SUP Biomechanics: Shoulders Volume 2

Anatomy of a Paddle Stroke

Monday, January 10, 12022 HE

For the backstory to this post, see my first post on biomechanics. And for more background information on general shoulder anatomy and biomechanics, see this post: Shoulders Volume 1.


Source: Schram, Ben et al. “A biomechanical analysis of the stand-up paddle board stroke: a comparative study.” PeerJ vol. 7 e8006. 1 Nov. 2019, doi:10.7717/peerj.8006
Source: Schram, Ben et al. “A biomechanical analysis of the stand-up paddle board stroke: a comparative study.” PeerJ vol. 7 e8006. 1 Nov. 2019, doi:10.7717/peerj.8006

For more of a kinematics-based take on Stand Up Paddleboard (SUP) stroke mechanics, check out this excellent video of the anatomy of a SUP paddle stroke by the team at Quick Blade Paddles below. Two things to note are the individual difference in stroke mechanics and the emphasis on burying the blade into the water. The individual differences arise from individual biomechanical factors, differences in equipment, learning, and technique acquisition. For example, a modifiable factor like the length of someone’s paddle will alter their stroke biomechanics. Similarly, non-modifiable factors like someone’s arm-span-to-height ratio, the so-called ape index, will influence their paddling technique. Leonardo Da Vinci‘s classical piece, the Vitruvian Man depicts the ape index phenomenon, as described by Vitruvius. For most people, their arm span will be equal to their height, a ratio of 1. But for some individuals, the ratio will be significantly larger or smaller. This can be seen in specific sub-group populations. For example, the sportswriter David Epstein in his book The Sports Gene devoted a chapter to “The Vitruvian NBA Player” and noted that the average arm-span-to-height ratio of an NBA player is 1.063. In the NBA there is a selection bias for players to have longer arms in relation to their height.

Leonardo Da Vinci’s Vitruvian Man.
Source: https://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Da_Vinci_Vitruve_Luc_Viatour.jpg/753px-Da_Vinci_Vitruve_Luc_Viatour.jpg

Burying the paddle blade is an important point not emphasised in the biomechanical video above as there is no water. In the study by Schram and colleagues (2019), the data was collected with the paddlers using a  KayakPro SUP ergometer. In real-life, the maximum power, and thus the greatest propulsion, is achieved when all of the surface area of the paddle blade is engaged in the water. To do this the blade must be efficiently submerged before initiating the drive/power phase of the stroke.

Also, SUPBoarder just released this video on their Pro site. It does an excellent job breaking down stroke mechanics in less anatomical and biomechanical language. Their content is well worth the membership, in my opinion, but they do offer a free 14-day trial if you want to check it out for yourself.

Schram and colleagues’ data showed that inexperienced paddlers appear more reliant on greater ranges of motion at the shoulder joint and less hip motion (2019). Experienced paddlers seem to utilize less total shoulder range of motion and more overall hip range of motion. Though, as is evident in the biomechanics video above, despite having overall lower ranges of shoulder motion, experienced paddlers had greater absolute shoulder flexion angles (see Table 1 from Schram et al., 2019).

The minimum shoulder flexion angle in the experienced paddlers was 75 degrees, compared to 35 degrees in the inexperienced paddlers. While, the maximum shoulder angle in the experienced paddlers was 132 degrees, compared to 114 degrees in the inexperienced paddlers. Thus, the average flexion range of motion was 57 degrees in the experienced paddlers but occurred through an overall greater range of shoulder flexion. Whereas inexperienced paddlers moved through a greater total range motion, 79 degrees, but through a lower overall arc of shoulder flexion.

To achieve efficient propulsion during SUP, the paddle face is best orientated vertically in the water. This is why the blade angle is offset from the shaft (for more on this check out this post by Moosejaw and this one by REI). The offset allows the blade to be vertical for longer in the water at entry or catch and through the drive/power phase of the stroke, making for a more efficient and powerful stroke. 

