Forward Stroke Conundrum

Hey all, sorry for the book report, but the forward stroke is complicated!

This is my third summer surfskiing (did a lot of sea kayak and sup before that). I had a decent forward stroke to begin with, besides having zero leg drive. After really focusing on leg drive the past two seasons with the help of Lawler videos and some great local surfskiers, my leg drive is strong enough that it is exposing other weak points in the chain.

Thus far I have religiously stuck to these principles:

1) Force primarily comes from the leg drive and your body weight
2) Your goal is to transfer these forces between the water and boat while minimizing losses

My current problem is: The harder I drive the legs the quicker my rear delts tire.
It make sense why...I keep my bottom arm extended while the paddle is in the water and only bend my elbow to take the paddle out like Lawler says. Combined with the diagonal path of the paddle, between one and two o'clock most of that rearward leg drive goes through the relatively small rear delt (not the lat) with a long lever attached to it (the arm).

Humans are not strong holding their arm out straight and pushing backward with their fist. Yes the top arm helps a bit, but is also very poorly leveraged for the rearward component of the vector. (It primarily transfers the vertical component.)

One of the principles of the forward stroke is to use the big muscles of your back and legs and not the small muscles in your arm. Well, the small muscles of your arm are all stronger and much closer to the paddle face (better leverage) than the rear delt.

Possible solution:
Use a J shaped stroke rather than diagonal. Lawler himself says to use a J, but if you follow everything else he says to a T, your stroke will be a straight diagonal. (I may be misinterpreting something of course.)
In order to make a J while rotating at the hips you must bend your elbow and/or break at the shoulder while the paddle is still in the water. (slo mo video of pros shows that most break at the elbow while the paddle is still pushing water.)

Advantages:
1) Your arm is extended at the beginning of the stroke when your lat is being loaded. When the loading shifts to your rear delt, the bending of the elbow shortens the lever and reduces strain on that small muscle and distributes it down the arm.
2) You can maintain vertical force on the blade longer. Lawler says the opposite...that bending the elbow pulls the hand toward the shoulder and reduces vertical force. What this ignores is that unless you have a straight diagonal paddle path, the paddle is closer to your shoulder in the middle of the stroke compared with the catch. This means the distance between your shoulder and hand can decrease while the vertical force you apply can stay the same or increase.
In this position it is actually a lot easier to transfer vertical force. Imagine doing a dip vs holding an iron cross.

Disadvantages:
1) Reduced 'gearing'. This doesn't break the 2 principles at the top but certainly bends them. All things being equal, shortening the effective lever in the middle of the stroke reduces the force transferred to the water. It is not lost however as it means your legs don't have to push as hard either.
But in biomechanics, all things are not equal. Your arm can hold more force without the muscles tiring, and thus your legs can drive harder/faster--like lowering the gearing on a bike to a more optimal level for climbing hills. The middle/end of the stroke is like a mini hill.
2) Unavoidable Loss?
My understanding of hydrodynamics/wing physics are rudimentary, but they tell me the lift/drag ratio is better in a straight diagonal than in a J path.
My gut tells me that drag is less of an issue in paddling than in flying, and maybe even gives you a little extra to push against. But I definitely feel less clean lift when using a J vs a straight diagonal where the paddle lock feels rock solid.

But as in many sports, more efficient biomechanics are often more important than more efficient end physics.
Anyone have other solutions besides the J? Anyone have a solution to wing paddle efficiency issues while using a J?

This is primarily aimed at marathon paddling as a long lever is usually a good thing in a sprint!

Greetings Belacqua

Very interesting observations you've made. That the rear deltoid overrides the much more powerful lat might suggest that the gluteus is inactive at leg drive

Leg drive, does it feel emerging from the gluteus? This muscle, a strong hip extender, has extensive myofascial connections with the lat. Contracting purposefully the gluteus will also help to engage the lat.

Forward shoulders, or weak rhomboid, or lower trapezious could also contribute to what you express.

Is your vertical vector on the "pulling arm" over riding the horizonal vector?

