Last updated: March 20, 2020
I am not completely sure what this concern is really intending to say, but I suspect it is something along the lines of
Holding a stress or a stretched position for a long time is damaging to the body.
A big part of the Yin Yoga style of practice is the long-held stresses applied to the tissues. We remain in the postures for 1 to 10 minutes or even longer. So, it is certainly true that we do stress tissues for quite a while in Yin Yoga. However, this is not a bad thing. It might be a bad thing, for some people, especially if they ignore warning signs that indicate they are remaining too long in the pose. If a student is getting sensations of pain and ignores these signals, then damage could happen. But long-held stresses are a normal and natural part of life!
Think about how long you sit every day. You certainly sit longer than 5 minutes at a time. Sitting for hours at a time can be unhealthy, but in Yin Yoga we do not remain in any single posture for hours at a time. When you sit for 20 or 30 minutes, stress is created along the spine and in the hips. For the vast majority of people, this is not dangerous. We do it all the time. However, if you go from sitting for a while to a load-bearing posture, like carrying heavy boxes after driving in your car for an hour, you could put too much stress on the weakened connective tissues. For this reason, counterposes are important in yoga; the body needs time to reduce the creep that has set in during the postures.
Think of how often you stand for minutes at a time. Standing places a constant stress on the knees, hips, ankles, feet, etc. Certainly, stressing these areas for 5 to 10 minutes at a time has not damaged you. We do not hold stresses longer in Yin Yoga than you are holding at other times, but in our Yin Yoga practice we are deliberately creating these static stresses in certain targeted areas that may require or desire some added focus. Stressing and resting tissues is how the body adapts and gets stronger. (Scientists even have a term for this: SAID, which means Specific Adaptation to Imposed Demand. When we impose a load, stress or demand on our tissues, they adapt and become more able to tolerate the demand.)
Holding a stress for a long time can be dangerous if we are too deep in a pose, which means the stress is too strong, or if the tissues are already damaged in some way. Generally, the body will warn us if this is risky by either sending pain signals or creating a level of discomfort that is obviously too much. If you practice with attention, you should be able to notice the pain or discomfort and use your own judgment to modify the pose or come out. It is not a long-held stress that is a problem, but rather a long-held inappropriate stress that can be dangerous. When you practice with both intention and attention, these stresses can be healing and helpful.
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 There are several studies that have shown extended sitting every day can increase mortality rates, unless they were accompanied by daily exercises or if the sitting was interrupted every 30 minutes with some mild movements. For more information, visit this site at the Mayo Clinic online: https://www.mayoclinic.org/healthy-lifestyle/adult-health/expert-answers/sitting/faq-20058005.
 There are students who, unfortunately, are in chronic pain. For these students, hearing a teacher say, “Don’t stay in a pose if you are feeling pain” is unhelpful. For these students, life is painful, and all poses involve some degree of pain and discomfort. In these cases, the intention becomes to notice if the degree and quality of the pain changes for the worse while remaining in the postures. If so, then the pose needs to be modified or abandoned. However, if the normal pain/discomfort does not become worse while remaining in the pose, it may be okay to stay and see how the stress of the pose changes the situation over the days to come. This decision is always up to the discretion of the student.