When you grip the paddle too tightly you feel tense, your forearms tire and cramp and you promote tendonitis or carpal tunnel syndrome. If your hand falls asleep during or after paddling, or your wrist and forearm are swollen and sore from paddling, you're probably gripping the paddle too tightly. If you are using a feathered paddle, and you think you are developing carpal tunnel problems, try adjusting your grip on the paddle shaft so that very little wrist movement is needed to feather the blade. Always try to keep your wrist, forearm, and shoulder in a straight line for the pushing part of the stroke. Try stretching tight forearms by bending your wrist while pulling your fingertips towards your elbow.
Most kayaks have flexible back rests to prevent back injuries.
The drawback to this kind of backrest is that they give little lower back support. Consequently, people who lean back in the seat are quickly uncomfortable. Leaning back or slouching while you paddle destroys the symmetry of your paddle stroke too. A slight forward lean while you plant the paddle gives you a longer stroke and lets you rest your back. Sitting up straight strengthens your abdominal muscles and allows for the torso twist and shoulder thrust that make up the efficient forward stroke.