First, age matters. People 65 and over are more likely to be injured while doing yoga than other age groups, usually from a sprain or strain.

"With any history of orthopaedic problems—especially shoulder or back—or if you're older, please have a conversation with your doctor," says Dr. Knowles. "They may recommend a physical therapist first, one well versed in yoga."

Yoga classes range from intense physical workouts to light stretching—make sure you choose one that fits your abilities.

And avoid classes that are too large.

"Pick a small class where your teacher can make sure participants execute moves safely and properly," says Dr. Knowles.

"Having the teacher demonstrate a pose is even better. Most teachers will ask, 'Does anyone have anything going on health-wise that I should know about?' Then they'll suggest modifications. When in doubt, ask."

"Runners, swimmers, and endurance athletes do yoga to strengthen and stretch," she says. "I find that strong people like to get stronger and flexible people like to be more flexible."

"Ideally, you'll create the right balance of both. If you're very strong, muscles can get tight, causing imbalances around your joints and making you more prone to injury."

"If you're very flexible? You can still over-stretch ligaments around your joints and incur an overuse injury from doing the same thing over and over incorrectly. That's why proper technique matters."

Go slowly and intently. Remember: Practice makes perfect. Save the backbends and head or shoulder stands for when you're more proficient.

Dr. Knowles wants you to try to lengthen and lift through your spine, to make yourself taller and lighter—use your imagination here—from the top of your head to your tailbone.

Imagine a line running through you—your center of balance. Pull in your abs and ribcage like you're "holding it all in." Now you're in proper alignment, she says.

"We roll our shoulders over at the computer and look down at our phones."

"We don't reach up much, but seem to crouch down more."

"Most of us are slightly curved forward, creating tight pectoral muscles or chest muscles. Pull your shoulders together in the back and position your ears over them. 'Open up your world' as you also open your chest and ribs for better posture—for yoga and for every day."

Props aren't just for theater. Your teacher probably uses blocks, straps, or a folded blanket. They prevent you from pushing too hard too soon, and they don't make you "less of a yogi."

"We can all have some slight discomfort when we exercise," Dr. Knowles says. "When it feels more than 'this is really hard,' stop."

"The shoulder is a very mobile joint, but lightly attached to the body, so it can be fragile. With poses that open and stretch, pushing too far past your normal range of motion—the full movement potential of a joint—might strain a rotator cuff, the muscles and tendons around the shoulder."

"Too-tight hamstrings or hip flexors from sitting can affect your lower back, and so can a forced arch in a pose like Chaturanga."