Hope this is of help,

JD

Sounds like a very thoughtful examination. I’ve learned almost everything about surfski paddling from watching videos from the Mocke brothers, Oscar, Lawler, and observing fellow racers. I paddle solo 100% of the time. I’m sure my stroke is technically crap, but I can tell when the mechanics are working correctly. Coming from a sea kayak background, leg drive was new to me but makes sense. It’s amazing how much more efficient my stroke is using leg and back muscles to drive the boat. I can tell when I’m doing it wrong because my elbows are sore. Probably meaning I’m transferring too much power from the stroke to the joints instead of the blade.

One thing I needed to adapt was where my foot pressure was on the sea kayak footpeg vs where the pressure goes on a surf ski footplate.

With the skegged kayak and its seat position, it was the ball of my foot that pushed on the footpeg. With the ski, I need to consciously remember to push on the foot’s bottom half, which also happens to work better for getting more help from the glute. At the same time, I also have to check on myself to really weight the active blade, to use my own body weight on that in-water blade. If I have I relapsed into not doing that and then correct it, there is an immediate increase in propulsion. Great feedback from the ski itself. Trying to burn this into muscle memory is an ongoing thing.

Someone recently posted an exercise that involved sitting in a chair with feet planted on the floor and alternately pushing down on the heels, which naturally promoted rotation. It was kind of an Aha! demo for me, and I thank whoever posted that.

re putting body weight on the the blade, my advantage is also paddling k1 where the dip to leg drive side is more obvious when you put weight on it, than a ski is due to less buoyancy, makes it easier to practice this principle. My boat generally doesnt dip side to side much when paddling so i drill with just a slight dip to drive side as leg bottoms out (straight in a k1 as there is no hump)

I always think of the j stroke with the tail of the j really being the start of the take out rather than power phase. This side slice out also helps with the body weight shift onto drive leg (and subsequent dip mentioned above). The side slice exit also reduces tendency to late exits

I think you also have to take into account when following Ivans style is that he is primarily a flatwater paddler where longer paddles and straighter arms are the norm, with constant paddle rates. Lower elbows and and bending of the elbows is more of a shorter paddle low CoG of the surf ski paddler who needs a more flexible position for rapid shifting into skim bracing and sudden acceleration.

"One of the principles of the forward stroke is to use the big muscles of your back and legs and not the small muscles in your arm. Well, the small muscles of your arm are all stronger and much closer to the paddle face (better leverage) than the rear delt."

... and than your leg muscles ...

This is what I repeatedly thought throughout my midpacker carreer. I had used a lot of leg drive as soon as I paddled kayaks with suitable footplates, but I never overcame my suspicion that, with the expection of an all-out sprint, too much leg drive might be a waste of energy due to biomechanics. Why recruit a big muscle when its power has to be transformed via several energy-consuming corners (aka joints), and when the motion induced is actually quite small. Just try yourself how much you can move your paddle blades just by leg drive, everything else held fast. In my view many kayak coaches put the cart before the horse by claiming the legs generate great forward power. In my perception this is nonsense, legs can do little more than transferring and directing the power generated by arms, shoulders and torso to the important contact points (footplate => transfer, paddle blade entry point => direction). They help a bit, but they are not the big source. Compare paddling only with leg drive with torse and arms almost fixed, and with legs fixed and torso and arms free to do the best. In the former case you will hardly move, in the latter you will have not much less than your marathon pace. If it were otherwise, 90% of seakayakers wouldn't get anywhere, ever.

I suspect that a lot of forward stroke technique principles originate from sprint kayak, with limited relevance for longer distances. It's as if Eliud Kipchoge were told to aim at the stride length and other tec details of Jusain Bolt.

