Yoga is Different than Exercise, Asana is Different than Stretching
Yoga is a process of integration. For some it is a sacred practice, for others a useful tool on a quest for health. A practice of finding balance, stability, ease. To reduce yoga to exercise is to lose a great deal of its purpose and meaning. To reduce it to stretching is to miss out on physical benefit and even to risk injury. To discuss asana only in terms of alignment may also be misleading - a challenging thought for those of us that practice 'alignment-based yoga'. While yoga inspired work has infiltrated gyms, studios and on-line exercise portals, bringing asana into the exercise science realm brings with it some particular problems.
The aim of a particular asana (lets use the ever popular uttanasana) is rarely just to stretch or strengthen one particular section of the body. A pose may be utilized to bring the nervous system into a particular state, to calm the mind, assist digestion, to prepare for pranayama or meditation, to affect the vayu, nadis or marma, to soothe or stimulate, inspire or ground the practitioner. Uttanasana opens the entire back body, ideally in an integrated way, it quiets the mind, strengthens the legs, assists the exhale and may serve as a transitional pose. Iyengar suggests that 'the spine is given a deliberate and intense stretch' in uttanasana. A similar looking exercise is a standing forward folding hamstring stretch, which requires the pelvis to rotate anteriorly at the hips and the spine to remain neutral, rather than to move into flexion. It's an important skill, but these are not the same thing. One action is about isolation of an area, the other is about integration. So while it's important to understand how the body works from the anatomical perspective, and to understand how to hone in on hamstrings for example, it doesn't mean a yoga pose exists for the purpose that a similar looking exercise does. Similar shapes, different aims.
Lengthening the hamstrings is a worthy endeavor, much needed in certain populations, quite over-done in others. What is optimal length and what are the best practices to achieve optimal hamstring length is another discussion, one sure to get many worked up over!