Why Can't My Body Move That Way? A General Overview of Joint Mobility

Hey Readers! Have you ever wondered why certain positions or poses (in yoga or otherwise) are hard, painful, or seemingly impossible? Well, it is very possible that movement in your body is being affected by the mobility of your joints.


The term "mobility" is used quite often in the health and wellness fields. But, do you really know what it means, why it matters, and what influences it? Let's dissect it a bit, so you that you can better understand how, and why, your body moves the way it does.

The Basics of Skeleton Movement in Humans

Let's begin with the basics of how your skeletal muscles move your bones (note: a discussion of movement created from smooth or cardiac muscle is beyond the scope of this post).

  1. You have an intention, or desire, to move a part of your body. Based on your intention, your brain generates a motor plan that includes what muscles need to move, when they should move, and how they should move. This occurs in the Premotor cortex of your brain.

  2. The information from the Premotor cortex gets transmitted to the Primary Motor cortex of the brain. Neurons in this part of the brain send signals, via the Somatic Nervous system, to your skeletal muscles to initiate movement. Axons (long slender extensions of neuron cell bodies) carry the nerve impulse from the Primary Motor cortex, through the spinal cord, to the skeletal muscle, where both the axon and muscle fiber meet at the Neuromuscular junction. To simplify things greatly, chemicals are released from the axon of the nerve that tell the skeletal muscle how, when, and where to move.

  3. The skeletal muscle generates force, either through contraction or tension (again, this is highly simplified).

  4. This force gets transmitted from the muscle to the tendon (i.e. connective tissue that connects muscle to bone).

  5. The tendon transmits the force to the bones of the joint, causing the joint to move in some way (e.g. flex, extend, rotate, etc.). Thus, movement in your skeleton occurs at joints, through the combined actions of neural signaling, skeletal muscle contraction, and force transmission across tissues.


For example, movement at the elbow joint is created via contraction of the Biceps Brachii muscle. When this muscle contracts, after input from the nervous system, the forearm bones (i.e. ulna, radius) move closer to the upper arm bone (i.e. humerus), causing the elbow joint to flex.




Conversely, when the Triceps Brachii contracts, via input from the nervous system, it causes the elbow joint to extend - that is, the Ulna and Radius (i.e. forearm bones) move further away from the Humerus (i.e. the upper arm bone).


So What Actually is a Joint?

So obviously a discussion on joints can get quite complex - I mean, there are doctors that specialize in this area. So, for the purposes of this blog post, I will keep the discussion to the basics as best I can. A joint is the junction or pivot point between two or more bones (or cartilages such as in the larynx). Our joints have two fundamental functions - they give our skeleton mobility and they hold it together. Movement of the skeletal body as a whole occurs primarily through rotation of bones around individual joints. Depending on the type of joint, either ligaments or cartilage (both types of connective tissue) connect bone-bone at a joint.


This image shows a general example of a Synovial joint, which is a type of joint that contains a fluid-filled space between the articulating bones and allows for some degree of movement.

What is Joint Mobility?

Joint mobility is the degree to which a joint can move, effortlessly and without pain - essentially, its range of motion. Adequate joint mobility is important because it allows your body to execute a movement properly without having to compensate with inappropriate muscles. Compensation means that you use a muscle (or muscles) to complete a job that it is not designed to do. For example, if you have reduced hip mobility, you might compensate pelvic movements by using your lower back, potentially causing low back pain and injury. Good joint mobility also helps to increase circulation within the joint capsule, allowing fresh nutrients to enter and cellular waste to be removed. Also, using a joint's full range of motion has been shown to stimulate the production of collagen within the connective tissue of the joint (although the mechanism behind this action is not yet understood).

What Factors Influence Joint Mobility?

Many factors influence the range of motion at a joint (i.e. mobility), including: joint type, joint structure, bone structure, ligament and tendon structure, muscle properties, and nervous system state. Let's chat about each one of these in more detail.