Impala wrote:

"One of the principles of the forward stroke is to use the big muscles of your back and legs and not the small muscles in your arm. Well, the small muscles of your arm are all stronger and much closer to the paddle face (better leverage) than the rear delt."
... and than your leg muscles ...
This is what I repeatedly thought throughout my midpacker carreer. I had used a lot of leg drive as soon as I paddled kayaks with suitable footplates, but I never overcame my suspicion that, with the expection of an all-out sprint, too much leg drive might be a waste of energy due to biomechanics. Why recruit a big muscle when its power has to be transformed via several energy-consuming corners (aka joints), and when the motion induced is actually quite small. Just try yourself how much you can move your paddle blades just by leg drive, everything else held fast. In my view many kayak coaches put the cart before the horse by claiming the legs generate great forward power. In my perception this is nonsense, legs can do little more than transferring and directing the power generated by arms, shoulders and torso to the important contact points (footplate => transfer, paddle blade entry point => direction). They help a bit, but they are not the big source. Compare paddling only with leg drive with torse and arms almost fixed, and with legs fixed and torso and arms free to do the best. In the former case you will hardly move, in the latter you will have not much less than your marathon pace. If it were otherwise, 90% of seakayakers wouldn't get anywhere, ever.
I suspect that a lot of forward stroke technique principles originate from sprint kayak, with limited relevance for longer distances. It's as if Eliud Kipchoge were told to aim at the stride length and other tec details of Jusain Bolt.

I always thought this, and that the leg drive was a more a reactive way of positively transferring body force. I find rotating opposite hip forward causes the leg drive to become a reactionary fulcrum between the blade and the hip . Force is transferred down the leg rather than generated by the leg. Opposite hip and shoulder going forward is just more body weight momentum on an extended lever more than sea kayak paddling style allows.

Afterall many top distance kayakers do not have monster muscles in their legs and arms.

JD, thanks for the focus on the lat! I think it has provided other possible solutions.

Taking your comment plus a little backyard biomechanics I reason the following:

To start with I get great engagement of the lat at the beginning, further my rhomboids and traps are currently fully capable of resisting forces applied and can put my scapula in whatever position I choose. The problem occurs at manipulation of the ball and socket part of the joint which the rhomboids and traps have little influence over.

Focusing however on how the lat inserts onto the humorous explains the situation perfectly! The lat essentially draws the elbow towards the body, or from a passive standpoint, resists it being pulled away from the body. That's where most of its leverage is, the direction its fibers run and what it can do with little help from other muscles.

From a paddling perspective this means it handles most of the vertical force. But I am struggling with horizontal forces. If we isolate those, it pulls roughly parallel to the direction of the humorous. From this horizontal perspective, the rear delt has the most leverage at pulling perpendicular to the humorous.

I have drawn a bird's eye snapshot of the paddling position at the moment the shoulders are perpendicular to the boat and added two possible arm positions. In the one where the angle of the arm with the line of the shoulders is smaller, the lat component of the paddle vector is providing more force, and the rear delt is dominant when this angle is larger.



I personally have a lot of flexibility rotationally. (If I held my arms forward at 90 degrees I'd be putting the paddle in the opposite side of the boat.) Therefore at the catch I have a very large angle between the horizontal line of my shoulders and arm. In order to reduce this angle, I could either rotate less or break at the shoulder at the beginning of the stroke. The first would solve the issue while maintaining a more extreme out ward paddle path/less J shape. (Don't worry, I do keep my torso locked and rotate from primarily the hips and not spine.)

Waverider and Impala, thanks for support in resisting the tenet that bigger muscles are always good!

Thus far, the legs are so much stronger that despite being in a poorly leveraged position they do not tire as quickly as the arms. I have thought that perhaps all of the blood they use might be a bit wasted, but because paddling is less cardiovascularly taxing than other sports, I never have to slow down cause I am tired. It has typically been once my upper body fills with lactic acid, and currently is when my rear delts get sore.

Waverider,

I guess what I mean by J is a path angle more parallel to the boat at the beginning and saving a lot of the lateral motion outward for the exit like you say, Making the overall path concave. Without actively making a J, the arm forms a slightly convex arc if using rotation alone.

Arcturus,

I have a Nordic skiing background, so vertical force is the one thing I do naturally paddling!

