The terms flexibility and mobility are often used interchangeably. The terms are similar, yet there is a subtle difference. In this article, I am going to describe the differences between mobility and flexibility and their relationship to strength.
What is flexibility?
Flexibility is the ability of a joint to lengthen. The ability of the joint to lengthen is a function of its potential to move. In other words, flexibility is where a joint can be taken into its full range of motion with the assistance of another limb or through the force of gravity.
What is mobility?
Mobility is the ability of a joint to move by itself. In other words, there is an engagement of the musculature and connective tissue around the joint that enables it to move through a given range of motion.
What is the difference between flexibility and mobility?
If I were to summarise the key difference between the two ideas, it would be this. ‘Passive versus Active’. Flexibility relates to passive range of motion. Mobility relates to active range of motion.
In order to illustrate this idea, a good example is the ability of the arm to move across the body (i.e. adduction).
If you take your arm and draw it across your body, there will come a point when the arm stops. It can go no further. Yet, if you use your other arm to pull the arm across the body, you will be able to take the arm further. The first movement demonstrates how mobile the shoulder joint is. The second movement demonstrates how flexible the shoulder joint is.
What is the relationship between flexibility and mobility?
Often a yoga or other strength and conditioning class will focus on ‘mobility’. That in it of itself is a good thing. It works to build more active range of motion around the joint. This means strengthening the musculature and connective tissue that support it.
Yet, if you are not flexible in the first place, you won’t be able to increase your mobility. That is, a joint will not move by itself unless there is a sufficient degree of passivity around the joint.
The opposite is also true. If you focus exclusively on flexibility, your ability to move the joint passively will be limited by the degree of strength and control of the musculature and connective tissue around the joint.
Why? Because, unless you have sufficient strength around a joint, the central nervous system (CNS) will not permit a greater degree of range of motion through the joint unless it feels safe. Rather, when a joint is strong, the CNS will permit a greater degree of range of motion since there is sufficient strength to the move the joint safely.
Is being stiff inherently bad?
‘Stiffness is a gift’. One of my yoga teachers, Graham Burns, likes to attribute this quote to the famous American yoga teacher, Eric Schifman. Being stiff means that there is the significant potential to move through a whole range of motion. That said, a stiff joint might also mean a weak joint, so it is important to create strength at the same time too.
If you are stiff, you are also more likely to sense and feel a body part’s movement, location and force (ie proprioception) when you encounter resistance. In Image 1 when it was taken, I had limited range of motion in my lower back as the angle between the top of hips and knees is relatively shallow. Certainly when I do backbends, I can feel the compression of my lower lumbar vertebrae.
However if you are very flexible, perhaps to the point where you can move a joint through its entire potential range of motion, then its proprioceptive ability may decrease. At this point, the risk of injury to that joint may increase the further such joint moves towards its maximum range of motion. In other words, the movement becomes ‘excessive’. This is further described below.
Image 1: ‘Wheel Pose’
How reflexes in the body work
In order to better understand the topic of flexibility and mobility, it is important to understand how the nervous system responds to stimuli known as a ‘reflex’. A reflex generally speaking is any mechanism that responds to stimuli in order to maintain equilibrium in the body.
Upon activation of a muscle group, the tension that arises will try and prevent the stretch. It is thought that this is a protective mechanism for the body against any extreme movements that may cause injury.
When you stretch a muscle, the receptors in it (called ‘muscle spindles’) will send a nerve impulse to the spinal cord. if the nerve signal is strong enough, the spinal cord will send an impulse to the muscle to stretch. This is known as the stretch reflex or (‘myotatic reflex‘). So in other words, there is a reflex action in the muscle that results in the contraction of muscle fibres.
This is illustrated in Figure 2 below. The term ‘afferent’ relates to information that is carried from sensory organs and soft tissue to the central nervous system (CNS). (ie the brain and spinal cord). The term ‘efferent’ relates to signals from the CNS to the muscles and glands of the body. ‘Interneurons’ are a type of neuron that connects sensory and motor neurons in the CNS.
Figure 2: The Stretch Reflex
After a period of time (approximately 12 – 15 seconds), the muscles will relax before the risk of an injury occurs. This is known as the tendon reflex or ‘inverse myotatic reflex‘. These reflexes contains receptors known as ‘golgi tendon organs‘ (GTO). They are located around the furthest end of a muscle where the muscle becomes tendon (ie the musculotendinous junction). Their role is to sense the amount of tension a muscle is exerting.
How does the Golgi tendon reflex work?
When there is a prolonged increase in tension in a muscle, the GTO will send an impulse to the spinal cord. This impulse results in the inhibition of the motor neuron that innervates the muscle of the GTO. This results in the relaxation of the muscle (even though the muscle does not necessarily stretch). Such relaxation protects the tendon from excessive tension which would otherwise lead to damage.
In someone who is very flexible, then there may be less sensory awareness of the joint’s location towards its end range of motion. The tendon reflexes do not activate and the practitioner may be at a higher risk of injury.
Should we focus entirely on mobility rather than flexibility?
It is true that a stronger joint will generally be more mobile. A stronger joint may also tend to be more flexible if there is a balance of muscular engagement around a joint. On the other hand, a body builder for example may become inflexible if the concentration of muscle bulk actually prevents the joint from moving through its full range of motion.
Body builders build muscles in parallel, whereas yoga practitioners build muscle in series. So in this example, if the concentration of muscle fibres results in strong (and large) biceps, then the ability to fully flex the elbow may be limited.
So mobility is good. That is not to say that mere flexibility is bad. Quite the opposite. Being flexible may bring qualities that allow for more down-regulation of the autonomic nervous system. This can help induce a relaxation response in the body where the particular stretch is held for a longer period of time. Practices like yin yoga are very beneficial in that respect.
When thinking about flexibility and mobility, the key difference relates to how active or passive the movement of the joint is. If a joint can move by itself without assistance, it is active range of motion and ‘mobile’. If a joint can move through its full range of motion with assistance, it is ‘flexible’. The terms are related insofar as flexibility will help create mobility through the ability to move the joint in the first place. In the opposite case, focusing on active range of motion will help build flexibility through its relationship to the CNS which controls neuromuscular activity.
If I can help you become more flexible or stronger through yoga, please feel free get in touch or email me at firstname.lastname@example.org
Simon Borg-Olivier & Bianca Machliss. Applied Anatomy & Physiology of Yoga. Australia, Yoga Synergy Pty Limited, 2013.
Bernie Clark. Your Body, Your Yoga. Canada, Wild Strawberry Productions, 2016.
Cecilia Cristolovean (www.yogaandphoto.com)