. This joint is very much responsible for giving us the ability to raise our arms above the head. The AC joint is a gliding synovial joint which is important because by acting like a strut it aids the movement of the scapula leading to a greater range of arm motion. Unlike the glenohumeral joint which has the benefit of being stabilized by a combination of ligaments and muscles (rotator cuff), the AC joint must solely rely upon ligaments to prevent any kind of extraordinary mobility this is due to the fact that there is no muscle that crosses directly over the joint. The AC Joint relies on three ligaments specifically to maintain proper stability within the joint these are the Coracoclavicular ligament, Coracoacromial ligament, and the Acromioclavicular Ligament. To the left is a model picture of the Acromioclavicular Joint. Due to this reliance on ligaments alone to stabilize the joint one can safely assume that it would take very little to cause the joint to become excessively mobile.

The most often onset of AC Joint issues is correlated with a traumatic injury involving some kind of high force impact. Occurrences of this nature are most commonly seen in physical contact athletes such as Mixed Martial Artist, Hockey, Football, and Lacrosse. Ligaments are the most elastic tissue in the body and once they have been pulled on by externally applied forces the ligament becomes permanently stretched leading to the increased mobility we observe. More common though and probably more relevant to the general population is the onset of AC Joint issues in a gradual manner. In general this onset slowly develops as an individual loses scapular stability in addition to experiencing shortness in the pectoral muscle primarily the minor pec this leads to an anteriorly tilted scapula that is slightly winged out. This dysfunction leads to some major issues between the acromion and the clavicle most commonly presenting in increased separation between the two. Individuals suffering from AC Joint pain may also be lacking in thoracic mobility causing them to compensate and gain back their lack of ROM by overusing the scapula.

When trying to correct such issues you must take into account all possible dysfunction and focus on slowing improving upon each issue. It is not relevant though what has brought about the issues it is only important that you focus your attention on improving scapular stability, increasing lower trapezius strength and anterior serratus strength in order to pull the acromion back in working order with the clavicle. One should also employ foam rolling and stretching in the pectoral muscles to decrease anterior forces.

Now that we have discuss what the AC Joint is, what are some of the dysfunction and causes of this joint, and how they should be addressed it is time to offer some considerations to be taken into account when training to train around such an injury. Typically there are a lot of variable factors to be taken into account when training an individual with AC dysfunction, but three good rules I’d start with is avoid adduction of the arm especially in a horizontal path, do not apply any kind of direct contact or force onto the joint as most individuals experience pain to the touch, and lastly avoid any kind of full extension with the humerus. These are simple guidelines one can use to train around issues involving the acromioclavicular joint.

Well now that you have had your crash course in AC Joint dysfunctions and possible ways to both improve them and train around them you are ready to head off to the gym and keep a close watch on the all important AC Joint.

Grip is often a much discussed topic when it comes to various exercise techniques. I tend not to get to caught up in it myself especially coaching clients. I mean when I am just beginning to teach an individual how to properly press or row I am usually more concerned with teaching them the foundations of the overall movement, such as maintaining proper scapula position at the top of a row, while keeping a packed neck, and avoiding humeral extension rather than what particular grip their using. I often here claims from a wide variety of individuals including other fitness professionals about how using this particular grip on an exercise versus the other grip activates such and such more percentage of this and this part of a muscle. Now while studies have shown that foot placement, angle of the movement, and hand position can all affect percentages of recruited muscle fibers in a given area I think for the most part individuals and coaches would be better off insuring the movement as a whole is perfected before starting to add variations in an effort to get more muscle fiber recruitment in a certain area.

