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The Organization of Movement
The Organization of Movement
Four Talks on the Primary Control
Part 2: Stretch Reflexes and the Musculoskeletal Framework:
How Stretch Reflexes Convert the Musculoskeletal System
into a Spring-like Framework
by Theodore Dimon
In Part 1 (AmSAT Journal #3,
Spring 2013) we saw that muscles
form a complex webbed system that,
working in conjunction with the
latticework of bones, produces a
network of support based on an
architectural structure of elastic
stretch. This background system is
crucial to how we move; no specific
movement can take place without
this larger network of support.
For the system to work,
Theodore Dimon
however, it is not enough for
muscles to be elastic, important
though this may be. They must also maintain tone in response
to the stretch exerted by the bony members of the skeleton in
order to support the skeleton and also to shorten to produce
movement. How do they do this? If I lift my arm, the
movement is produced by the contraction of muscles that are
operated by motor nerves. A nerve impulse beginning in the
higher cortical centers of the brain carries a motor message to
the muscles that combines with other nervous impulses to
produce coordinated movement.
But movement is more complicated than that. We saw in
Part 1 that specific movement takes place in the context of a
larger system of muscular support. For instance, if I simply
raise my arm, many muscles are involved in the support of the
shoulder girdle and in the postural support of the body as a
whole. This process is far too complex to be directed piece by
piece. We never just contract one muscle; the entire support
system must constantly adjust itself in relation to whatever we
are doing as the background against which the specific
contraction takes place. This overall support, which produces
what we all know as posture, is the work of stretch reflexes.
Muscle Sensors and the Stretch Reflex
The stretch reflex is the automatic contraction of a muscle
in response to being stretched. The most familiar example of a
stretch reflex is the knee-jerk test: the doctor uses a rubber
mallet to tap the patellar tendon just below the kneecap,
stretching the quadriceps muscle, which contains sensors that
register the change in length due to the stretch. These sensors
send an impulse to the spinal cord reporting the change in
length, which in turn excites the motor nerve innervating the
quadriceps and causes the muscle to contract, eliciting the kneejerk response (Fig. 1).
In this example, the doctor artificially produces a stretch
reflex for the purpose of testing muscles and reflex responses,
but the real function of the stretch reflex is to maintain stability
of body parts. When you are standing, gravity is acting on your
body and causing many of your joints to buckle, including the
knees. This buckling of the knees causes the quadriceps muscle
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to stretch––just as the doctor’s mallet did. The muscle,
registering this change, sends an impulse back to the spinal
cord, which in turn sends an impulse to the muscle telling it to
contract. This contraction of the quadriceps keeps the knee
from buckling, thus maintaining stability in the leg. In its
simplest form, then, the stretch reflex is a basic reflex arc
designed to respond to stretch—not just in the knees but
throughout the body—so that parts of the body that are
buckling can be stabilized, thus maintaining posture. In this
way, the body’s elegant elastic system maintains constant
support and tone.
Fig. 1. The knee-tendon reflex
There is a great deal more to say about this process, but
first I want to address the basic nature of a reflex arc. Figure 1
shows the knee-tendon reflex. The quadriceps muscle on the
thigh is charged with the task of keeping the leg straight at the
knee. When the knee buckles, the quadriceps is stretched and,
in order to maintain the support of the leg, contracts in
response. Why should it be necessary for a nerve in the muscle
to send a sensory signal to the spine, and for another nerve in
the spine to receive this impulse and send a motor signal back
to the muscle, when the muscle could just respond directly and
avoid all that unnecessary and apparently redundant signaling?
To understand this, we must remember that muscles are
served by motor nerves with cell bodies located in the spinal
cord and axons that project out from the spinal cord as
peripheral nerves to the muscles. Signals for muscles to
contract originate either in the spinal cord and travel to the
muscle through the peripheral nerves or originate higher up in
the brain and travel down to these peripheral nerves. In any
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The Organization of Movement
case, the muscle receives the impulse to contract from a motor
nerve located in the central nervous system. The muscle does
not know when and how much to contract; that information can
only come from sensors in the muscle that register stretch in the
muscle. So the spinal cord must first receive an impulse from
the muscle sensor telling it when contraction is required, and
then send the motor signal to the muscle (Fig. 2) telling it how
much to contract.
hands, which do not seem to need postural support, have
background tone that is maintained by stretch reflexes. In this
sense, every part of the body participates in postural support––
the arms, hands, and face no less than the legs and back.
