A student in my online Yoga Anatomy Course asked a simple question in reference to a lesson about the muscular action of the diaphragm. My response is a good example of how studying basic kinesiology can help us understand that muscle relationships are always contextual and complex.
The question was: “The fibers of the diaphragm are oriented vertically, do the fibers contract on the inhale or the exhale?”
Here’s my answer:
Well, the simple answer is “on the inhale.”
Problem is, there’s no such thing as “the” inhale. Every inhale (and exhale for that matter) places a unique demand on the body’s musculature – depending on what movement we’re doing, what position we are in relative to gravity, and what our intentions are (among many other factors).
Mechanically, inhaling is the act of increasing volume in the thoracic cavity through muscular action. The key muscle involved in that action is the diaphragm, and a concentric action shortens the distance between its lower and upper attachments, thereby increasing all three dimensions of thoracic volume.
But, the diaphragm can also be actively contracting during an exhale when an eccentric action allows its attachments to move away from each other in a controlled way (think of doing a very slow curling sit-up as you exhale).
Even more confusing is the fact that the diaphragm can be relaxed and relatively passive during both inhaling and exhaling – as in Kapalabhati.
These are just the basics of how complex an answer to that question could be.
So, for now, let’s just go with “on the inhale.”
This is the kind of fun stuff we get to geek out about in my anatomy courses.