Epicpaddler,

That's kinda the same position I am in right now, my rear delts light up way before anything else so I must be doing something wrong

Arcturus wrote:

...With the ski, I need to consciously remember to push on the foot’s bottom half, which also happens to work better for getting more help from the glute. At the same time, I also have to check on myself to really weight the active blade, to use my own body weight on that in-water blade. If I have I relapsed into not doing that and then correct it, there is an immediate increase in propulsion. Great feedback from the ski itself. Trying to burn this into muscle memory is an ongoing thing.
Someone recently posted an exercise that involved sitting in a chair with feet planted on the floor and alternately pushing down on the heels, which naturally promoted rotation. It was kind of an Aha! demo for me, and I thank whoever posted that.

Greetings Arcturus

Your observation about the glute is spot on. The glute is a mayor player, in hip extension, which produces leg drive.
If we see the weighted paddle as being fixed on the water, as many coaches recommend, then it is the ski what becomes the free end of the chain of bodily motion.
Engaging the glute, upon catch, connects the leg drive with the rotation of the hip and torso. And in effect, this creates the chain of motion that ultimately drives the ski ahead.

Yes, I'm looking at the paddling motion from outside the box, with the glute being a mayor player, by radiating energy into leg drive as well as supporting lat action and torso rotation.
It may seem a lot to ask from the glute, but looking closely, it connects via myofascial tissue to the whole leg and the thoracolumbar region.

Hope that these concepts shed some light and can remain in memory while paddling. Glad that you found the chair rotation drill useful!

JD

Belacqua wrote:

JD, thanks for the focus on the lat! I think it has provided other possible solutions.
Taking your comment plus a little backyard biomechanics I reason the following:
To start with I get great engagement of the lat at the beginning, further my rhomboids and traps are currently fully capable of resisting forces applied and can put my scapula in whatever position I choose. The problem occurs at manipulation of the ball and socket part of the joint which the rhomboids and traps have little influence over.
Focusing however on how the lat inserts onto the humorous explains the situation perfectly! The lat essentially draws the elbow towards the body, or from a passive standpoint, resists it being pulled away from the body. That's where most of its leverage is, the direction its fibers run and what it can do with little help from other muscles.
From a paddling perspective this means it handles most of the vertical force. But I am struggling with horizontal forces. If we isolate those, it pulls roughly parallel to the direction of the humorous. From this horizontal perspective, the rear delt has the most leverage at pulling perpendicular to the humorous.
I have drawn a bird's eye snapshot of the paddling position at the moment the shoulders are perpendicular to the boat and added two possible arm positions. In the one where the angle of the arm with the line of the shoulders is smaller, the lat component of the paddle vector is providing more force, and the rear delt is dominant when this angle is larger.

Greetings Belacqua

Glad that my comments were useful.

I just replied to Arcturus, describing an alternative way of analyzing the paddling motion. In it, I describe how a fixed paddle creates the point from where the chain of motion is born. The motions of the arm, in this analysis, would be within a closed kinetic chain: this means that the arm is not being pulled towards the body, because it is holding a fixed paddle in the water. Rather, a closed kinetic chain implies that the hand is fixed in space and the body is being pulled towards it. As it happens, via lats, the glute also engages and radiates energy via hip extension towards the leg, which will rotate the hips, all of which eventually pushes the ski forward. The ski will become the open end of the kinetic chain then.

Nowhere in this analysis is the rear deltoid considered as a prime mover. Closed kinetic chain movements happen along gross patterns of motion, with big muscles being the main players. Think about that stuck door that you pull to open, using your bodyweight.

Possibly, the rear deltoid becomes active at the end of the power phase, to assist in the retraction of the paddle from the water. This is an open kinetic chain motion.

The rear deltoid operates as an arm abductor, weak extensor and assists in external rotation. These motions are more associated with paddle retraction, rather than being used as a mayor player in the kinetic chain that pushes the ski forward.

Another possibility that could explain why your rear delt is being targeted, and fatigued, might point to it being assisting the lateral excursion of the paddle in the water.

The lat is a powerful arm extensor and internal rotator, the rear deltoid is not built to perform the power phase.

