Shoulder Tendons – Customized Mechanics Series

The previous two articles, which you can read here and here, I’ve written about how the human body is designed to throw by being built to take full advantage of the elasticity.  To get a refresher of what elasticity is you can check out this article here.  This article looked at the elasticity of the lower body while this one looked at the core while today’s will focus the elastic energy of the muscles and tendons that surround the shoulder.

This will be my fifth article in a row covering this topic of elasticity and I feel as though I am just scrapping the surface of this vital power source.  I have enjoyed reading and learning about this topic and I’ve even participated in a study at local University that looked at the difference in tendons surrounding the elbow of baseball versus non-baseball subjects and after 100 throws with imaging ultrasound.  I’ll be sure to write about the results and what they mean in the future but for now let’s look at the tendons responsible for the fastest action that the human body can produce; internal rotation of the shoulder when throwing.

Tendons of the Internal Rotators

The muscles responsible for internally rotating the shoulder are the Pectoralis Major (pic below), Latissimus Dorsi, Teres Major, Deltoid and the Subscapularis.

Image result for pec major tendon variation

Their tendons on average are 58 mm long, as a comparison the Achilles tendon is 120 mm.  When we just look at the length however we only get a 2D look at something which real life is 3D.  One of the main reason that these tendons can store and release so much energy is because of it cross sectional area (CSA) which gives us an idea of the tendon’s girth.  By comparison this group of internal rotators has a CSA of 3.6-5 times larger than that of either the Achilles Tendon or the Patellar tendon (48mm) in the knee which are both used to store and release elastic energy when jumping. This gives these short tendons more volume that they need to exchange the high amounts of energy when the forearm is laid back into ER right before it springs into IR at speeds in excess of 7,500 deg/sec.

To highlight how these tendons are designed for throwing read the hypothesis from Dr. Roach whose study on human evolution and throwing I’ve be referencing for the last two articles.

“we propose that as a complex, the short length and large aggregate CSA of the numerous ligaments and muscle tendons act in parallel across the shoulder allowing these elements to return large amounts of elastic energy very rapidly

These short but thick tendons are made for the fast action of throwing but this is just an observation about humans in general.  We should know by now that not everyone’s the same and there is sure to be some variation in both the length and CSA of these tendons from person to person allow some to have naturally gifted arms.

Since different people will have slightly different tendon size and quality not everyone will use the same strategy to load into external and unload into internal rotation.  Here we see two very different arm actions, first is the 6’5″ and 220 lbs Nate Jones with a very short and quick arm action.

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Next we have the long circular arm action of 6’6″ and 180 lbs Chris Sale.

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Just to be clear I don’t know exactly what kind of tendons these guys have in terms of size and quality plus there are a lot of other factors that go into how they use their arm.  But I wanted to use this example to draw a parallel to what we learnt in a previous post about how tendons played a huge role in determining the biomechanics to two different elite high jumpers.

This first clip shows how Stefan Holm uses his short and extremely stiff tendon to get himself off the ground fast!!!  This is similar to how Nate Jones throws the ball.

Next is Donald Thomas with his longer but not as stiff tendon going through a slower jumping motion with more range of motion to produce the same type of results.  This is more along the lines of a Chris Sale type arm action.

The slightest variations of tendon length or girth (CSA) of the internal rotators might play a huge role in how a pitcher optimally loads their throwing arm back into external rotation by stretching the tendons of the internal rotators.  The reason I say “might” is because this is just a theory at this point since I haven’t been able to find any type of research to back this up.  But as a theory it makes sense to me and would put some scientific, objective and quantifiable reasoning behind the classifications that are already used in the baseball world when we say “he has a quick arm” or “he has a long loose arm action”.

The difficulty is that measuring the length and size of these tendons isn’t that easy.  Differences in length are easy to see  in the Achilles Tendon and you can even get a rough estimate by just using a measuring tape.  The variation with the internal rotators won’t be as drastic and easy to see with the naked eye but that’s not to say that their isn’t variation between tendon size of the internal rotators from person to person because their is.

The only way of truly measuring this is with imaging ultrasound like we see in the picture below where they are where they are measuring the Achilles Tendon length.

Image result for how to measure tendon length

If you have the ability to measure tendons you can see how they big they are and more importantly how stiff they are.  Tendon stiffness is a subject that is worth its very own 2000+ word article but for now you just need to know that its a good thing that can provide you with a lot of power.