I do however have one particular area of my programming in which I do get very specific about which grip is to be used and this is with the majority of pressing movements. I prefer that my clients do a good amount of their initial pressing movement with neutral hand positions. This is what I refer to as the position between a supinated wrist and a pronated wrist where the palms face inward toward one another. I sometimes allow my more experienced clients to use a hybrid position where the wrist is pronated to about a 45 degree angle to the contact of stability (this is what I refer to as the bench or ground depending on your orientation to gravity and the exercise being performed). Just about every client I have programmed for has asked me why I prescribe this hand position over the much more popular and more frequently practiced pronated grip (palm is facing away from you). The initial question asked every time is "does this work a different part of my muscle". Now while changing hand position will shift the percentage of muscle fibers recruited within the stimulated muscle, I often don't share that with them because I do not want them getting into a mindset that they need to do 6sets of a pressing motion all with various hand positions in order to sculpt the most beautiful well rounded muscular physique. I mean come on last time I checked your goal sheet you didn't put down that you want to be the next Mr. Olympia. While grip variations and angle adjustments are great for a change of pace for most of my clients I want their focus on the holistic movement not what part of their muscle is being innervated. So normally I lie to them (its for your own good people don't hate). I tell them no it is not because it works a different muscle or part of a muscle, the reason I have you use that hand position is because it will end up saving years of wear and tear from being piled on to your shoulders. So let me explain my explanation a little further so everyone understands why a neutral grip hand position is more beneficial to shoulder health than a pronated grip hand position.

Now I know I have made the focus of this post the differences in hand positions but let me let you in on a little secret really I could care less where an individuals hand is position during a pressing movement. What I really care about is the degree of the angle of abduction from the elbow to the side of the body. You see the further the arm moves away from the body the greater the degree of external rotation in the shoulder. When the shoulder is forced to maintain a high degree of external rotation for a long period such as during a set of the bench press with the arm abducted from the side of the body more than 45 degrees it places much stress on your rotator cuff to stabilize the head of the humerus in its very shallow socket on the glenoid. Now add increased load to that position such as two 45 pound plates in the bottom position of the bench press and you have a bad situation for any body lacking proper strength in the rotator cuff (which is a large amount of the General Population Clients I see). Due to this situation I find myself being very particular about what angle my clients abduct their arm from their side to perform a pressing movement.

You are probably think but wait you were just talking about hand positions and now you have jumped to the angle at which the arm is abducted aren't those two different factors that play a role in exercise technique. Well yes they are two separate factor that are not directly related. You can maintain pronated grip with keeping the angle of arm abduction in a press very small, such as during a close-grip bench press. But what I have found is that during dumbbell pressing movements whether they be horizontal or vertical when I cue my clients to use a neutral grip they automatically assume a preferable angle of abduction which I place somewhere less than or equal to forty five degrees. Now when we are talking barbell variations of pressing movements a better cue is to adjust the grip width to allow the elbows to get to a preferred angle of abduction.

Anyway the take home message here is that I find a neutral grip when performing dumbbell variations and narrow grip width when performing barbell variations to be more shoulder friendly because it allows for a more preferred angle of abduction between the arm and the rest of the body. This smaller angle places the shoulder in less external rotation and places less stress on the small tiny rotator cuff muscles that your shoulder joint relies on heavily to maintain the humeral head in a stable position against the glenoid. So for the majority of pressing movements I would recommend a neutral hand position. Use a pronated grips sparingly and accordingly.
Let me preface this post with a little warning. 

NERD ALERT: the following reading is filled with words not used in daily-common language and if read may result in droopy eyelids and drooling.
With all joking aside I would like to take some time to talk about a process that is widely unknown among all the local gross and functional anatomy experts I run into on a daily basis at my local gym. I would like to discuss the proposed process that creates movement within a muscle. This process is most commonly known as the Sliding Filament Theory. But wait before we detail the process I must first lay out the basic structure of a muscle in order for you to understand what exactly I am referring to when we go deeper into the sliding filament theory.

Basic Structure of a Muscle

Most individuals think or visualize each muscle within the body as a single unit. This is a logical conclusion because the muscle functions as a single entity, but actually skeletal muscle is much more complex than than this portrayal. Lets approach the anatomy of a muscle as if we were dissecting it. We would begin by cutting through the outer most connective tissue covering the muscle called the epimysium. This connective tissue surrounds the entire muscle holding it and its components together. Once the epimysium has been severed small bundles of fibers will become visible, which are also wrapped in their very own connective tissue sheath. These bundles are referred to as fasciculi and their connective sheaths are perimysiums. Once we finally slice through the perimysium and place the muscle under a magnifying lens you will be able to observe individual muscle fibers. These fibers are also thought of as the individual muscle cells. But wait each fiber is also covered by an endomysium which is it's own connective tissue sheath. Now that we understand where and how a muscle fiber fits into the entire muscle we can dive into the individual muscle fibers even more, but it is important to remember moving forward that muscle fibers do not run from one end of the muscle to the other. Fibers are often divided into compartments or more transverse fibrous bands.

