A lot of words float around the yoga, fitness, and health communities, but they're not always used correctly. Let's talk about "mobility" vs. "flexibility" and "stability" vs. "strength". Many people use “mobility” and “flexibility” interchangeably. Although related, these words have different meanings that are important to note.
Mobility is the amount of movement a joint has before it becomes restricted by surrounding tissue (muscle, tendon, ligament, scar tissue, etc.). According to physical therapist Gray Cook, having adequate mobility means we are able to move the body or segments of the body without restrictions, limitations, or pain. Flexibility refers to the absolute range of motion of a muscle group or joint, regardless of how it interacts with other parts of the body. Therefore flexibility plays a role in mobility, but it isn’t the whole story. Take malasana (deep squat), for example: ankle (calf) flexibility plays a role in your ability to sit down into this pose with your heels down, but flexibility across other joints in addition to control, balance, and coordination also play a role in your ability to perform this movement. You can have good flexibility without good mobility. Mobility is the "bigger picture". We create ideal mobility through developing not only flexibility, but control, balance, and coordination.
On a similar note, “strength” and “stability” are also related but distinct. Having stability means we are able to maintain and control the alignment of our bodies as we move (neuromuscular coordination). Strength is the production of force against resistance (including external weight or body weight). To achieve stability, you still need to have a certain level of strength to work against your own body’s resistance, but it also requires coordination. Think about a chaturanga; the core must be stable and therefore strong (among other things) to keep the hips from sagging down. You can’t have good stability without some strength, just like you can’t have good mobility without some degree of flexibility.
Yoga is about striking a balance between strength and flexibility as well as mobility and stability. We all fall somewhere along the stability-mobility spectrum. Some of us are naturally more mobile, flexible, and bendy, while others of us are naturally more stable, strong, and/or tight. People are attracted to that which they are already good at, so unfortunately because people think you must be flexible to go to yoga, the vast majority of yoga-goers are naturally bendy people. Let’s dispel this myth now — everyone can benefit in some way from yoga! Those who need to gain more flexibility and mobility can find it. Those who could benefit from more strength and stability can find that too! Think about where you fall on this spectrum. When approached with your goals and tendencies in mind, yoga, perhaps with the help of other modalities, offers the opportunity to achieve your true balance — lengthening what needs to be lengthened, strengthening what needs to be strengthened, and restoring what needs to be restored.