Teacher Training - Muscle Spindles & GTOs

Although the Asana's (poses) of Hatha Yoga are not a system to increase flexibility (it is a system to create balance), flexibility is a consequence of asana and thus one of its main attractions. Many people look to do yoga to improve their flexibility. The theory being, the greater the range of movement at a joint, the less rigid the joint becomes. This leads to freedom of movement and increased health. When a muscle stays partially contracted (or short), an abnormal state of prolonged shortening called contracture develops. Contracture not only shortens the muscle, it makes the muscles less supple, less strong and unable to absorb the shock and stress of various types of movement. The most appropriate remedy for this is to stretch and relax the muscles. A relaxed muscle is defined as the "cessation of muscular tension".

Ultimately, optimum flexibility, which is simply proper range of motion at a joint increases efficiency of movement which is the key to physical health.

As an analogy, the great B.K.S Iyengar (1979) explained:

"To the Yogi, his body is the prime instrument of attainment. If his vehicle breaks down, the traveler cannot go far. If the body is broken by ill-health, the aspirant can achieve little. When the body is sick or the nervous system is affected, the mind becomes restless or dull and inert and concentration or meditation becomes impossible".

Whatever the goal is for the practising "yogi", an additional side benefit of asana will be the benefit of flexibility. With that being said, lets look at some science behind flexibility.

What are Muscle Spindles?

The muscle spindles are specialised muscle cells that are wrapped in nerve fibres. These spindles are distributed throughout the muscles tissue and lie parallel to the muscles fibres.

The muscle spindles are sensory receptors within the belly of a muscle, which respond to changes in length and tension (mechanical events), and signals this information to the central nervous system. You could say that muscles spindles are the way in which the central nervous system stays in touch with the muscles.

The muscle spindles send messages to the brain via the spinal chord regarding the length and the velocity of the stretch within the muscle. If the stimuli is enough for the brain to perceive it as a threat to the integrity of the muscle, the spindles will create a nerve impulse that sends a message to the brain, the brain then sends a corresponding signal back to the muscle, stimulating a “contraction”.

This muscular “contraction” is known as the stretch-reflex. This reflex inhibits the muscle from lengthening further. Although the muscle spindle is often thought of as a control system, it is more accurate to think of it in terms of a communication system.

To give you a yoga example of the stretch reflex, lets look at Trikoṇāsana - Triangle pose (below), the more I lower my left hand towards the ground, the greater the intensity of the stretch on my front leg hamstring. The depth of the pose can be overwhelming on the front hamstring for some, so to ease off and give the hamstring a rest, most people will bend into the front knee.

So what actually transpired?

If the hamstring in the front leg is under more tension than the muscle can tolerate, the stretch capacity at the hamstring has been reached. The hamstring is feeling tight and vulnerable so the muscles spindles within the hamstring will contract, this tightens the muscle ensuring the hamstring does not over stretch and tear. What actually happens is the muscle spindles within the hamstring send a message via the nerves and spinal chord to the brain, warning the brain that the hamstring is under too much stress. The brain then sends a corresponding signal to the spindles in the hamstring, via the spinal chord and nerves telling them to make any necessary adjustments. In the case of the Triangle example, the brain tells the hamstring to contract (tighten). This “contraction” makes it impossible to keep the front leg straight, this could result in muscle spasms or a feeling of tightness, so perhaps subconsciously the front knee bends, this releases the intensity of the stretch and ensures the hamstring does not tear.

To avoid the 'stretch reflex', the key is to not go so far that the muscle senses more stress than it can tolerate. Still using the above Triangle as an example, is I lift my torso higher and perhaps place my left hand on a block, the intensity of the stretch is reduced. This then allows my hamstring to stretch, but not stretch so far that the muscle senses danger. When the hamstring receives no nerve impulse from the brain, it can relax and you can feel yourself going further in the pose. Essentially what has happened is the elastic force that has accumulated in the muscle is released.

The muscles spindles are also sensitive to the rate of length change. So they will respond quicker if the stretch is forced or if the pose is entered too quick. Lets look at Uttānāsana - standing forward fold (below).

Image used courtesy of https://www.bandhayoga.com/

In this pose, I theoretically stretch my hamstrings and lower back muscles. If I go into my forward fold too quick and without thought, the muscles that I am trying to stretch (the hamstrings) may “contract” and try to pull me back. The length change of the muscles was too sudden. The hamstring is trying to protect itself. It does this via the muscles spindles. This is why I instruct to move a little at a time on each exhale. You want the muscle spindles to not sense a sudden change in length. Essentially you want them to relax and allow you to go further in the pose.