Joint Type: The amount of movement allowed at a particular joint varies based on the type of joint. Some joints are completely immovable, such as the joints that connect your different skull bones together. In these immovable joints, there is a minimal amount of connective tissue fibers that connect bone-bone. Some joint types allow a very limited amount of movement, such as the joint between both of your lower leg bones (i.e. tibia and fibula). In these limited movement joints, the connective tissue fibers at the joint are slightly longer than in immovable joints, but shorter than in a freely moving joint. Speaking of freely moving joints, there are some joints in the body that allow a wide range of movement, so-called, movable joints. In these movable joints, the ligaments or cartilage are longer and more numerous than immovable or slightly movable joints. There is also usually a space between the articulating bones that contains a fluid (synovial fluid), which helps to keep the joint lubricated and moving well.

Joint Structure: There are many different structural plans for joints, and based on the general structure of the joint, the joint will have differing mobility. For example, a hinge joint, such as the elbow joint, only allows flexion (bending) and extension (straightening). In contrast, ball-and-socket joints, such as the hip, are the most freely moving joints, allowing movement in all axes and planes (e.g. flexion, extension, rotation, etc.). Thus, the structure of the joint will dictate how much movement is permitted at that joint.

Bone Structure: The shapes of articular surfaces (i.e. the portion of the bone in the joint itself) also determines what movements are possible at a joint. Bone shape is permanent and cannot be changed once the growth plates are closed (during adolescence). The shapes of bones varies from person to person, based on a combination of genetics and lifestyle. For example, the elbow joint is formed by a shallow depression on the bottom and back side of the humerus (i.e. upper arm bone), known as the Olecranon fossa, and a protrusion on the top and back side of the ulna (i.e. one of the forearm bones), known as the Olecranon process. If a person's Olecranon fossa is really shallow, he/she might not be able to extend the elbow joint (i.e. straighten the arm) all the way because the Olecranon process of the ulna will compress against the humerus. Conversely, if a person's Olecranon fossa is really deep, he/she may be able to extend the elbow joint past the normal range of motion simply because there is more room for the bones to move. Bone-on-bone compression should not be changed; if you try to surpass bone-on-bone compression, you might break some bones.

Ligament and Tendon Structure: Ligaments (connect bone-bone) and tendons (connect muscle-bone) are made up of collagen fibers. Collagen fibers have some elasticity, allowing them to stretch a little to allow movement to occur (be careful not to overstretch your ligaments and tendons because once they stretch past their natural length, they never go back). The composition of the collagen fibers can vary based on genetics, age, lifestyle, and/or injury. When the composition of the collagen in a ligament or tendon changes, joint mobility can be affected. For example, as we age, the collagen in our bodies starts to break down. This causes ligaments and tendons to lose some of their elasticity, reducing joint mobility. Also, smoking and sunlight can damage collagen fibers, causing a reduction in joint mobility. Injury can also affect the mobility of a joint by influencing the composition of ligaments and tendons. Some people who are hypermobile (i.e. have increased range of motion at a joint) have a collagen makeup that provides more elasticity than the average person, and this increases joint mobility.

Muscle Strength: Muscle strength, the amount of work you can do with a muscle at one time, can affect joint mobility. If the muscle that moves a joint is weak, it can limit how much the joint can move, simply because the muscle is too weak to transmit adequate force to the joint. For example, if your biceps muscle is weak, it can reduce mobility re: flexion (i.e. bending) in the elbow joint. Additionally, if the triceps muscle (on the back of your arm) is weak, it can reduce mobility re: extension (i.e. straightening) in the elbow joint.


Muscle Flexibility: Muscle flexibility, the ability of a muscle to stretch and relax, can affect joint mobility. If an antagonist muscle (i.e. a muscle that stretches in opposition to a contracted muscle) is inflexible, joint mobility can be reduced. For example, if your triceps muscle (i.e. antagonist to the biceps) has reduced flexibility (i.e. meaning the muscle cannot stretch and relax with ease), it will affect how much flexion you can achieve when you contract your biceps muscle at the elbow joint. Conversely, if your biceps muscle (i.e. antagonist to the triceps) has limited flexibility, it will reduce the mobility of your elbow joint to extend when the triceps muscle contracts.


