Originally Posted by skigirl84
It helps strengthen and tone muscles, specifically stabilizer muscles.
HOW though? Anatomically and mechanically how, not the "you do it and it has this outcome" - how does the process act on the body to produce an outcome? Why does it make you tired? Most people can sit up all day without feeling extreme fatigue and pain and that uses muscles to stop yourself falling down but we are not in agony of fatigue doing that (I am, but that's another story!) so is it the unfamiliar action of holding a wall sit that makes it do something? What does it do to the muscle that renders it stronger? What is the process by which holding a pose makes your heart rate change, makes you sweat, makes you tired? I want to get a bit more into it.
I'll try an example. If I were to be asking about how do you bake bread, one answer is that you follow a recipe and you put it in the oven and when you get it out it's bread. The "why" answer is that strands of gluten are elastic but firm and are able to expand due to the action of the yeast, trapping air bubbles inside, where the baking process then solidifies the gluten and other parts of the gluten mixture by baking out the air, thus the bread has risen due to the incorporation of air bubbles into the mixture.
What would be the equivalent "how and why" answer about how holding a fixed pose actually has any kind of result - why is it that your body has to work harder at holding a pose in a wall sit than holding a pose lying down, both are essentially static positions, what is the process that makes one into total inactivity and the other into an exercise?