Here’s another from my Instagram Stories Q&A, plus a bit more exposition:
Q: How do you know when you’re (sic) body is anatomically not built for a certain pose (and when to accept this)?
A: This is a key question that involves a deep practice of swadhyaya (self-inquiry), and it never has any final solution, because as our body ages, the answers will constantly change.
My expanded commentary: Some questions cut right to the heart of yoga, and this is one of them. The second chapter (Sadhana Pada) of Patanjali’s Yoga Sutra offers a brilliant, succinct three-part definition of yoga practice (Kriya Yoga). Two of those parts (tapas and isvara pranidhana) are referenced in this question and the third (swadhyaya) is in my answer.
The practice of poses (asana) can be seen as a kind of tapas. Although the term tapas is usually translated as “austerity,” a more useful view derives from its primary meaning of “warmth” or “heat.” My teacher T.K.V. Desikachar described the heat of tapas as a fire which removes impurities. Asana practice accomplishes this by working our physical body and breath against the grain of our embedded habits (samskaras). The assumption behind this idea is that we are working with something that actually is changeable – like how we breathe or hold tension in certain muscles – and this is how our bodies adapt to the practice. By contrast, we sometimes discover that some poses are made difficult (if not impossible) by some aspect of our body that is not going to change – like the proportional relationship of our arm-to-torso length, or the orientation of our hip joints – and this is when we must adapt the practice to our bodies.
Through practice and self-reflection (swadhyaya) we can discover some things about ourselves that are not subject to change – that’s when acceptance of that reality needs to become our focus. This is isvara pranidhana, a surrender to that which is not changeable or within our control. Or, as Desikachar put it: “…in the final analysis, we are not the masters of everything we do.” (from Heart of Yoga)
To re-state what I said in my original answer, everything about our embodied existence is subject to some kind of change, so we must always maintain a self-reflective attitude that allows us to constantly re-evaluate what we are working to change, and what we need to stop trying to change. Surrender is itself an act of will.
Another well-known formulation of this principle is Reinhold Niebuhr’s Serenity Prayer which seeks to find “the strength to change the things we can, the serenity to accept the things we cannot change, and the wisdom to know the difference.”