A Closer Look at a Muscle Fiber

Muscle Fibers are extremely small usually ranging in diameter from 10 to 80 micrometers meaning they are not visible to the naked eye. But using a magnifying lens we can observe the structure and components of a muscle fiber. Once under a lens the first observable structure will be the muscle fibers plasma membrane, or the sarcolemma which encases the entire muscle fiber. The sarcolemma is the part of the muscle fiber that fuses into the tendon at the end of each fiber and allow for the transfer of generated force by the muscle across the tendon and into the bone to create movement. 

Inside the sarcolemma the on-looker will be able to be that the space with-in is filled with a gelatin like substance called the sarcoplasm, which is the fluid part of the muscle fiber. The sarcoplasm contains mainly dissolved proteins, minerals, glycogen, fats, and necessary organelles. Yet sarcoplasm differs from cytoplasm of most cells because it contains high levels of glycogen and myoglobin instead of hemoglobin. Within the sarcoplasm is housed an extensive network of transverse tubules which are extensions of the sarcolemma that pass laterally through the muscle fiber. These tubules are interconnected and pass among myofibrils allowing nerve impulses received by the sarcolemma to be transmitted to individual myofibrils. Another network of tubules is housed within the sarcoplasm called the sarcoplasmic reticulum. This system runs next to around the myofibrils. These tubules serve as storage site for calcium which plays essential role in muscle contraction.

While observing the sarcoplasm one with also notice that their are smaller subunits of bundled fibers within the muscle fiber, which are called myofibrils. These are the contractile elements of skeletal muscle. They appear as long strands of smaller subunits called sarcomere. The sarcomeres are the basic functional unit of a myofibril. Each myofibril is composed of numerous sarcomeres joined end to end. Each sarcomere is defined as everything from one Z disk to another Z disk which includes: An I band, A band, H zone, rest of A band, and second I band. Here is where we stumble upon the most important structures for muscle movement. Under a much more powerful microscope one can differentiate two types of small protein filaments in a myofibril. The thinner of the two filaments are actin and the thicker of the two are myosin. Each myosin molecule is composed of two protein strands twisted together and one end of each strand is folded creating a globular head referred to as the myosin head and each filament contains several of these heads which protrude from the whole filament to form cross bridges that can interact with other filaments. Each actin filament contains active sites that interact with the myosin filaments.

I know probably more information than you wanted and a little bit of an anatomy nerd overload, but you know can now understand what functions will what that creates movement and causes the muscle to shorten and contract.

How Muscle Creates Movement

It is important to remember that there are several important events that preceded this next several steps I am going to outline, but they are performed by the nervous system and would require a much more in-depth discussion of motor units and action potentials that frankly is just to complex for our purposes. We will jump into a muscle contraction after the myosin cross-bridges have been activated. Once primed these cross-bridges bind tightly to the actin filaments via the active sites on the actin. This binding results in a conformational change in the cross-bridge and causes the myosin head to tilt toward the arm of the cross bridge and also drag the actin and myosin filaments in opposing directions. We refer to this tilting as the power stroke. The pulling of the two filaments results in the muscle shortening and force being generated. When the fibers are done contracting after the power stroke the myosin head remains in contact with the actin's active site, but the molecular bonding is weakened. Immediately following the myosin heads tilt it breaks away from the original active site rotates back and binds to a new active site that is further along the actin filament. Repeated attachments and power strokes cause theses two filaments to slide past each other and cause the muscle to shorten. This process will continue until the ends of the myosin filament reaches the Z disks.

And finally there you all have it the structure of a muscle and a muscle fiber and the components of these structures that work together to create movement. I apologize for the anatomical overkill, but I think it is imperative you understand what structures are being referenced in the sliding filament theory and how they integrate into the overall picture.