If your Biceps Brachii muscle, on the front side of your Humerus, has reduced strength, it can limit how much flexion occurs at the elbow joint; if it has reduced flexibility, it can limit how much extension occurs at the elbow joint.



Conversely, if your Triceps Brachii muscle, on the back side of your Humerus, has reduced strength, it can limit how much extension occurs at the elbow joint; if it has reduced flexibility, it can limit how much flexion occurs at the elbow joint.


Muscle Tone: Muscle tone, low levels of contractile activity in relaxed muscles that keep the muscles healthy and ready to react to stimulation, can affect joint mobility. Excessive tone, or tension, in a muscle can reduce the ease and fluidity of movement at a joint, hence, limiting joint mobility. A muscle might have excessive tone for several reasons, such as trauma, disease, or increased sympathetic nervous system activation (e.g. when stressed out or panicked). Excessive tone can be temporary or a permanent feature of a person's musculoskeletal makeup. Conversely, low muscle tone can cause increased joint mobility since the muscles are more relaxed with less tension.

Nervous System State: The human nervous system consists of the brain, spinal cord, and peripheral nerves. The autonomic nervous system (ANS), which is a branch of the peripheral nervous system, is divided into the sympathetic and parasympathetic branches. The sympathetic nervous system (SNS), often referred to as your "fight, flight, or freeze" system, is active when we are exercising, excited, stressed, scared or nervous, or in an emergency situation. Re: skeletal muscles, the SNS causes increased blood flow to skeletal muscles AND it causes the release, and subsequent binding to muscle receptors, of Norepinephrine and Epinephrine. These neurotransmitters cause an increased force of contraction in skeletal muscles. This effect would be totally fine if you were trying to escape real danger; however, in our busy, "go go go" society, we tend to live in a sympathetic nervous state on-the-regular. This can cause increased tension, or tone, in our muscles, which can limit joint mobility over time. The parasympathetic nervous system (PNS), in contrast, is often referred to as the "rest and digest" system because it increases activity in the digestion and urinary systems. Heart rate, respiratory rate, and blood pressure decline with PNS activation, causing a general feeling of relaxation in the body. While the PNS does not directly innervate skeletal muscles, it does have an effect on skeletal muscles. Since the PNS is responsible for bringing about an overall state of calmness, it allows your skeletal muscles to relax a bit. As we discussed earlier, relaxed muscles move more easily, causing mobility at joints to increase.

Summary

Skeletal motion in the body is the result of the combined actions from the brain, peripheral nerves, and transmission of force from muscle, to tendon, to bone. Movement in the skeleton occurs at joints, which are locations where two, or more, bones or cartilages meet. Adequate joint mobility is essential for your body to move safely, efficiently, and without pain. Many factors contribute to joint mobility, some of which are under your control, and some of which are out of your control. Stay tuned for future blog posts describing ways to improve mobility in various joints of the body. As always, the information presented in this blog post is derived from my own personal study and practice of yoga and human movement. If you have specific question regarding your own joint mobility, please consult with your physician, physical therapist, or athletic trainer. And a special shout-out to my amazing husband for doing the artwork in the pictures. Thanks for reading!

~Namaste, Jackie Allen, M.S., M.Ed., CCC-SLP, RYT-200, RCYT

References:

Clark, B. (2016). Your Body Your Yoga. Bernie Clark.


Fetters, K.A. (2018). "Here's the Difference Between Flexibility and Mobility - and Why It Matters." US News. Article link here.

Long, R. (2008). "Your Guide to Functional Anatomy in Yoga: The Key Poses of Yoga." Scientific Keys, Volume 2. Bandha Yoga Publications.

Marieb, E.N. (2004). Human Anatomy & Physiology, 6th Edition. Pearson.

Neumann, D.A. (2017). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation. 3rd Edition. Elsevier.


Roatta, S., & Farina, D. (2010). Sympathetic Actions on Skeletal Muscles. Exercise and Sports Science Reviews. 38(1): 31 - 35.

Thompson, N. (2019). "Joint Mobility and Stability." ACE. Article link here.


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