There is an undeniable connection between yoga and flexibility and, according to a survey by Yoga Alliance (2016), flexibility is the number one reason people start yoga. However, there's a recent trend (if you follow the same people I follow on Instagram) of focusing on mobility over flexibility. Read on to find out hte difference between the two in this article which is adapted from my book The Physiology of Yoga.
There are multiple definitions of flexibility and mobility depending on the source and the context. Recently, flexibility has come to refer to a muscle’s ability to lengthen passively through a range of motion while mobility has come to refer to the ability of joints to move actively through their range of motion. But scientists refer to these as passive range-of-motion and active range-of-motion.
Basically, all joints have a certain range-of-motion (ROM) through which they can move. Joint ROM is affected by many factors including bony architecture, connective tissue mechanics, and importantly muscle extensibility which is primarily governed by the central nervous system. Joint ROM can be measured using a goniometer (basically a large protractor) and is usually described in degrees. While standing, if you can lift your leg out in front of you so that it is parallel to the floor and straight, you are at 90 degrees of hip flexion. This ROM is dependent on both the ability of the hip flexors (the agonist) to create hip flexion as well as the hamstrings (the antagonist) to lengthen.
ROM can also be further described as active ROM or passive ROM. The example above uses active ROM and is a variation of Extended Hand-to-Foot Pose (figure 1a). Imagine this same pose but in a reclining position. The pose then becomes a variation of Reclining Hand-to-Foot Pose (figure 1b). It is essentially the same pose but with a different relationship to gravity. In the reclining version, if you take hold of your foot and pull the leg toward your chest, the arms generate a force that deepens the stretch on the hamstrings and allows you to explore your passive ROM. Additionally, if the leg is at more than 90 degrees of hip flexion, then gravity will help to bring the leg closer to the chest, just like the Leaning Tower of Pisa. Adding a bind or changing your relationship to gravity will affect whether you are using active or passive ROM.
Our passive ROM will always be greater than our active ROM, not just in a hamstring stretch but throughout the body. Consider a seated twist (figure 2). If you rotate your spine without using your hands to push yourself deeper into the twist, you might find you can rotate your trunk about 30 degrees, which is within the bounds of normal ROM. This rotation would come from your core musculature with the abdominal obliques playing a major role. If you then force yourself deeper into the twist using your hand against your knee to rotate or using a hand on the floor behind you, you would very likely rotate farther, perhaps to 45 degrees of thoracic rotation.
While there is crossover between the two, the development of active ROM requires a different approach to movement than the development of passive ROM. Consider Side Lunge (Skandasana). How low you can sit in Side Lunge would be a measure of your passive ROM in terms of hip flexion, knee flexion, and ankle dorsiflexion. How low you can transition into and out of your Side Lunge without using your hands on the floor would be a measure of your active ROM in those same joints. One way you can increase your active ROM in a pose is by having fewer points of support (e.g., try hovering your bottom hand above the floor in Triangle) or by transitioning in and out of a pose with control.
Is active ROM better than passive ROM?
Many movement teachers, yoga teachers included, have begun to focus on active ROM over passive ROM — in other words, prioritizing mobility over flexibility. Some teachers make claims online about how mobility is better than flexibility and that being too flexible can actually be dangerous. But this is simply not borne out in the scientific literature. This comes from the idea that stretching can destabilize joints, which simply isn’t the case (and that’s a topic for a separate blog post). In reality, the “best” type of stretching (just like the “best” type of training) depends on the goals you wish to achieve. And the best type of exercise is the exercise that gets done! If you love static, passive stretching and you want to increase your flexibility, then go to town, my friend!