Flexibility is dependent upon structure. The shape of the join, how much cartilage is there to protect the joint, ligament/muscular support, and whether there is damage due to disease or injury (ex., osteoarthritis and excessive repetitive motion), all contribute to the potential flexibility of the joint.
Flexibility is the ability of the joint to have appropriate movement of the ligament that holds the joint together, and the muscle surrounding the joint needs to be able to stretch to accommodate the movement. There are different types of joints (i.e., ball and socket, pivot, hinge, and etc.), which will dictate the healthy stretch of the ligaments and muscles, and the direction of movement. For example, a hinge joint (knee) has safe and healthy flexibility in flexing the knee so the heel of the foot moves towards the buttocks. Unhealthy stretch is the knee moving into hyperextension so the knee buckles backward. This creates too much stretch on the ligaments and muscles that support the knee joint, destabilizes the joint, and can lead to improper movement of the joints above and below the knee (hips and ankles).
So range of motion is combining strength with healthy flexibility of the joint. Someone who is very flexible may not have full or adequate range of motion due to lack of muscle strength to take the joint through full range of motion unless assisted manually. For instance, lay on your back and flex one knee up to the chest while holding on the the back of the thigh. You might be able to really take the joint through a lot of movement, but release your hands and try doing it with the power of your muscles and not the support of your hands/arms. The range of motion may be very different than the flexibility of the joint.
In order to be flexible and increase range of motion in a joint, the nervous system has to believe the body is safe to execute the movement. The nervous system will immediately contract the muscle to protect the joint if going to far, or too quickly. The joint needs to be pain free otherwise the nervous system will respond with contraction to protect the joint from further injury or pain. When working with developing flexibility it’s important to remember the role of breath to help relax against this immediate reaction to contract. It is also important that one stays within the pain free area of the joint. Burning, deep achy pain should always be avoided. That is not a good sign.
So flexibility doesn’t equate to being strong or coordinated. Range of motion needs the combination of healthy flexibility of the joint, the strength of the surrounding muscles, and the willingness of the nervous system to be accepting of the increased movement. Range of motion is what keeps stability and balance within the body.
Yoga-asana should develop range of motion, not just flexibility. This is why the standing poses of yoga such as the warrior series, triangle, side angle and lunges are great places to begin developing strength (unless there are issues, then non weight bearing would be the starting point). Holding the body upright in good posture, while breathing comfortably and effectively through out the asana creates stamina, coordination, range of motion and flexibility. Not to mention the mind muscle that standing poses develop: awareness of the state of the body, and the strength of mind to remain attentive and focused.