Sherrington found that stretching a muscle not only caused
the muscle to resist the buckling of the limb by contracting, but
also caused related muscles to contract and support the activity
of the extended limb. At the same time, activity in the
antagonistic or opposing muscles was inhibited or prevented––a
phenomenon he called reciprocal innervation.4 Although the
stretch reflex forms a simple reflex arc, it is wired up to similar
and opposing groups of muscles so that, even at the spinal cord
level, the coordination of the reflex begins to get rather
complex (Fig. 3).
Fig. 2. The reflex arc
Sherrington and the Study of Posture
C.S. Sherrington, a major founding figure in the study of
neuroscience, was the first to identify and describe the stretch
reflex. In trying to determine what parts of the brain were
responsible for movement, Sherrington conducted experiments
on rabbits and cats by operating on the brain. When he
transected the brain of a cat just above the level of the midbrain,
so that only the brain stem remained, the animal was incapable
of spontaneous action; yet it extended its limbs in an
exaggerated way and would stand indefinitely in forced
extension until it was knocked over––a condition he called
decerebrate rigidity. Furthermore, trying to flex the cat’s limbs
heightened the contraction of the muscles so that the leg
forcibly resisted his efforts. 1 His experiments demonstrated
three things: First, the exaggerated extension of the limbs was
not caused by the voluntary part of the brain, but by automatic,
constant signals from the brain stem that maintained tone in the
limbs––what Sherrington called tonic activity, and which he
distinguished from more active contraction of muscles.2 This
concept of constant, low-level tone in muscles is still widely
accepted today.
Second, when he tried to flex the decerebrate animal’s
limb, the resistance to flexion indicated that receptors in the
muscles themselves must be activating the muscles that
extended the limb, which meant that the main source of this
activity must be proprioceptive outflow from stretch receptors
in the muscles.3
Third, these stretch reflexes clearly played a role in
maintaining posture, since the extended limbs were resistant to
buckling at the joints and therefore helped to maintain the
animal’s upright support against gravity.
It is important to mention that the stretch reflexes are not
confined to the extensor muscles that keep our joints from
buckling. In fact, stretch reflexes act on virtually all the
different parts of the body, helping to maintain its stability. For
instance, the shoulder girdle is supported by muscles acting
upon it from various directions. Even freely hanging arms and
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Fig. 3. The spinal circuits in the stretch reflex system at the
elbow: a. motor neuron to same (homonymous) muscle; b. motor
neuron to related (synergistic) muscles; c. inhibition of motor
neuron to opposing (antagonistic) muscle
Since all this was discovered many years ago, we have
learned much more about posture and reflexes. T.D.M. Roberts,
for instance, points out that balance is maintained not only by
automatic stretch reflexes, but also by anticipatory and learned
responses,5 which play an important role in posture and support.
But underlying all of these responses are stretch reflexes, which
are operating constantly throughout the body as the basic
functional unit of the postural system. As we have seen, these
reflexes are wired at the spinal cord level so that even the
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The Organization of Movement
higher functions are built upon and work in conjunction with
stretch reflexes, adjusting some and turning off others to
produce a wide variety of voluntary movements in the context
of postural support. Basically our complex neural wiring
involving muscle spindles, afferent nerves, spinal cord, and
motor nerves keeps the whole body taut and the limbs engaged
but flexible and fluid when necessary, like the strings of a
marionette. The higher centers of the nervous system––the brain
stem, cortex, and cerebellum––work with and build upon these
circuits.
The entire muscle system, then, is invested with sensors
that play a crucial part in our postural support by enabling
muscles to sense changes in length and send messages that, in
turn, trigger muscle activity designed to counteract these
changes in length and maintain postural tone. Coupled with the
elastic and energy-storing potential of muscle tissue, the stretch
reflex system converts the musculoskeletal structure into a
spring-like framework capable of automatically supporting the
body against gravity and stabilizing all the parts of the body in
movement.
Wrapping around the muscle spindle is an afferent nerve
that carries sensory information to the spinal cord. The part that
wraps around the spindle is called an “annulospiral” receptor
because of its shape (Fig. 4b.). When the larger muscle fibers
are stretched this also stretches the intrafusal fibers that make
up the muscle spindle and elongates the annulospiral receptor
endings wrapped around it. This activates the nerve, which
sends an impulse to the spinal cord, where it synapses with the
motor nerve innervating the same muscle, telling the muscle to
contract. In this way, the muscle spindle functions as a very
sensitive organ for detecting changes in the length of a muscle
and sending an impulse to the spine, which in turn “reflects
back” a motor impulse to the same muscle.
Muscles Spindles and the Reflex Arc
How do stretch reflexes work? We saw a moment ago that
stretch reflexes operate as a reflex arc that begins with a sensory
signal sent by a stretch receptor in the muscle. The stretch
receptor, called a muscle spindle, is an amazing little organ
(Fig. 4a.). It is called a muscle spindle because of its
resemblance to a textile spindle; that is, it is shaped like a
cylinder that bulges at the middle and tapers toward the ends.
