Definition & origin
Most of us go to the mat with one rule for reading our own body: the more you feel, the more is happening. A strong stretch means the pose is working; no sensation means you are not there yet. It is the rule that sends people deeper, and it is wrong often enough to matter.
Some of the strongest sensation in a pose is the nervous system refusing: the brain senses a region that is weak or poorly controlled and holds it tight on purpose, and pulling harder rarely talks it round, because what it wants is strength, not length. And a pinch at the front of the hip that feels like progress is often bone meeting bone, the kind of stop that does not change and should not be pushed. Sensation on its own cannot tell those two apart. A map can.
So physical anatomy here means how the body's tissues, joints, and nerves actually behave in a posture, not a catalogue of Latin names. The aim is a working map: enough to feel what's happening and respond well, no more. It draws on modern movement science and yoga-specific anatomy (notably the teaching of Jason Crandell), with one rule running through it: big picture first, detail only when it helps someone move and feel better in their own body. This is the outer map, and only one of three: Three maps of the same body sets it beside yoga's own and Chinese medicine's.
Why it matters
Knowing what tissues do and where joints stop lowers the risk of injury, and it does something quieter long before anything is hurt. The more the mechanics make sense, the less you fight your own body, which frees attention for the breath and the mind, the real work of the practice.
What this works on
Mostly the physical body, the most finite layer, and the one whose aches shout loudest. But a clear map also steadies manas (the senses, the channel that carries sensation) by turning raw feeling into information you can read. And it settles buddhi (the thinking mind): given a simple, honest model, the intellect can stop catastrophising every twinge.
Core concepts
The body shares load. Bones are the framework, muscles the engines, tendons and ligaments the connectors and limits, fascia the continuous web tying it all together. A posture works best when stress is shared across the whole system rather than dumped into one joint or one muscle.
Muscles pull — in three ways. A muscle can shorten as it works (concentric, like lifting into a curl), lengthen as it works (eccentric, like lowering slowly), or hold at a fixed length (isometric, like holding a plank). The eccentric and isometric kinds (control on the way down, and the steadiness to hold) are what keep joints safe through transitions.
Mobility and flexibility are not the same. Mobility is how far a joint moves under its own power; flexibility is how far it can be taken with help from gravity, a hand, or a strap. Some flexibility is useful, but strength at the far end of your own active range matters more: that's the range you can actually control.