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Pick the most appropriate yoga class. Ask the instructor about the best poses for the shoulders. Don't forget about your neck and mid-back. Continue with yoga for relaxation and injury prevention.
Bear in mind that there are different kinds of yoga, ranging from fairly gentle and less challenging to vigorous and very challenging. Most yoga taught in the United States is a form of hatha yoga, which involves the use of classic poses along with controlled breathing. For beginners, hatha yoga may be a little too difficult — unless you find classes tailored to total newbies. Instead, Iyengar yoga (also widely practiced in the U.S.) emphasizes precise physical alignment and makes extensive use of bolsters, blocks or blankets to help you move easily into each pose. Iyengar yoga is great for learning about proper alignment and modifications. Perhaps the most vigorous form of yoga is called ashtanga, as it involves assuming a series of postures called sun salutations in rapid, flowing movements. All the classic and modified poses in yoga challenge multiple muscle groups and joints simultaneously, so none isolate just the shoulders. Having said that, some are more challenging or stretch the shoulders to a greater extent than others. Ask your yoga instructor about the poses best suited to strengthening or stretching the shoulders and then focus on them later at home. Make sure you understand the alignment before trying to replicate postures at home while unsupervised, otherwise injuries might result.  There are many yoga poses (almost all with unusual names!), so decide if you want to primarily stretch your shoulder muscles or strengthen them. If stretching is more your desire, then pay attention to where your pain is located, such as if it is in the front, side, back, or top of your shoulder area. Research poses on the internet to become familiar with them before you join a class. Yoga poses that primarily stretch the shoulders include Bow, Camel, Cow Face, Extended Puppy, Gate and High Lunge.  Yoga poses that primarily strengthen the shoulder include the Dolphin, Half Frog, Cat, Cobra, Firefly, Handstand, Scale and Side Plank. Your shoulder pain may not only involve your shoulder joint and related muscles. Additionally, your neck and mid-back may also be contributing factors, so don't forget to do some yoga poses tailored for these areas also. For example, the levator scapulae muscles attach your shoulder blades to your upper neck and are a common source of pain due to "stress neck" and habitual postures such as computer work and driving a car. Another example is the rhomboid muscles between your shoulder blades, which are commonly strained by slouching.  Yoga postures great for your neck and mid-back include the Warrior, Supported Shoulderstand, Revolved Triangle, Plow and Noose, among others.  It's important not to try to attain poses or postures if they increase your shoulder pain. Some muscle ache or discomfort from a good stretch is likely okay, but sharp and shooting pains are never a good sign and an indication you should stop and take a break. Find a way to modify the pose or consult your doctor. Yoga may certainly help with your mild-to-moderate shoulder pain, but it's ability to reduce stress, increase overall muscle flexibility and strength, lower heart rate and blood pressure, and relieve anxiety, depression and insomnia are all proven by studies. As such, yoga is not just a short-term fix for shoulder pain, but rather as a long-term lifestyle commitment to better your health and well-being.  Realize that yoga in its full form combines physical poses, breathing exercises, meditation and a distinct philosophy regarding nutrition and lifestyle choices. Remember that practicing yoga should not replace conventional medical care when it comes to musculoskeletal injuries or other health conditions.