Posted on 19 Mar 2009 17:16
By Ground Up Strength
Please note: The purpose of this entry is to provide simple explanations and should by no means be considered comprehensive or above reproach. These topics are much more complicated than many trainers maintain, so while this may seem highly technical to some, it is in fact, highly basic.
The Myotatic Reflex
The stretch reflex, or myotatic reflex is a neural mechanism that responds to changes in muscle length (stretching) by attempting to resist the change in length. The changes in length are detected by proprioceptors called muscle spindles. Changes in muscle tension are detected by another important proprioceptor, the Golgi tendon organ (note: there may be other processes at work).
Myotatic reflex is part of a feedback system of proprioception that is highly important for a sense of body position and for posture maintenance and motor control.
For instance, if one stands for long periods of time and the extensor muscles maintaining position relax and thus lengthen, causing the body to lean to one side or other, the muscle spindles cause the muscles to contract and thus return the body back to correct positioning without conscious involvement.
The joints of the body have two opposing sets of muscles, extensors and flexors. The myotatic reflex, together with reciprocal inhibition, allows these muscles to work in synchrony, as opposed to against each other.
The myotatic reflex is initiated by receptors in muscles called muscle spindles which convey information to the spine used to trigger the response.
Muscle spindles are fusiform structures scattered widely throughout the muscle body (but not everywhere). They are attached to the tendon, the endomysium, or the perimysium in parallel with the extrafusal fibers (regular fibers outside the spindle).
These spindles contain two types of intrafusal (within the fusiform spindle) fibers: nuclear bag fibers and nuclear chain fibers. Nuclear bag fibers are also divided into dynamic and static fibers.
Dynamic nuclear bag fibers are highly sensitive to the rate or change or velocity of the change in muscle length. Static nuclear bag fibers and nuclear chain fibers are more sensitive to static or steady state muscle length change.
When you stretch a muscle there is an initial stretch reflex which is quick and strong. The strength of this reflex depends upon the rate of change. This is the dynamic component. There is a second, weaker but longer acting, static component which responds to the new length. The static component persists as the new length is held. The static fibers respond to the new length in a sustained, tonic fashion and the discharge of both static fiber types is continued for as long as the new length is held.
One misconception of the stretch reflex is that the muscle spindles are only active when stretching or otherwise stimulating a muscles. In fact the job of the spindles is to signal muscle length continuously as part of the proprioceptive system. Both primary and secondary fiber types discharge tonically when the muscle is at it’s resting length and increase their rate of discharge when there is a change in length. The muscle spindles themselves, due to their short lengths, size, and other factors, do not generate any appreciable muscle tension.
The Golgi tendon organs are proprioceptors which, instead of monitoring change in muscle length, monitor change in muscle tension. There is an active and passive component here as well, with the threshold for tension developed through stretch relatively lower than that of active contraction. Previously it was thought that the golgi tendon organs only reacted to extreme changes in tension and were simply a protective device. But it is now known that the receptors are simply more insensitive to passive tension but extremely sensitive to active tension. The gogi tendon organs initiate autogenic inhibition in the stretched muscle.
The misunderstandings about the stretch reflex are based on this. Different muscles can have different tonus. Some muscles may be hypertonic and show a marked increase in reaction to stretch. Whereas some muscles may be flaccid and have a muted response. It is important to note that “normal” tone in a muscle is a very ill-defined concept. All muscles in any given individual do not react the same to stretch.
The relaxation response is part of a cyclical reaction. Once the muscle is “relaxed” via autogenic inhibition is is free to stretch and thus lengthen again and thus the muscle spindles are free to react to stretch again thus initiating another stretch reflex, upon which time, at a certain point, the golgi tendon organs may react again, so on and so forth.
It is not well excepted that the golgi tendon is sensitive enough to passive or static stretch conditions to contribute greatly to muscle relaxation via autogenic inhibition. It is certainly not an optimal condition for GTO involvement.
So Is Response to Static Stretching a "Neuromuscular" Response?
Earlier theories have suggested that one response to prolonged passive stretch was the subsequent relaxation after contraction of the stretched muscle, by one means or another, which should theoretically lessen resistance to further stretch.
Although there is indeed less resistance during slow, static stretching, these simplistic theories about muscle relaxation should be considered incomplete. The idea that complete relaxation is needed to increase flexibility certainly doesn’t hold up in real training.
