I’ve put here all the informations I think would be interesting for owners to know about their horse’s shoulder anatomy. In this article I’m going to talk you through the important muscles, the movements, and the nerves and vessels relevant to the area. There’s a lot to go through so please feel free to read the bits you’re most interested in or just look at the pictures 😉 And as usual, you can leave a comment below to let me know your thoughts or questions!
Part 1: joint, muscles and biomecanics
About the shoulder:
There’s 2 bones that we’re going to be looking at, the scapula (also called shoulder blade for humans) and the humerus. One major difference that horses have compared to us is that they do not have a collarbone. This means that there is no bone linking the shoulder joint with the ribcage, only muscles. I mean, I say only but there’s actually A LOT of muscles linking the 2!
The actual shoulder joint has 2 ligaments to hold the scapula and humerus together. There’s no need for more because of all the muscles actively working to keep everything together. Some people actually consider that there is 2 joints, one “real” that link the scapula and the humerus, and one “fake” that links both bones to the thorax. The first one is called scapulo-humeral joint, the second one would be the scapulo-thoracic joint. As I said it depends on who you talk to, but I thought it would be worth mentioning as it shows the importance of the muscular system that holds the shoulder and the thorax together.
This is where it gets fascinating: when I first started writting this I was going to focus on the muscles the are technically just around the shoulder, and not anywhere else. This idea lasted for about 2 secondes, as I mentaly flicked throught the muscles in my mind and realised that I would be leaving out a lot of muscles majorly important in the mobilty of the shoulder. I’m not going to describe all the muscles here as it would be tedious and boring to read, instead I’ve added pictures from my own lessons at uni and have put the names of the muscles relevant to the shoulder. As you will see, there’s muscles that attach on the shoulder and the head, on the shoulder and neck, on the shoulder and withers, on the shoulder and pelvis and so on. This, to me, is the best way to show you how much your horse moves as a unit. Even when you try to focus only on one area, you find that it brings you to other areas of the body. It also gives you an idea of how a practitioner sees your horse, and how we link tension and restricted mobility from one area to another. It’s fascinating and I always get super excited about it 🙂
I’ve made a little picture especially for this, hopefully it will be easy to read, I’m thinking about doing it slightly differently next time to make it easier BUT you will have all the informations about the biomecanics of the shoulder joint, including the muscles used for each movement. I’ve actually reproduced the drawings from a book I have, hopefully I’ve done an okay job 😊
To summarize it: the movements of the horse’s shoulder are going to be fairy limited compared to ours, due to the fact that the joint is against the rib cage.
The main ones are going to be flexion/extension, where the joint is going to slide against the ribcage either forward or back. The flexion happens in stance phase, extension during swing phase.
You then have adbuction/adduction, that you can see clearly in lateral movements.
What’s interesting for me here, is that the shoulder can be maintained, due to unbalanced tension, in one position or the other! Your practitioner then has to indentify wich muscles/joint/fascia is holding the position and work towards regaining the right range of motion. I love my job 😎
Part 2: The nerves
Today I’m going to cover the main nerves that are relevant to the shoulder. Just like for the muscles, you’ll see that some nerves come from another area of the body, which is, as usual, really interesting to know.
Before we start I just want to remind you a few things that you need to know to understand the nervous system.
– The nervous system is divided in 2 major parts, central and peripheral. The first one is the brain and the spinal cord, the second one is all the nerves that reach into muscles, organs and tissue.
– That second part is itself divided in motor and sensitive nerves. The first ones bring information from the spinal cord to the tissue, the second from the tissu to the spinal cord. So the motor part is the “orders” from the spinal cord and brain to the tissu, and the second one is the feedback given by the tissu to the spinal cord and brain.
– Finally, the motor part of the peripheral nervous system is divided between voluntary and unvoluntary nervous system. The first one deals with the muscles related to movement, the other one with your fight and flight responses, as well as your gut function, heart contraction and others. This is a quite simplistic way of putting it, but it’s not the main focus for today so I’ll just leave it at that for now.
I’m going to describe only the voluntary nerves that are related to the shoulder, as I think the unvoluntary side of things could be a whole topic on its own 🙂
The plexus brachial:
A plexus is a big crossroad of nerves. There’s a few plexus in the body, one of them being the plexus brachial, which is located between the last cervical and the first thoracic vertebrae, in front of the first rib. It’s about 6-7cm large and 1cm thick, and receives branches from the nerves C6,C7,C8, T1 and T2. From this plexus leaves all the nerves for the foreleg, some for the shoulder and the chest. It basically covers the nerve supply for most of the muscles we talked about last week. But that’s the interesting thing here. It only covers MOST of them 😉
It’s rare to have shoulder tension and not tension in C7/T1 as well, around the main part of the plexus. This usually causes tension in the opposite shoulder as well as up the neck. It’s really a key part in front end biomechanics and function 🙂
Muscles covered by the plexus brachial:
I’m only going to talk about the ones I labelled in the pictures last week.
