Avoidance of positions that contribute to shoulder injury or dislocations
Many kayakers experience shoulder injuries at some point in their paddling hobby. Most of the time, this injury can be prevented. The best prevention is strong adherence to the Paddler's Box. The Paddler's Box uses the arms like struts and ALWAYS keeps the center of the paddle shaft visually in front of their face. If you can't readily see the middle of your kayak paddle shaft, you are risking injury. Another good rule of thumb is to never let the center of the paddle shaft rise above your eye balls. Paddlers that violate this rule often separate their shoulders when a paddle blade gets caught on something like a rock. A loose paddle grip also helps to avoid injuries. When executing a draw or sculling stroke, watch where you are going. This requires you to turn the upper part of your body to your side (twisting your torso). When you perform a kayak roll, your head follows your lead paddle blade. Here is a great video demonstrating weight bearing exercises you can use to strengthen your shoulder muscles: Gym Exercises for Shoulder Strengthening. Here is the Davey Hearn shoulder Rehab article: Davey Hearn Shoulder Rehab. Watch the following video on a kayaker running National Falls on the Upper Yough and dislocating his shoulder. Watch closely and you will notice that his paddle shaft is way above his head - a big no-no: Kayaker dislocating their shoulder. We wrap this up with a very extensive article on shoulder dislocation.
Arms as struts connecting paddle to torso
A good way to think of paddling is to compare it to ballroom dancing - the man's side if he is doing it right. When dancing, we maintain a nice erect posture. We use our arms pretty much like struts to signal to our partner where we want to go. We don't crush her hand, we have a nice relaxed grip. We try to keep an eye on where we are going so we don't run into other obstacles (dancers for instance). Although we are sitting in a kayak, the basics are pretty much the same. This approach makes good use of our core muscle groups instead of pulling with just our arms. Here is a great article on paddling posture and the Paddler's Box: Paddling Posture.
Boarding, three points of contact, weight kept low, etc
In river kayaking, we often have to put in at non-optimal locations along river banks. There may be a drop-off, slippery surfaces, moving water, or other challenges. A good practice is ensuring you have at least three solid points of contact when entering/exiting your craft. This is often two feet on land and a hand holding the craft or two hands stabilizing the craft and placing one foot at a time inside the boat. The paddle float rescue in deep water depends on this principle - tripods can be very stable. Here's an article on this subject from the ACA: Boarding a kayak from shoreline. Keeping your weight low is also far less tippy. That is also demonstrated quite clearly in the paddle float rescue.
Use of larger torso muscles
In the previous article, we talked about using the whole body. In this lesson, we will concentrate on the core muscle groups. Beginning paddlers use their biceps more than any other muscle group. They tire very quickly, especially on long flat water paddles. Racers and experienced boaters use the arms more like struts and pull the paddle blade through the water with their abdomin, pectoral, shoulders, and upper back muscles. Compare the size of those muscle groups against the much smaller biceps and you can easily understand why seasoned boaters can paddle all day and make it look effortless. Here is a good article on an efficient forward stroke from a racer's perspective: Racer's Efficient Forward Stroke.
Effective Body Usage and Bio-Kinetics
Many new to the sport think paddling is all about the arms. Although you can paddle pretty much with the arms, you will tire very quickly if you do so. Seasoned paddlers use their torso as much as possible to take the load off their arms. They also push off the foot bulk head with the same side foot with each paddling stroke. Most of the body works together to get the maximum power out of each stroke. Here is a good video demonstrating torso rotation: Torso Rotation Demonstration.