An image showing the paddle blade offset angle from the shaft.
Source: https://www.moosejaw.com/content/info-paddle-anatomy

A similar phenomenon holds for the hip to shoulder biomechanics. A longer stroke requires the trunk to flex forward to achieve greater reach. This re-orientation of the torso and shoulder girdle necessitates greater absolute flexion angles of the glenohumeral joint. As a thought experiment, consider paddling a SUP from a laying down position, completely horizontal. To execute any semblance of a paddle stroke, the shoulder angle required would be hyper-flexed beyond 180 degrees. And there would be next to no length of the drive phase of the stroke. At the other extreme, consider paddling completely vertical throughout the stroke. At high degrees of shoulder flexion, your paddle wouldn’t be in the water, and it is only in the middle range of the shoulder where the biomechanics would allow for paddle entry. Inexperienced paddlers demonstrate lower degrees of shoulder flexion but a larger range of shoulder motion within that lower arc range for this reason. Their more erect torso position due to limited hip flexion obligates the greater shoulder ranges of motion at lower absolute angles.

In fairness, this conclusion is more based on the representative animations above. The data on hip angles is only slightly different between experienced and inexperienced paddlers. The average hip flexion range of motion was 64 and 50 degrees, respectively. And the maximum and minimum hip flexion angles in the experienced group were 134 and 68 degrees, respectively. In the inexperienced group, the hip flexion values were 131 and 81 degrees, respectively. This difference, showing the similarity of the datasets between groups, yet the discrepancy between the visual example, may be explained by both the variance of the dataset and the selection of representative examples. The standard deviations for the inexperienced paddlers are larger, both for the hip and shoulder motions, indicating more variability in the group. Meaning that some inexperienced paddlers had greater ranges of motion while others had lesser.

The question arises as to what are the implications given that the experienced paddlers in this group paddled with less shoulder range of motion, albeit at higher joint angles, and tended to use more hip motion. The first conclusion is that anyone wanting to get into SUP or currently participating in SUP should work toward maximizing or maintaining their shoulder range of motion. This will ensure that they can comfortably have the shoulder in a high degree of flexion. As mentioned in my Shoulders Volume 1 post, I believe that non-pathological shoulder impingement is a natural joint state at extreme ranges of flexion and abduction. Scapular mechanics are paramount to limiting the risk of developing symptomatic shoulder impingement. As we saw in the volume one post, the scapular biomechanics play an integral role in the overall mechanics of the shoulder complex. An inability to upwardly rotate, or posteriorly tilt the scapula can be a predisposition to developing pathological shoulder impingement of the subacromial type. At the same time, the scapula needs to be able to protract and medially rotate (i.e., turn its anterior surface toward the mid-line of the body) to clear the coracoid process for flexion and horizontal adduction of the humerus. When paddling, the top arm that holds onto the t-bar grip of the paddle must cross the body (horizontally adduct) in a flexed position. If the scapula is not free to protract and contour the natural curvature of the rib cage into medial rotation then the humerus will repetitively compress into the coracoid process and surrounding structures. This is a recipe for coracoid impingement.

The shoulder has complex neuromusculoskeletal interrelationships with the cervical and thoracic spine (see Shoulders Volume 1). An efficient paddling technique requires contralateral trunk rotation with respect to the side of the paddle. This allows the bottom, or shaft, hand-arm greater reach for the entry phase of the stroke. Though it is possible to over-reach and lose the effective power face verticality of the paddle blade (more on this later). Some of this motion comes from the hips and will be covered in more depth in a future post. Part of the hip motion is to counter-balance the shift in the body’s centre of mass that accompanies getting the paddle to the outside of the board. This requires a degree of contralateral trunk list to the paddle-side. It is at this point that the contralateral paddle arm is in flexion and horizontal adduction. The blade must then be driven into the water to engage the medium. This is a spearing-like action and requires the extension of both shoulders, particularly the contralateral arm holding the T-grip of the paddle. This recruits muscle activity from the latissimus dorsiteres majorpectoralis major, and triceps brachii as prime movers (agonists) in addition to the dynamic stabilizers (synergists) of the shoulder girdle.

