Bulky biceps and horseshoe triceps are awesome, sure, but contrary to popular belief among beginner standup paddlers, there are far more important muscles to a strong SUP stroke.
While Hulk-like guns are certainly helpful, proper stroke technique relies on a symphony of the body's muscles working together through each phase of the stroke. If you're doing it right, your back and core will actually be doing most of the work, which is a relief since those tend to be much stronger muscle groups than the arms.
If your arms tire quickly when you’re paddling, you’re probably doing it wrong. The good news is that problem can be resolved by transferring the brunt of your stroke’s resistance to the bigger, more powerful muscles of your body. Here's a breakdown of the main muscles involved in a proper SUP stroke and how you can use them to paddle faster, farther and with less fatigue.
While it may seem counter-intuitive, the main source of power in a proper SUP stroke is the core. Rather than relying on arms and shoulders to pull the paddle to you after the catch phase of your stroke, try twisting slightly in the chest and dipping your stroke-side shoulder to engage your oblique and abdominal muscles, using them to pull your body forward to the planted paddle. Note the combination of back and core involved in this technique, as both are engaged when it’s done properly.
A good SUP stroke involves hinging and twisting at the waist to pull your body toward the planted paddle, a technique that takes pressure off your upper arms and places the bulk of the effort on your back and core--two of the body’s strongest muscle groups. To fully engage your upper back (deltoids and trapezius muscles), keep your lower arm straight as you plant and pull your paddle and use your lower hand as a fulcrum for the leverage of the paddle shaft. Your upper arm should hinge slightly at the elbow and push the paddle grip forward, leveraging the blade backward toward your feet. If you're doing it right you’ll feel a squeeze between your shoulder blades and far less fatigue in your arms and shoulders.
To optimize your stroke technique and get the most power possible out of each muscle group, it's important to think of your shoulders as anchors between your power muscles (back and core) and your leveraging limbs (upper arms and forearms). Your shoulders should be constantly engaged as pivot points for the swing of your paddle, but not as primary power sources for the stroke themselves. That said, constantly lifting, dipping and pulling your paddle demands a lot from the shoulders and their strength will improve the performance of all other muscles involved in your stroke.