QUESTION:
Is it harmful to draw knees into chest after a series of back bends or inversions?

Especially Urdhva Dhanurasana, Setu Bandhasana and Salamba Sarvangasana? For me, it feels better to let the spine naturally settle on the floor and close the knees together. Some of my students need more of a release in the low back than that offers to them.

THE POSEMASTER'S ANSWER:
This, drawing the knees to the chest, is a very common occurrence, for beginning students, during a backbends sequence. And, the word harmful may be a bit dramatic, saying it is not beneficial to the intention of the practice is a more truthful description of this occurrence. The potential for harm is greater if the the person draws their knees to the chest during a backbend sequence and then continues with more backbends. Counter-posing is one thing, sending contradictory neuromuscular signals though the body during a sequence is something completely different.

The way of practicing that we are discussing here is setting an intention on one group of postures, in this case backbends and deepening the intensity and effect of backbends for the entire practice with minimal counter-posing at the end. Each way of practicing has its own unique effects. Vinyasa is a little bit of everything. Muscles and organs get stretched and contracted in full range of motion. To practice a specific group of postures creates an entirely different effect. To practice one group of postures in a single session can increase the yoga practitioners ability to do those postures, but even more importantly the person will realize the effect of this group of postures on their body and consciousness.

Example: If one is Pita aggravated, i.e. hot by nature, inflamed, and what psychologists call a Type A personality (i.e.) aggressive and competitive, how would a backbends sequence effect this person? It would probably increase their inflammation and feed their aggressive ego. But the only way to really know how back bends would effect you is to practice backbends exclusively and find out. After a backbends practice what is your energy/fatigue level? What foods are you attracted to? How do you interact with people? How do you sleep? What dreams do you have? If you practice a little of everything you will never really know how one group of postures effect you. With this knowledge you can practice with the awareness of what you need at that moment and how to get it.

Each grouping of postures has its nature. Back bends; heating and stimulating. Forward bends; cooling and nurturing. But just because the pose has its nature that does not mean that the pose will have the same effect on each individual who practices it. If one can easily stretch forward in Pascimottanasana (seated forward bend) and lay their chest and stomach on their legs and stay there, pain free for several minutes, the forward bend will most likely have the cooling and nurturing effect that it is intended to have. If one is very stiff in the low back. tight in the hamstrings, and the sacro- iliac joints do not move efficiently, this person will not feel cool and nurtured in Pascimottanasana. The same could be said for the nature and effect of back bending. If pushing up from the floor is impossible for a person, they do not feel heated and stimulated by the attempt, they feel blocked, and frustrated and the only heat that occurs comes from the anger that accompanies frustration. The extremely flexible Vata/Pita types who proclaim their love of backbends and practice intensely, get an effect of simply feeling normal after back bends practice. It is the person who can do backbends, but feels very challenged, that will receive the heating and stimulating effect of backbends.

The mechanism for these effects is the nervous system. The nervous system is a constant interplay of motor nerves that send signals from the brain and spinal cord out to the skin, muscles bones, organs and connective tissue, and the sensory nerves that bring the signals back to the brain and spinal cord. These signals can travel smoothly back and forth or they can have a more troubled path. So if your intention is to train the path of the nervous system signals for the maximal effect of back bending, what does it serve to send the signals in the opposite direction? And, what about the denser matter of muscles and joints? Bending joints back and forth to maximum can have the effect of breaking a wire coat hanger. The protocol for this type of practice is to establish your theme at the beginning of your practice, follow through with postures and actions that support the theme, take it to the limit that you feel challenged, neutralize the action and then practice the minimum amount of counter-posing, so that you are not in pain after your practice.

Just how much counter-posing you need to practice depends on how experienced you are. You said in your question that you feel fine when you lay on your back and let the spine settle. That is fine for you. A position like that is what I would call a neutralizing posture. Lay on the back, knees bent soles of the feet on the floor and knees together. Also, prone Savasana is a good neutralizing posture, just lay on your stomach and chest. Flow the erector muscles next to the spine away from the spine. You say that is not enough for your students, but, how long do they stay in a neutralizing position? Maybe one or two minutes is fine for you, they may need 5-7 minutes, or more. As your students continue to practice for months and years, the body will not need as much time to neutralize.

To abbreviate all of this: DO NOT pull the knees to the chest during a back bends sequence. After the sequence in finished lie in one or both of the above positions until the tension in the back is relieved. Finish with MILD twists and forward bends. Practice the minimum amount and intensity of twists and forward bends so that you are not in pain after your practice.

Regarding the inversion practice, headstand is a mildly heating posture and can be practiced as preparation for back bending. If the theme of your practice is inversions, shoulderstand must be practiced after headstand. Shoulderstand is a mildly cooling posture and must not be practiced after a backbends sequence. The same idea about understanding the effect of a posture is true. If you want to know the effects a posture has on your system, do not pull the knees to the chest to erase the effect.