There's a common misconception that yoga is all about stretching. Not. First of all, it's important to remember that yoga isn't, primarily, a physical practice. The asana practice is just one—and not even No. 1—of the eight "limbs" of yoga. Second, the asana practice aims for strength in equal measure to flexibility. For any muscle that is being extended, there is a muscle that must be flexed to facilitate and support that extension. Indeed, injuries occur if a muscle is yanked into maximum stretch without the reciprocal contraction of a complementary muscle. To safely and effectively stretch your hamstrings (the muscles at the back of your thigh), you must firm your quadriceps (the muscles at the front of your thigh).
One of the core "scriptures" of yoga, Patanjali's Yoga Sutras, is a compendium of short, pithy, sometimes enigmatic aphorisms—a strand of pearls of wisdom. One sutra, "Sthira sukham asanam," can be translated as "Asana is steadiness and ease." In just three words (in the Sanskrit version), Patanjali gives you yoga—and health—in a nutshell. Every position must combine strength (steadiness) and ease (relaxation)—or flexion and extension. Together, these produce balance. Strength, flexibility and balance are the foundation of physical health. If you've ever been to a physical therapist, you've probably been assigned exercises that increase strength as well as those that increase flexibility. The yoga practice is like physical therapy for your whole body.
Of course, since yoga is not all about the body, the principle of Sthira sukham asanam applies to other domains as well. Steadiness, flexibility and balance are pretty much the definition of mental as well as physical health. However, whereas in the body you can identify specific muscles to be flexed and others to be released, the mental sphere is subtler. And therein lies the beauty of yoga. Like a metaphor, the physical practice spells out in concrete terms the elusive intangibles you're seeking in your mental life. Having the concrete image helps you find the counterpart mental "muscles" that must be worked to achieve emotional balance. It's like an osmotic process, in which the wisdom achieved in the physical sphere effortlessly seeps into the mental sphere. That may be one reason you step out of yoga practice exhilarated, light, hopeful.
Converts? Anyone ready to accept yoga into her heart (and onto her mat)?