There is a fundamental problem with the practice of asana or 'yogic postures'. That is, when an individual geared to exercise (and so many who attend yoga classes are) applies the same ambition they have to increase fitness levels to their yoga practice, something happens which should never happen. They go beyond the point of tension, and into pain when it comes to stretching.
Students push and pull when they should relax.
It's not that you shouldn't work in yoga postures but many find it hard to differentiate between when to push themselves to increase strength and when to hold back when stretching.
Even if you were to hold back you might find a teacher pushing you into a posture! This actually astounds and devalues yoga in the eyes of those in the fitness industry. I've witnessed this myself.
In order to increase your fitness levels you need to increase the work load, whether this is running faster or longer, lifting heavier or whatever the activity. However, if, like many are doing in yoga today, you push yourself into a yoga posture to increase your flexibility, stiffness or injury are bound to happen eventually. You are going to create weakness in the joints and injure yourself over time.
I attended a kettlebells training earlier this year and the instructor noticed my flexibility and asked me 'You don't want to be anymore flexible, do you?' No, not if it's going to cost me my stability.
The muscle receptors actually create more stiffness if you push too hard into a stretch if you don't know how to do it correctly - simply knowing the antagonistic muscle and how to work them etc. I have found over the years that the more strength training I combine with my yoga practice, the more open my body becomes, as a firm structure can make one feel safer stretching beyond the usual range of motion.