Could your rear deltoid be telling you something else? Maybe the paddle is being held in the water too long, or too deep, or swept out too far.

Food for thought.

Thanks for your excellent analysis and thought provoking posts.

JD

RE: the exit slice movement

Waverider mentioned the slice out and up for setting up the next stroke on the other side—YES, that’s another thing being more deliberate about gave tangible positive feedback on.

The blade exit used to be almost an afterthought to me, playing second fiddle to planting the blade. But “tuning it up” really did allow the other blade to go in naturally. Allowing more of my weight to go onto the active blade—timed with the glute-powered propulsion described earlier—eliminated the annoying light PLOP that I sometimes got upon blade entry. Basically, weighting the blade securely delayed it just a tad before moving backward relative to the ski’s movement.

I guess the entire coordinated string of movements means the most effective use of my limited power goes to the active blade at the right time. Without being careful about the exit, I think I was rushing the next bite.

This has only coalesced within the last few weeks, and I assume it’ll take a lot more checking on technique as I go. A good side effect is that the sitbone pressure pain that was bad the first year and lingered some last year no longer threatens. I’m still keeping the pad on, though.

As for working the leg muscles, there might not be many sports that tax legs AND cardio to the extent that running, cycling, x-c skiing, and climbing do.

balance_fit wrote:

Could your rear deltoid be telling you something else? Maybe the paddle is being held in the water too long, or too deep, or swept out too far.

That's what came to my mind when reading the OP. I remember reading/hearing that 80% of the power is in the first quarter of the stroke. At that point my lower arm is doing zilch, it's just hanging on for dear life trying not to loose the paddle. I'm exaggerating a bit, but the arm should be basically extended and shoulder low and locked. Power comes from body rotation (w/leg assistance) and weight. The only way I can imagine getting tired deltoids is either holding the shoulders too high up or trying to pull too hard too late in the stroke in some odd way?

I was thinking about a similar issue last weekend. I searched for what a J stroke is. For me, pulling straight back does not feel as fast. I cannot catch waves with that stroke for sure.

The wing paddle stroke definitely seems more powerful. The problem with the wing paddle stroke is the potential for the arms to be doing the work, and your arms don’t have good leverage. Solution is to make the rest of the body do work, and not the arms.

From reading tips on this forum and watching video, this is what works for me right now. Hold the upper hand near the shoulder. The upper hand pushes the paddle thru the water as your torso rotates. The lower hand is straight. Think of the lower hand as a piece of rope. Is just holding the paddle. The shoulder, upper and lower hand are frozen in place. It forms a rigid triangle. None of the joints are moving at all thru the entire power stroke. What powers the boat is your lower body. That is a combination of hip rotating, and waist rotation. If the upper body is frozen, you will automatically do some kind of leg drive and twist at the waist to move the boat. When you let your body figure it out for you, you end up with a powerful stroke. You are using the core muscles, your hands are not tired.



One other trick I stumbled on is paddle with the force of grandma for 20 minutes. Get the boat moving as fast as you can with a tiny bit of force. Then you get the feel of the water. Force yourself to end the power stroke at 3 o’clock position. Any mistake you make in the catch or stroke will be obvious because your boat will not move at all. If you can get the boat to move at a satisfactory speed with the force of grandma, then when you apply full power, it will feel like you are flying with little effort.