From there the drive/power phase of the stroke is initiated. Here efficient form is to keep the elbows extended so that the force generated from the larger muscles of the trunk, hips, and legs can be transferred to the paddle. The hips are extended, and the trunk list and rotation are reversed toward neutrality. Any flexion in the knees is extended. And if the movement is performed at speed, there is an unweighting of the body as the centre of mass is rapidly raised, allowing the board to rise in the water in a dolphin-like swimming action. It is somewhat counter-intuitive, but with the paddle submerged into the non-compressible aqueous medium, the paddler then pulls their body and board past the relatively stationary paddle.

The paddle is removed from the water when the stroke is near the feet. Beyond this, the paddle blade offset angle and mechanics of the stroke result in water being lifted superiorly rather than pushed posteriorly, which is inefficient forward propulsion. To disengage the paddle from the water (release phase), the blade shaft must be raised vertically from the water. This is predominantly executed by flexion of the shoulder, which is accomplished by the deltoid and pectoral muscles as well as accessory muscles like the biceps brachii and coracobrachialis. The exit/release of the paddle can be combined with a gentle medial rotation of the shaft by a slight ulnar deviation at the wrist of the top, t-grip hand, and loosening of the grip and/or slight flexion of the wrist of the bottom, shaft hand. This can help to disengage the paddle from the water and limit any shovelling of water or paddle drag.

Depending on the paddling goal (e.g. speed versus endurance) the recovery portion of the stroke may vary. When paddling for speed, the goal is to keep the paddle pulling water as much as possible. As such, the faster the recovery phase of the stroke is, the quicker the next reach and catch/entry and power/drive phase are initiated. In this context, keeping the paddle more vertical minimizes the duration of the paddle stroke recovery. To do this, the shoulders are kept at a higher angle of flexion and the trunk (i.e. the hips) are used to recover the paddle stroke. In a more leisurely paddling setting, the arms can be allowed to drop and there is more of a circumduction of the shoulder that is combined with trunk movements to recover the paddle stroke.

Harkening back to my allusion about the interrelationship between the shoulder and thoracic and cervical spine, the mobility and stability of these structures is important. The thoracic spine needs to be able to extend and rotate in order to have optimum shoulder mechanics. For example, try slouching your posture and then flexing both arms to overhead. You’ll find that your range of motion is limited and that you can only achieve the full range of 150-180 degrees of shoulder flexion by having an erect/extended spine. This comes down to the scapulothoracic junction and the tilt of the scapula that is dependent on the orientation of the thoracic cage.

Source: Kapandji, I A. The Physiology of the Joints: Volume 1 The Upper Limb, 2007.

Another point on the function of the shoulder centres around open and closed kinetic chain movements. An open kinetic chain movement is one where the distal segment is free and not fixed to a stable surface or object. For the shoulder, this would be an action in which the hand is free from a stable object, like in throwing a ball. Whereas, a closed kinetic chain movement is one where the distal segment is fixed to a stable structure. An example for the shoulder would be a pushup in which the hand is resting on the floor. However, this distinction is not always a practical one, as in many functional settings open and closed kinetic chain activities are combined or are not easily categorized. In the case of SUP, the movement is both open and closed kinetic chain depending on if the paddle is engaged with the water. However, even when the paddle is engaged with the water it is not necessarily a stable surface due to the strange and intriguing properties of the liquid form of dihydrogen monoxide (check out The Weirdness of Water Part 1) and how the thin blade edge can interact with water. So for SUP, it is important to train both open and closed kinetic chain exercises as part of a cross-training regime for shoulder health and performance.

File:Water molecule (1).svg
Dihydrogen monoxide molecule.
Source: https://commons.wikimedia.org/wiki/File:Water_molecule_(1).svg

One last interesting point is on the evolution of the shoulder. The shoulder is a joint that is quite different from our other ape ancestors. The uniqueness of the Homo sapien shoulder in this regard gives us the advantage of throwing and tool manipulation. But it does come at the price of being prone to shoulder injury. Knowing this history and having knowledge of our current understanding of the shoulder, positions you better to be a happier, healthier, and better SUPer (and maybe even a happier, healthier, and better human).


Stay tuned for a future post on the hip joint…

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: