Hello anatomy friends this is Dr Alsup and in this video we will be discussing
the deltoid muscle. And I know it seems a bit odd to just be focused on one muscle in a learning
objective video but this is an important one so we want to spend some time discussing the location
innervation and of course the actions associated with the deltoid.
So my guess is that even if you have never had anatomy before
or musculoskeletal anatomy you likely have a vague idea of where the deltoid muscles are located.
Sometimes when associated with exercise you may hear this referred to as your "delts".
I often lovingly refer to it as your "thick shoulder cap". It sits superficial too and covering
the glenohumeral humeral or shoulder joint and it's really that muscle that gives the shoulder
region the distinct rounded contours that are visible. So if you put your hand over your shoulder
and palpate you can feel your deltoid muscle deep to the skin and subcutaneous tissue.
Approximately this muscle has an attachment and we're looking at a posterior view here
you can tell because you can see the majority of the scapula, so approximately this muscle
has an attachment to the bones of the pectoral girdle so both the clavicle
as well as the scapula specifically on the acromion and the spine of the scapula.
And the distal attachment is actually on the shaft of the humerus and a region referred to as the
deltoid tuberosity. So this muscle crosses and can directly affect the actions of the glenohumeral
joint to that shoulder joint. So speaking of actions let's explore this a little bit more.
And what I want you to take away regarding deltoid actions is that it is the primary abductor
of the shoulder joint so pulling the arm away from anatomical position in
the coronal or frontal plane so think lifting your arm to point to the side.
So by far the most powerful and primary abductor of the shoulder but it needs a little help at
the beginning of the movement so initiating the action of abduction and it gets that through the
contraction of the supraspinatus muscle which is one of the rotator cuff muscles it will be located
in this general region right here we'll talk more about that in upcoming videos.
Now I do want to note that the deltoid is actually pretty complex in composition
it has three separate parts that can contract independently, and those different parts can
actually produce movements that are antagonistic or opposing actions, so one part can oppose the
action of the other, so you can have flexion as well as extension and medial or internal rotation
as well as lateral and external rotation dependent on which part of the muscle is contracting.
So if you look at the movements that are possible with contracting or with contraction of the
different parts of the deltoid it actually is capable of almost every type of action possible
at the shoulder joint except for adduction. So you can see how hugely important this muscle is to the
shoulder region. I do want to note that no muscle works in a vacuum. Typically multiple muscles are
working in unison to produce an action so there are almost always more than one muscle that can
play a role in a specific action so there'll be more than one muscle that produces flexion
at the shoulder joint or extension. However there is typically a prime mover or most
powerful muscle for certain actions and this can change based on positioning of the joint
or other factors but really the deltoid is almost always and I would almost err on saying always the
prime mover in terms of abduction of the shoulder joint past the first 15 degrees of that movement.
Another important relationship we want you to walk away with is the fact that the deltoid muscle
is innervated by the axillary nerve so one of those terminal branches of the brachial plexus.
And this nerve is located deep to the muscle so you can see the deltoid muscle has been reflected
and you can see the axillary nerve right there. If there's an injury or damage to the axillary
nerve this can affect the deltoid muscle and potential for movements associated with that
and it can even lead to atrophy of the muscle if severely damaged.
One thing also to note the deltoid muscle is a notably thick and coarse fibered muscle
and because of this in its superficial location it's a common site for intramuscular injections.
And so because of that you need to be aware of the location of that axillary nerve
deep to the muscle and taking care to avoid the axillary nerve in its more posterior position.
Excellent so that is the deltoid muscle in a nutshell, those big picture concepts we want
you to walk away with. I hope you take time to review this and always feel free to reach
out with any questions. Thank you for your time and attention and have a great rest of your day