In yoga, inversions (which can be considered any posture in which the hips are higher than the head) are some of the most risky and most popular postures. Inversions, in theory, can be very good for the body. They improve core strength, stimulate the lymph system, and provide an energy reversal. Inversions, like spinal twists, are generally not part of very many other exercise programs outside of yoga, and that’s one of the reasons they are so popular among practitioners. But the problem with inversions is that they are so often practiced incorrectly.
One popular (and complete) inversion practiced in yoga is exactly what it sounds like: the headstand. Now, the problem with the headstand is that the head and neck were not meant to take the weight of the body. Headstand, when done properly, places very little weight on the crown of the head, and most of the weight in the forearms. It also includes the use of appropriate padding. When done improperly, headstand, shoulder stand, plow, bridge, and other inversions place too much pressure on the head or neck (and the cervical vertebrae). Over time, these postures can do serious damage if practiced incorrectly.
Even a partial inversion like downward dog must be practiced with care and attention. One of the problems identified by William Broad is that, with many hot yoga practices (and indeed outside of hot yoga as well) a hyper-mobility of the joints means that there is a lack of stability. Lots of people have the sense that yoga is about flexibility. But it’s actually about balancing strength and flexibility in order to maintain the integrity of the posture. The shoulder joint is one of the “complex joints” Broad mentions in his NPR interview which is easily compromised through incorrect practice of common yoga postures such as downward dog.
Now, I’m not saying that inversions shouldn’t be practiced at all, or that they can’t be practiced safely. When done properly, they are safe for most people without injuries or contraindications (such as un-medicated high blood pressure). The problem is that most of the time they aren’t done properly. It takes a lot of time, a lot of warm-up, and a lot of set-up (in terms of blankets and other props) to do these postures correctly. Thus, they are postures best suited for workshops or advanced classes.