Repetitive strain/stress injuries can result from knitting or crocheting for long periods of time. Especially when working one repetitive stitch. Carpal Tunnel Syndrome falls into this group. Neck and joint pain can also result.
There are ways to avoid these injuries by exercising your hands and wrists to stretch your muscles and tendons. This will help reduce stiffness and help to retain a good range of motion.
A nice warm-up before you knit is to massage your thumbs and palms of your hands. Flex your fingers by stretching them wide then clenching them. Grasp your left hand fingers with your right hand and press your hand backwards. Repeat this on your other hand. You should feel the stretch on the inside forearm. Gently press the thumb of your left hand down towards your forearm with your right thumb. Repeat this with the other thumb. Rotate your hands in a circle, first one way then the other. Repeat these exercises several times.
There are seated yoga postures to help reduce tension. Make sure you knit in a comfortable chair with your work resting in your lap. A heavy project, if not in your lap can put undue stress on your arms and hands. Use hooks and needles that are comfortable. Some knitters and crocheters find hooks and needles with cushioned handles to be beneficial.
There is a benefit to having more than one project going at a time. Then you can switch them periodically, allowing slightly different pressure points to be exercised. Have a project that uses large needles and one that uses small. Change up the weight of the yarn. Use different stitches.
Be kind to yourself. If you feel pain, you might have to either limit your knitting or possibly stop it for a time.