Pārighāsana - Gate Pose below is a good example from my personal experience. Unless I enter a side-bend with slow controlled movements, I can irritate my lower back. Perhaps this is because the very muscle I am attempting to lengthen ends up tightening and pulling on my spine and ribs. My intention is to stretch my 'QL' muscle (above my left hip looking at the image below). If the side-bend movement is too sudden, maybe the change in length of the QL muscle is too sudden. So the muscle spindles within the QL muscles will send a message to the brain which in turn will send a sudden message back signalling the QL to contract. This is the contraction that suddenly pulls me back and "jars" my lower back. When the side-bend is done slowly and more length is achieved on each exhale, there is no discomfort.

This form of stretching is known as static-stretching. Most of the hatha yoga poses are static poses. The key when trying to increase the length of muscles is to approach the pose as a slow static stretch. Static stretches has shown to demonstrate the least amount of motoneuron excitability which is turn allows the muscles being stretched to relax increasing flexibility. In theory anyway.

Another simple example of muscle spindle "activity" and their function is the knee jerk reflex (Patellar reflex). This is a sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon, which lies just below the kneecap.

Tapping on the tendon of the quadricep (just below the knee) causes activation of the muscle spindles. Exaggeration or absence of the reaction suggests that there may be damage to the central nervous system.

So how does a basic understanding of muscle spindles help the budding yoga teacher? Well, the science of the spindles etc is easily forgotten over time and that is fine. What is key to remember is that the muscles will find a way to protect themselves if the stretch is forced. The very muscles that you are attempting to lengthen can contract and contradict your movement if you attempt to stretch too far or too sudden. In the case of a forward fold, the response under stress can make you exit the pose maintaining the integrity of the hamstrings etc. In triangle, because of the relationship with gravity, its harder to exit the pose safely if you stretch too far or too quick. Some poses are safer to exit and others are not. So the key when trying to increase flexibility, is to enter the pose slow and controlled. The aim is to relax the muscles you are trying to stretch.

If the muscle spindles sense a change in length and relay this information to the brain, what senses change in tension? That would be the Golgi Tendon Organ.

The Golgi Tendon Organ (GTO) is located within the tendon, close to the musculotendinous junction. Thats the site of connection between tendon and muscle (below).

The GTO is said to act as a feedback monitor which sends signals to the central nervous system in response to tension. In contrast to the muscle spindles, the signal sent by the GTO to the brain is suggested to result in a relaxing (inhibition) of the muscles. Specifically, following six seconds of stretch, the GTOs are believed to send sensory impulses to the brain which lead to relaxing of the corresponding muscles. This inhibition, or reflex relaxation, allows the main muscle (or agonist) to be stretched further, reducing the risk of damage. This theory demonstrates that stretching for a short duration, specifically maintaining a static stretch for less than six seconds at a time, is of little use. This is due to the inability of the Golgi Tendon Organ's to respond to the changes in length and tension.

This is just all theory of course. I have read elsewhere that all the above is nonsense and is a very common misconception about the GTOs. This same research said that they are still unclear what the GTOs actually do. This is the wonderful world of anatomy and physiology.

In the next practical element of our Teacher Training, we will do our own research using our bodies as our laboratories (as advised by B.K.S Iyengar). We will look at static stretches and hold them for the various prescribed times and see how our bodies respond.

Additional reading:





As much as I dislike the Yogajournal, here is a helpful article on stretching:



1-J. Alter, M., 2018. Science of Flexibility. 3rd ed. Human Kinetics(ADVANTAGE) (Consignment); 3rd Revised edition edition (15 May 2014): London.

2-Adina T. Michael-Titus (Author),‎ Patricia Revest (Author),‎ Peter Shortland (Author), B., 2010. The Nervous System: Systems of the Body Series, 2e. 2nd ed. Churchill Livingstone; 2 edition (29 Mar. 2010): London.

3-Chalmers, G., "Do Golgi Tendon Organs really inhibit muscle activity at high force levels to save muscles from injury, and adapt with strength training?", Sports Biomechanics, July 2002

4-McGill, S.M. (2002). Low back disorders: Evidence based prevention and rehabilitation, Second Edition, Human Kinetics Publishers, Champaign, IL.

5-National Academy of Sports Medicine (NASM), B., 2016. NASM Essentials Of Personal Fitness Training (National Academy of Sports Medicine). 5th ed. Jones and Bartlett Publishers, Inc; 5th Revised edition edition (23 Jun. 2016): London.

6-www.sciencedirect.com. 2018. Understanding Muscle Contraction*. [ONLINE] Available at: https://www.sciencedirect.com/topics/neuroscience/golgi-tendon-organ. [Accessed 1 February 2018].

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