Active flexibility (AF) is a mixture of strength and flexibility, while passive flexibility (PF) is pure flexibility. AF is the ability to engage our muscles while in the end range of motion, or under maximal stretch. While in PF we are unable to get into the end range of our capability without external support, as our muscles can’t engage while being stretched. An example to illustrate this is to imagine doing a side split with your legs in two aerial silks. Someone with active range flexibility will be able to bring their legs back together from the split without any support, whereas someone with passive range flexibility will require to pull themselves up with their hands (external support) to bring their feet together.
When we train to engage our muscles while in maximal stretch or in the end range of our movement capabilities, our practice is more secure and stable, protecting our joints. We are less susceptible to injury and don’t develop instability or over-flexibility in the joints. It is important to incorporate AF into our practice for the sake of longevity. To do that we need to incorporate some form of strength training into our yoga practice. Even if it is not muscle-based strength work, it is important to do joint based strength work. PF is also important and quite helpful in opening our ranges of motion quickly. (Adjustments are also a form of PF). Though to prevent injury and imbalance it is always good to augment passive flexibility with something active. Especially if one practices Yin or similar forms of yoga it is crucial to prevent degeneration in our joints as we age.