Fig. 4b. Annulospiral receptor wrapping around the
intrafusal fibers that make up the muscle spindle
Fig. 4a. Muscle spindles located on the main muscle fibers
The spindle is made up of a bundle of very tiny contractile
muscle fibers––about three to ten in number––that attach where
they taper at each end to the connective tissue that binds
together the fibers of the main muscle. These tiny fibers,
sometimes called intrafusal fibers to distinguish them from the
extrafusal fibers that make up the main muscle, are much
smaller in diameter and length than the extrafusal fibers––they
are only four to ten mm in length.
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This reflex arc operates as a negative feedback loop. When,
for instance, we are standing and our knees buckle from the
force of gravity, this buckling stretches the quadriceps muscle,
which stretches the muscle spindle, activating the annulospiral
receptor wrapped around the spindle, which sends impulses to
the spine and synapses with motor nerves that, in turn, tell the
quadriceps muscle to contract. This muscular contraction
reduces stretch on the spindle so that the annulospiral receptor
stops firing––a negative loop that is turned on by stretch and
stopped when the muscle contracts and the spindle is no longer
being stretched. This feedback loop thus operates continuously
to maintain stability in the joints (Fig. 5 ).
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AmSAT Journal / Fall 2013 / Issue No. 4
The Organization of Movement
Fig. 5. Negative feedback loop of the stretch reflex arc: a. flexor muscles keep the arm flexed at the elbow; b.
muscle is stretched, muscle spindle fires and activates reflex arc; c. motor neuron fires, muscle contracts, spindle
is no longer stretched and stops firing
Antagonistic Action and Stretch Reflexes
But this stretch reflex system cannot work properly if the
muscular system is not working efficiently. In Part 1, we saw
that the musculoskeletal structure can be described as a system
of tensile components and struts in which the tensile
components support the struts, but the struts maintain length on
the tensile components. If this system is interfered with––in
other words, if we tighten the neck and interfere with head
balance so that the back muscles are shortened––muscles
throughout the body must compensate to maintain upright
support. When this happens, the muscles can no longer lengthen
and the spindles no longer register stretch. Parts of the stretch
reflex system, quite literally, shut down.
When this situation occurs, the remedy is to restore length
to the system. This can be achieved by placing the skeleton in
supportive positions in order to restore length to particular
muscle groups that have forgotten how to maintain length and
perform their supportive function. This restores natural
tensegrity support and increases the stored potential energy in
muscles, which imparts more spring and elasticity to the
framework. But because the tensegrity system is invested with
sensors that register stretch, restoring muscle length also has an
effect on the stretch reflex system. Stretch reflexes that
previously were not operative begin to work, the body begins to
feel lighter, the spine regains a kind of inward buoyancy, and
muscles that were flaccid and tight spontaneously tone up and
release. These changes in the musculoskeletal system can be
observed and felt from head to toe. We begin, as we say, to “go
up.”
What has happened is that the condition of elasticity and
release in muscles, which is part of the tensegrity design, has
stimulated stretch reflexes. We might assume, because muscle
spindles are capable of adjusting to the shortened condition of
muscles, that the stretch reflex is always operative. However,
when muscles become too shortened, the spindles cannot adjust
and the stretch reflexes shut down. To work properly, the
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stretch reflexes require both the elasticity of muscles and the
dynamic relationships between muscles and bones that are
established when the system works as a whole. Without these
conditions, the reflexes are inoperative; when these conditions
are restored, the stretch reflexes are stimulated, and the
muscular system regains its automatic, buoyant support.
Stretch reflexes, then, do not work independently of the
condition of muscles, but only function properly when muscles
are elastically supporting the skeleton. If this network is not
operating elastically, the muscular support system is not
properly activated, and effortless support is replaced with
chronic contraction of muscles, which are now needed to
maintain upright posture; some stretch reflexes end up operating
in a compensatory mode and many do not operate at all. When
the system is restored, the stretch reflexes are activated, muscles
no longer need to forcibly contract, and muscles perform their
naturally supportive function with more length and tone. In
short, elasticity in the context of the architectural whole is the
key to the stretch reflexes operating effectively to provide
muscular support of the whole with a minimum of effort.
Stretch Reflexes and Muscle Tone
Knowing how stretch reflexes work is essential to
understanding what makes muscles healthy. In Part 1, we saw
that we cannot describe a muscle as being healthy in the
absence of elasticity. A muscle is certainly not healthy simply
because it is built up and can perform work or because it is
relaxed and can be mechanically stretched. For muscle tissue to
be healthy, there must be an active relationship between length
and tension. For the stretch reflex system to operate fully, and
for muscles to contract most effectively, they must function
antagonistically within the context of a skeletal framework that
imposes stretch so that the muscle spindles can sensitively
register changes in muscle length and respond with a lively,
spring-like activity. This is a key part of what makes muscles
healthy and toned.