All the pectoral muscles
Serratus ventralis thoracic
So who covers the other muscles? You might find it surprising to discover that quite a few muscles get their nerve supply from the pole!
One of the cranial nerves, number 11, is called the accessory nerve and it covers a fair few important muscles.
The accessory nerve:
For those of you who don’t know, there’s 12 pairs of cranial nerves that leave the brain around the pole. Some stay in the area to take care of things like the face, but others, like the accessory nerve, go much further. It’s not rare to have a horse that has a shoulder restricted in movement due to the pole. Of course there’s other reasons for that, but the accessory nerve is often part of the issue.
Muscles covered by the accessory nerve:
How cool is that?! Think about it, the pole influences the nerve supply of the shoulder! A lot of people are focused on getting the neck round whilst riding, which we all know isn’t how you actually get a horse to work its back, but now you also know that the pole can actually affect the shoulder muscles, so forcing the pole in a certain position will have a negative effect on the reach of your horse’s foreleg! Of course there are many other links between the pole and the shoulder, but how interesting is the nerve side of it!
As a therapist I sometimes have to work on the pole/insertion of the rhomboidus muscle, in order to release the shoulder. So make sure you don’t force your horse into a head carriage that’s not comfortable, but also make sure you don’t allow your horse to “lock” its neck really round, with the nose past the vertical line! Even if it’s not you forcing the head and neck, it still has a negative effect and usually is a way for your horse to avoid contact/working properly 😉
Part 3: the circulatory system
What is the circulatory system? Well, it’s both blood and lymphatic system. As I did for the nerves, I’m going to start with a few reminders and then get into the specifics of the shoulder area.
- Blood circulation is separated in 2 directions: from the heart to the tissues which is the arterial system, and from the tissues to the heart which is the vein system. As a little reminder, the arteries bring nutrients and oxygen to the tissue, and the veins take away the blood poor in O2 to the heart and lungs.
- The lymphatic system has all its vessels in one direction, from the tissue to the heart area, the cranial vein cava to be more precise. For those of you who don’t know what the lymphatic system is, it’s the one that is in charge of the immune response, of the detoxification of body tissue, and of carrying nutrients from the intestines to the blood, in which case it’s called chyle.
So for example, when you get swollen glands under your throat, that’s your lymphatic system fighting infection: there’s glands in different places of the body to take care of specific areas, as you’ll see on one of the pictures. Another good example is swollen legs in horses. That’s the sign of a lymphatic system that is struggling to do its job!
What’s important to know with the circulatory system, is this: any tension, whether it’s of a muscle, a joint or an organ, will affect the blood and lymphatic flow. Think of it as a hose with a knot in it. The water isn’t going to run the same way, is it? Chances are, it’s going to accumulate on one side of the knot and slow down on the other side. Well, a similar thing happens to blood and lymph when your horse has tension in its tissue. This is why it happens often that horse start itching during a session: the sudden rush of blood can make them feel itchy! There can be other reasons, but that’s a pretty common one 🙂
Now, this actually sounds a lot worse than it is, as the body knows the importance of all those vessels, hence it’s evolved through time in vessels and liquids, which have the capacity to adapt really well to how much space there is, and squize in tight spaces and carry on their journey. But still. Important to know that tension will influence all of that!
So if you look at the blood vessels of the shoulder, you can see that they’re mostly located between the shoulder and the rib cage. Remember that the shoulders are attached by many muscles to the rib cage as the horse doesn’t have a collarbone. So if you have tension around those muscles, you will have an impact on the blood flow of the area. And as horses always have a lot of tension in the shoulders, you basically always have less blood than you should. This means that the tissue is less capable of coming with metabolism, as well as less able to adapt to small tears or injuries. Finally, it also means that any actual tear will take longer to heal, as blood and lymph are absolutely major in the healing process. This is why it can be amazing for your horse, in certain cases, to see a practitioner when on box rest for injury. By working on the tissues around, your practitioner can ensure that the area get all the nutrients and cells it needs to heal properly. It will also avoid compensation patterns and overuse of other areas!
Okay, so I’ve gone past the 2000 word count here so I won’t say much else on the subject 😉As always, you can message me or leave a comment if you have any question!