Can I just try to clarify the orientation of the J in a J stroke?
As far as I can tell, the long limb of the J is not parallel to the side of the boat. It may be that any attempt to do this feels inefficient or fatigues some muscles unduly (eg posterior deltoid). The lower arm's elbow does move parallel to the side of the boat at first but the paddle blade is angling away from the boat, roughly parallel to the bow wave.
Never pull with the lower arm.....Beach ball drill....paddle shaft parallel to chest.
Slice that blade out nice and early.
Don't just push against the footplate with the leg on the stroke side. If you are deliberate about bringing the opposite hip forward at the same time, the leg drive push will create rotation of the torso (not just push you into the back of the seat).
I've only had 2 lessons in my life but the lightbulb moment for me was getting set up on a seated row weight machine in a gym, set up just like a paddling ergometer ie weights to cable, cable attached to only one end of a steel bar which is held with one hand at either end, just like a paddle shaft. Sorry if that is hard to visualise but it's just like Ivan Lawler's sliding paddle machine except that the seat doesn't move and the paddle pulls on a cable which lifts a pile of weights... ..........anyhow it is clear on that how the power is generated early in the stroke and that the power is generated in a chain from the leg to the oblique abdominals to the lats. With a heavy weight you can really feel which muscles are doing the work.
It's rightly compared to a shotput technique or a basketball longshot where the legs initiate the power which is translated through the core to create speed in the arms / paddle / shotput / basketball.
Power first, then speed. Thanks Birdy!
The other great advice I received but haven't done nearly often enough is to get someone to video you paddling from different angles. What you think you are doing and what you are actually doing are 2 different things. I was pretty shocked at my slouched posture (tight hamstrings, core still needs work) and late exit.
While I'm thinking about leg drive, I recall how much better it felt when I shortened the foot plate a bit closer and also how ineffectual it was when I had an old boat with a big hump that my calves were squashing into.
I don't agree that leg drive is only for sprint or marathon. 80% of my paddling is open ocean

Can I just try to clarify the orientation of the J in a J stroke?
As far as I can tell, the long limb of the J is not parallel to the side of the boat. It may be that any attempt to do this feels inefficient or fatigues some muscles unduly (eg posterior deltoid). The lower arm's elbow does move parallel to the side of the boat at first but the paddle blade is angling away from the boat, roughly parallel to the bow wave.
Never pull with the lower arm.....Beach ball drill....paddle shaft parallel to chest.
Slice that blade out nice and early.
Don't just push against the footplate with the leg on the stroke side. If you are deliberate about bringing the opposite hip forward at the same time, the leg drive push will create rotation of the torso (not just push you into the back of the seat).
I've only had 2 lessons in my life but the lightbulb moment for me was getting set up on a seated row weight machine in a gym, set up just like a paddling ergometer ie weights to cable, cable attached to only one end of a steel bar which is held with one hand at either end, just like a paddle shaft. Sorry if that is hard to visualise but it's just like Ivan Lawler's sliding paddle machine except that the seat doesn't move and the paddle pulls on a cable which lifts a pile of weights... ..........anyhow it is clear on that how the power is generated early in the stroke and that the power is generated in a chain from the leg to the oblique abdominals to the lats. With a heavy weight you can really feel which muscles are doing the work.
It's rightly compared to a shotput technique or a basketball longshot where the legs initiate the power which is translated through the core to create speed in the arms / paddle / shotput / basketball.
Power first, then speed. Thanks Birdy!
The other great advice I received but haven't done nearly often enough is to get someone to video you paddling from different angles. What you think you are doing and what you are actually doing are 2 different things. I was pretty shocked at my slouched posture (tight hamstrings, core still needs work) and late exit.
While I'm thinking about leg drive, I recall how much better it felt when I shortened the foot plate a bit closer and also how ineffectual it was when I had an old boat with a big hump that my calves were squashing into.
I don't agree that leg drive is only for sprint or marathon. 80% of my paddling is open ocean

Wombat, tve and Crab, this is precisely why it has been difficult to diagnose.

And why I think the door yank analogy and reversing the kinetic chain are possible solutions as well.

I follow directions really well, often too well! When paddling I have a short stroke; the paddle leaves the water between 2 and 2:15, never close to 3 o'clock. I keep both shoulders locked and down when the paddle is in the water. I do not punch with the top arm, and keep the blade facing down at the catch and neutral at the exit, never up. I focus the weight of the top arm (with the body hanging from it) straight down the shaft. I have very little hip/shoulder separation and drive from the legs. My bottom elbow does not bend until the moment it pulls the paddle out of the water. The paddle locks in the water like cement and doesn't release until I take it out (which is a hint at the problem).