Many people think that once a stretch is “held” for a long enough period of time then the stretch response simply vanishes. The idea is that the involuntary contraction of the muscle due to the stretch response can be relaxed by a slowly applied and held static stretch, thereby inducing relaxation in the muscle and making it accept a greater stretch. Instead, the response is part of a feedback circuit that continuously monitors muscle length and tension. Simply pausing a stretch does not eliminate the stretch reflex entirely, only ameliorates it. Furthermore, any great change in muscle activity has not been shown after a hold of 30 seconds or more and, indeed, significant activation of stretched muscles, even during ballistic type stretching, has not been shown! Longer durations will not result in greater acceptance of stretch by reducing the stretch reflex, and the stretch reflex itself has not been proven to be the governing factor in a muscles acceptance of a stretch. Neuromuscular relaxation has largely failed as a theory to explain the results of a stretching routine.
There have been a number of stretching and flexibility books and products based on the idea of muscle relaxation for better range of motion while stretching. A notable example is Relax Into Stretch by Pavel Tsatsouline. This book and others operates under the assumption that it is not possible to effectively stretch a muscle unless 'complete relaxation' is achieved.
The methods used to achieve this are usually based on altering the nervous response to the stretch by using simplistic applications of reciprocal inhibition. For instance, the antagonist muscle is made to contract preceding a stretch and this is believed to relax the muscle so that it can reach it's "true" ROM. In fact, it is just as likely that a prior contraction of the muscle will serve to facilitate and excite it, thus, making it prone to more tension, not less, if this muscular activity has any large effect at all, which it probably does not. Also used are relaxation sequences using imagery or other techniques to force relaxation of the muscles. There is no evidence that more ROM is garnered through relaxation combined with static stretching than with static stretching alone.
Stretching and Injury
Another misconception is that injury due to stretching is entirely due to velocity of stretching and thus only dynamic and ballistic stretching activities will lead to injury. However, the extensibility of muscles is not without limit and all tissues will eventually reach a point of rupture, regardless of the velocity of the lengthening. The stretch reflex together with the relaxation response due to inhibition is not an excuse for "extreme" stretching activities. Injuries can and will occur if muscles are stretched too vigorously.
On the other hand a spasticity condition known as the “clasped-knife” can occur in the extensor muscles of a joint, during certain physiological conditions such as “upper motoneuron” disease. See [http://www.uth.tmc.edu/nba/neuroscience/s3/iii6-1.html#clasp_knife_reflex]
The flexion is resisted due to the reflex response to stretch in the extensor but the reflex suddenly “melts away” and the muscle is relaxed due to an exaggeration in the response. Thus when a spastic limb is being passively moved though a range of motion there is a constant resistance and then it seems as if this resistance suddenly lets go, and the limb suddenly easily collapses into full flexion, much like a pocket knife snapping shut.
This response was originally thought to be caused by autogenic inhibition but it is now known to be exaggerated reaction and the activity of the Golgi tendon and autogenic inhibition cannot alone account for it. Even if this inhibition explains the relaxation of the spastic muscle it does not explain the failure of the return to spasticity, since once it relaxes it is free then to react to stretch.
Actually, this gives rise to another frequent misunderstanding. The difference between the stretch reflex and mechanical recoil in the muscles. When you stretch a muscle, mechanical energy is stored in the cross-bridges which basically results in a spring effect. Thus, during the eccentric portion of a bench press, energy is stored in the muscles that contributes greatly to the concentric.
However if one pauses for long enough at the bottom of the press, this energy dissipates. This has caused some to theorize that the stretch reflex is dampened or eliminated and the inhibitory action of the Golgi tendon organ makes the concentric action harder after a pause. This is likely to have very little influence.
How the GTO works and doesn’t work, it should be noted, is not entirely understood.
When flexibility in a muscle increases, it is because the response to repeated bouts of stretching dampens and modulates the reaction to stretch. Thus allowing greater changes in length over time. But it is the repeated exposure over time that is important and it is possible to injure the muscle due to too vigorous a stretch. This could result in a muscle that not only does not achieve the desired flexibility in a timely manner but that becomes less flexible and ameliable to stretch due to many small injures over time.
As stated previously any tissue can reach it’s point of rupture regardless of the velocity of stretching. The stretch reflex is probably not able to be “down-trained” over time and indeed has little influence on the outcome of the stretching program. Seeking to continuously down-train the stretch reflex is probably not going to contribute to athletic success. However, our understanding of just how a muscle's extensibility increases due to repeated stretch is currently in flux. Both neural and mechanical models fail to adequately explain.