If we think of tendons like the springs seen below, the shorter one on the left is very stiff and has the potential to return a lot of energy.  The reason I say potential is because it requires a lot of force to load up this spring enough to get all that energy back.  Where as the longer one on the right doesn’t need a lot of force to load it up but it won’t give too much energy back but its length is what allows it to produce more power.  In this case the tendon would be compliant instead of stiff.

Stefan Holm is said to have the stiffest tendons on record which is why he can produce so much force in such a short amount of time.  Stiff tendons require a lot of force to make them stretch enough to reap the benefits which is why he has the fastest approach speed of any higher jumper.  This is the reason why he can’t jump very high from a standstill (24 inches) but if you let him get a running start he is able to produce enough force to load up is stiff tendons in order to produce Olympic Gold Medal results.

Stiff tendons are a requirement for the sport of high jumping.  And while Donald Thomas’ aren’t as stiff he makes up for it by having a longer tendon which gives him the opportunity to build up more power and use whatever stiffness he does have.

Everyone is going to have a different combination of size (length & CSA) as well as stiffness and as a result there is going to be different ways to load up the throwing arm back into external rotation before unloading into internal rotation.

How to Get Loaded?  Into External Rotation that is..

This loading phase is known as a countermovement.  We’ve talked about the countermovement jump which is performed by squatting down which flexes your hips, knees and ankles before you rebound into the jump when those joints go into extension.  This is what they call triple extension. The countermovement stretches the tendons which adds power to the muscles contracting to help catapult you off the ground higher than if you only used your muscles if you tried to jump from a seated position.

Here is how Dr Roach described the countermovement of the arm:

“While the arm cocking countermovement is similar in a number of respects to the squat preceding a countermovement, we hypothesize that this counter rotation is driven by a different and novel mechanism (the inertial mass of the forearm)”

What this means is that the throwing arm isn’t being actively loaded into ER by the muscles that produce ER but rather the forearm is being whipped back into the layback position due to the momentum being produced by the rest of the body.  Here we see Zack Grenkie’s forearm being whipped back into the layback position.

While these two movements might differ in regards to their countermovement they share the fact that if the tendons are stretched further and faster it can potentially produce more power.

There’s that word “potential” again. There are lots of athletes who are big enough, fast enough and strong enough to load up their arm with the mechanics that precede the layback position but the lack the tendons (quality and/or size) needed to reap the benefits of this added stretch.

We learnt this concept in this video where the vertical jump was used as an example, go to the 2:50 mark.  When we perform a depth jump we should be able to jump higher than a countermovement jump because we get the benefit of a bigger and faster stretch of our tendons due to enhanced loading speed from stepping off a box.  In theory if we kept raising the height of the box that we are stepping off of we should continue to see higher and higher jumps due to the bigger and faster stretch of the tendons.  This however has a limit for each person because we end up getting to a height where we will see the jump heights go down rather than up.  If you can’t measure jump height you can watch closely and you will see the athlete spend more time on the ground as they transition from landing to take-off rather than bouncing off the ground.  Spending too much time on the ground causes an energy leak. This will happen if you don’t have enough isometric strength in the muscles or enough stiffness in the tendons.  Coordination also plays a huge role and if you have heard of “muscle slack” that’s what this is about which is another subject for another long article.

In order to assess this when it comes to throwing it’s a lot tougher to see if there is any lost energy or delay between the transition of ER into IR due to the high speeds but it is possible.  Let’s say we have a slow motion video of you pitching that’s a 150 frames per second and that your max amount of external rotation you get is 180 degrees.  In order to get the most efficient energy exchange we would only want to see that arm at exactly that 180 degree angle for one frame, anything longer and we are losing precious energy.

If this was the case it might make sense to get this pitcher to “slow down” their mechanics in order to produce a smoother stretch of the internal rotators because they are unable to handle this force that they’re producing.  It does you no good to produce a ton of force with your legs and torso if you can’t harness it and turn it into velocity.  As the old saying about earning money goes “it’s not about how you make its about how much you keep”

On the opposite end of the spectrum we are going to have pitchers that might have stiff enough tendons to produce lots of power but don’t have the size, speed, strength or mechanics to produce enough of a stretch to reach their potential.  These types of pitchers are the ones who really shine when you watch them long toss due to the extra stretch they produce with the added momentum from the crow hop but when they start from the stationary position on the hill they can’t reach the same velocity.

There are lots of other factors and even other tendons at play when we are talking about throwing a baseball.  The goal of this article was to highlight these internal rotators because in my opinion they are the limiting factor for producing velocity since they are responsible for the highest levels of speed we see when the shoulder joint goes through internal rotation during the acceleration phase.

Graeme Lehman, MSc, CSCS

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