In trying to diagnose it, I think one helpful thing is to look at changing variables. First, it is possible I am getting the muscle name wrong--but I am referring to a muscle innervated separately from the main delt, and the one that if you hold your arms out in front of you while locking your scapulae back and main delt facing upward, would function to bring your humeri away from your chest and out to the side.

It is only this spring where I have experienced the rear delt as a weak link. The two primary factors that have changed is I have worked on hamstring flexibility over the winter and my leg drive is much more dynamic (higher rom due to hamstring flexibility, plus I have been driving the legs much harder/faster to attempt to shift more fatigue into the legs).

I can't solve the problem by taking the paddle out earlier--cause I already am taking it out at 2 o'clock. I can't solve the problem by locking my arms and shoulders--because they already are locked.

Thinking about a reverse kinetic chain and yanking a doorknob appears to address the issue though because they focus the force held by the arm near the catch and release it back the other way down the chain afterward.

Near the catch, the effective horizontal lever arm from the rear delt (and shoulder in general) the the paddle relative to the direction of force is essentially zero. The effective horizontal lever arm from the paddle to the leg relative to the direction of force is essentially the distance between the ball of your hip and the ball of your shoulder in the horizontal plane--not much, but 'infinitely' more than the shoulder--which at this moment is lined up directly with the force applied.

By the time the arm reaches 2 o'clock, the effective lever arm of the shoulder is over half the length of the arm and only a few inches shorter than the lever arm acting on the leg. It makes sense, if am finishing the leg drive at 2'oclock with full force, that the rear delt is not going to be able to handle that power since shoulder and leg are now leveraged similarly.

This issue is exacerbated by increased hip rotation and thus a larger angle between humerus and chest during the stroke.

I think if I try to focus 80% or more of the force near the catch (yanking the door) and send it back into the boat immediately--the part of the stroke I am having issue with will be much more carried by momentum and require less holding power at the shoulder.

Imagine doing a single arm, single leg (same side) dumbbell deadlift in roughly the catch position. All of the force is going through your glutes and hamstrings/quads depending on your proportions, and you can hold a huge amount of weight in that position. Now try to hold that same dumbbell in the paddling position you reach at 2 o'clock. No matter how rigid your arm is, you can't hold anywhere near as much weight. (In fact if you let your self bend at the elbow, you can hold more weight!)

Wombat--the geometric point you make is something I hadn't thought about. The muscles working together to hold a rigid shape with the paddle shaft can resist forces in directions they could not otherwise. Said triangle however is mostly 2 dimensional and strongest in its own plane. Since the force I am struggling with is in the horizontal plane, I think I would need to bring the top arm down and make the plane of the triangle closer to horizontal, which would have negative effects on the blade angle in the water. But as I said earlier--better biomechanics often is more important than better end physics!

I used to get shoulder pain, I think it came from the jarring of initial application of full power. The way I got around is making sure arm is not dead straight, a slight band will absorb some of the initial shock but still stiff enough not to 'spill". Also when making the catch i make sure the offside (straight leg) is in full contact with bottom of boat and does not start to lift until blade is in the water. This prevents preemptive torso unwind and an easy identifiable marker.

It also makes for snappier leg drive rather than the slow vague leg drive that ski paddlers often end up doing, compared to say sprint kayakers. This vagueness is not just how much power is applied but also affects timing. This seems to transfer much of the initial shock from shoulder to leg. Worse case scenario is when you have started leg drive slightly before you have taken tension up in the stroke arm, That can be jarring on the shoulder which is the weakest link in the lever.

Shortening leg length forces more hip rotation just to be able to straighten leg, creating more power, of course it is subject to conditions as shorter leg length is less stable

I used to get pretty sore shoulders too. They now hardly bother me at all and I put that down to:
1. theraband exercises to strengthen rotator cuff
2. sticking to safe technique
3. change of paddle from an olympic-associated brand to the Gara Odin small with a more flexible shaft and slightly smaller blade.
4. avoiding flat out sprints with the associated loss of form.
When I do sprint for a wave I drop the paddle angle a bit and really try to make the legs and rotation do most of the work.