Interesting, and convincing article in the NYTimes today about the dangers of yoga, for some people - particularly those not brought up sitting in full lotus - which means most of us. Actually, yoga was originally more of a spiritual than physical undertaking, it meant 'the union of individual consciousness with universal consciousness', and was combined with other approaches to reach this end. In total, they consisted of: yoga (asanas), pranayama (breath control), mudra (subtle physical movements which deepen awareness and concentration), bandha (locking the breath in particular areas to redirect its flow for the purpose of spiritual awakening), shatkarma (purifiction practices ridding the body of toxins, and preparing a base for the higher practices of pranayama), dharana (concentration), and dhyana (meditation). Patanjali said yogic technique entailed many categories of physiological and spiritual practices which are prerequisites for samadhi, all of which were to be mastered before union with the higher reality could take place.
The yoga we know of today was developed as part of the tantric civilisation existing in India 10,000 years ago, tantra being the science of expanding one's consciousness. Lord Shiva, considered to be the embodiment of supreme consciousness, is said to be the founder of yoga. As the techniques of yoga have their source in tantra, they can not be separated, although they quite commonly are.
The ultimate purpose of yoga is to awaken Shakti, and the central pathway in the spine through which this (kundalini) energy ascends, until the higher centers of human consciousness, in particular, sahasrara chakra, are reached. When kundalini is established in sahasrara chakra, that is called yoga - the union of Shiva (consciousness) and Shakti (energy). It is said to be the greatest discovery of tantra and yoga. The awakening of kundalini is the subject matter of Hatha Yoga, but, by itself, and, as far as the article is concerned, for those for whom 'yoga' is no longer a possibility, asanas alone were a limited technique to begin with.