Yoga is mainly a pasive stretching?

“… even though connective tissues provide the outermost limits to stretch, it is the nervous system that provides the practical limits in day-to-day life.” (Coulter 2001, page 62)

Flexibility refers to the range of motion in joints due to ability to lengthen muscles, tendons and ligaments. Stretching is commonly understood as lengthening muscle fibres and associated connective tissues. This is obviously true, nevertheless a simple experiment of putting anyone under anaesthetics and switching off their somatic system shows that mobility in their joints, which becomes so great that they can be easily put in almost every yogic pose, primarily depends on the response of nervous system. Because nerve fibres are not elastic and can tear easily, lengthening of muscles is limited in order to protect the nerves.

This shows that although flexibility can be gradually increased by stretching muscles and facia, the mind can also influence our range of motion. When muscles are stretch passively, such as when we stretch ham strings in downward dog, some lengthening is possible because nerves meander through the tissues. But if we reach the limit which the nervous system defines as safe, it will activate this muscle to oppose further elongation; this response is called stretch reflex, or myotic reflex. In yin yoga slow stretching under the force of gravity is accompanied by relaxation, which helps to slowly overcome the stretch reflex. Yoga practice focused on breathing also helps to relax the body allowing much more flexibility. Such stretching is less invasive but ultimately much more effective. As much as forceful passive stretching is not counterproductive, when the force is so big that the muscle cannot cope, another mechanism of relaxation is activated. This however will lead to inevitable injuries.

In active stretching, in contrast, the antagonist muscle to the one which is stretched actively contracts to oppose active force, such Uthita Hasta Padangushtasana C in Ashtanga Primary Series, where one leg is lifted high (by contracting quadriceps) while standing on the other. When Padangushtasana C is immediately followed by stretching the hamstring of lifted leg, the limits can be pushed much further and with less effort. For this reason stretch would be much more effective if Padangushtasana C was followed by Padangushtasana A and B, not the other way. A similar effect is achieved when Utanasana follows Utkatasana, where both quads are activated.

Many contemporary vinyasa teaches are aware of the mechanism of active stretching incorporating is into their sequences, although traditional old vinyasas don't. Perhaps this was don't on purpose to protect the students from hyper flexibility and injuries. Although hatha yoga is commonly associated with increased mobility, it was healthy balance between strength and flexibility that was the main aim of traditional hatha yoga, and not flexibility alone.