The stretch reflex is sometimes confused with the mechanical "stretch response" which is the response of the muscle due to it's actual mechanical characteristics. Both the stretch reflex and the stretch response may be part of the stretch shortening cycle (SSC). It is unclear to what extent each component is important but the SSC is key in developing explosive power. As previously stated, it's importance in understanding the muscle's response to static stretch has been greatly overstated.
Viscoelastic Stress Relaxation
Skeletal muscles are viscoelastic. When a tensile load is applied to a muscle, its length changes. When the load is removed, the muscle resumes its previous length. This is a demonstration of a muscles elasticity. However, much like a liquid, a muscles response to tensile force is rate and time dependent. This is because muscle also behave viscously.
Several studies have been undertaken to determine the muscle's viscoelastic deformation response to repeated static/passive stretching. To some degree after stretching, depending on rate, duration, and frequency, the extensibility of the muscle increases. Some of this change in extensibility may be due to the viscoelastic response of the tissue.
The gradual decline in a muscle's resistance to stretch when it is held in a stretched position has been shown to be a viscoelastic stress relaxation by the aforementioned studies. This response is transient in nature and according to further studies, probably has no influence on subsequent stretches. However, this does demonstrate that the initial response to a static stretch can be explained by the biomechanical properties of the muscle-tendinous-unit itself rather than to a neural stretch reflex.
What is the Difference Between Passive and Static Stretching?
Since this article refers to passive and static stretches as two different types of stretches it may help to define them. Note, however, that different people use these two terms differently and some use them interchangeably.
When the terms are used interchangeably, then active or passive stretching is described as any stretch that involves a relaxed person's muscle is brought into a position by a partner or some other means or apparatus and then held in that position for various lengths of time. The term static-passive is also used to describe this.
Many however, refer to passive and static stretching as two distinct activities and it is the preference of this author (Eric) to keep them distinct and the following definitions will describe the terms based on this view. However, both types of stretching do overlap and it is completely unnecessary to always separate them, or to argue over the semantics of the term. The distinction becomes more important based on the circumstances.
Passive stretching involves a person who is relaxed and making no contribution to generating the force that is bringing the muscle into stretch. The body part is brought into position by a therapist, partner, or external apparatus such as traction equipment and the muscle is carried through its range of motion. The key is that there is no muscle activity involved in generating the position, thus the term passive.
Static stretching involves holding a muscle in a stretched position for a period of time and perhaps repeating. The position may be induced by an active muscle contraction, by an external apparatus, a partner, or be the assistance of gravity. The person stretching is in control of the stretch (not passive) and there is little to no movement involved, thus the term static. The lack of movement is the key distinction between static and passive stretching, the secondary consideration being the degree of muscle control.
This entry is by no means to be taken as a comprehensive theory or overview of reflexes, proprioception, the muscle spindles, the GTO or any other motor process.
For further reading see sources below:
Hole, John W. Human anatomy and physiology. Dubuque, Iowa: W.C. Brown, 1987.
"Spinal Cord Medicine." NCBI HomePage. Ed. Vernon W. Lin. 2003. Demos Medical Publishing. 15 Mar. 2009 <http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=spinalcord>.
"Chapter 11 - Muscle Receptors." University of Nebraska Medical Center (UNMC). 15 Mar. 2009 <http://www.unmc.edu/physiology/Mann/mann11.html>."Neuroscience Online (iii,2,1)."
Neuroscience Online: A Neuroscience Electronic Textbook | The University of Texas Medical School at Houston." Neuroscience Online. Ed. John H. Byrne. 19 Mar. 2009 <http://www.uth.tmc.edu/nba/neuroscience/index.htm>.
Joan C. Edwards School of Medicine - Marshall University. 19 Mar. 2009 <http://musom.marshall.edu/anatomy/grosshom/spinalreflexes.html>.
Weppler, C. H., and S. P. Magnusson. "Increasing Muscle Extensibility: A Matter of Increasing Length or Modifying Sensation?" Physical Therapy 90.3 (2010): 438-49. Print.
Alter, Michael J. Science of Flexibility. Champaign, IL: Human Kinetics, 2004. Print.
This page created 19 Mar 2009 17:16
Last updated 27 Feb 2016 22:09