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The Organization of Movement
An Alexander Technique teacher can detect the presence of
this lively tone and the life in the muscle. This is not just
fanciful or metaphorical. It is an observable condition in which
muscles are lengthening, the spindles are firing, and the
contractile components of the muscles maintain a healthy
amount of resistance against lengthening—in other words,
underlying electrical and chemical activity combines with an
elastic component to produce an overall lively, flexible quality
in the muscle that can be felt. And it cannot be achieved
through any sort of treatment, bodywork, or exercise, but only
in the context of active support and movement in the
gravitational field.
Because the system works as a whole, it cannot be put together
piecemeal by trying to correct one part or another; all the parts
must be readjusted in relation to gravity as an interrelated
whole.
Reciprocity of Nerves and Body
Although neuroscientists have explained a great deal about
how muscles spindles work, their work has not yet taken into
account the functioning of the body as a whole. When we
consider the reflex arc, it is easy to assume that it is basically a
two-way street—a sensory response to stretch in muscle
followed by a motor impulse that is reflected back to the
muscle. But in order to work properly, muscle spindles must
operate in the context of a system of muscle pulls and stretches
The Alexander Technique
Understanding this principle of muscle function was one of
that maintain the proper organic conditions under which the
Alexander’s great accomplishments. Fully one half of the
muscle spindles are at their most sensitive and the muscles at
Alexander work is about restoring the postural/stretch system to
their most healthy and responsive. Stretch reflexes are designed
its normal working; the other half is about gaining conscious
to respond automatically to stretch, but they function as part of
control over this system so that we
the dynamic interplay between the
can apply this knowledge in our daily
signaling to and from muscles and
“The idea is to restore the stretch reflex
activities. When we are born, skeletal
the state of the musculoskeletal
system, including the ability of muscle
support imparts a natural length to
system as a whole and cannot be
to sensitively register changes in length,
our muscles so that our inborn,
expected to work normally if this
natural reflexes can respond to
system is imbalanced.
which is a big part of their function––and
stretch all over the body, making it
One of the things this shows is
a big part of what the Alexander
possible for the system to provide
that awareness alone cannot improve
Technique is all about.”
support in an efficient and effortless
muscular and motor functioning. We
way. Habitual tightening of muscles
often hear Alexander Technique
however––as well as other influences in life––begin to interfere
spoken of as a kinesthetic method, which of course it is. Other
with this system, and the body has no other alternative but to
methods, such as Feldenkrais and relaxation techniques, also
maintain support by replacing stretch with muscle tension. At
utilize kinesthetic awareness to reduce unnecessary muscle
first, this may not be a problem; but over time, the
tension. We receive proprioceptive information from muscle
compensations become chronic and the muscles do not have the
spindles, but if muscles are chronically tight, this information is
length they need for the stretch reflexes to be activated. We are
drastically reduced and less accurate. This means that unless we
then faced with the dual disaster of losing tone in many of the
establish lengthening of muscles as a precondition for healthful
muscles needed to support the body against gravity, and other
function, trying to be aware of muscles is virtually useless, not
muscles working far too much and becoming chronically tight.
to mention misleading. Before we can rely on kinesthetic input,
The body cannot find a way to recover and the postural system
we must first restore length to muscles and, as much as
cannot work correctly. New activities, meanwhile, are
possible, remove the interferences with normal and natural
undertaken in an increasingly harmful way until, with time,
muscle tone. Awareness is not the first step in the process––it
dysfunction and collapse result. This disarrangement is most
depends on basic organic conditions of elasticity in muscles,
easily seen in the pulling back of the head, the shortening of the
which is why awareness must be based on re-education.
spine, the overworking of the lower part of the back, and
Summary
collapse of other parts of the body. The antagonistic stretch on
To summarize, the body is organized as a tensegrity
the muscles is compromised, the stretch reflex system cannot
structure; stretch reflexes convert this structure into a springwork properly, and the system, unable to work as it is designed
like framework. Essential to this system is the elastic condition
to work, goes into collapse and fixation.
of the muscles that erect the tensegrity structure, so that all the
It is at this point that re-educational work is needed,
muscles, tendons, and ligaments––the tensile parts of the
beginning with some form of mechanical support (such as
system, the guy wires––perform work appropriately and the
sitting with a supported back or lying in a semi-supine
workload is distributed across the whole network. This
position). The idea is to restore the stretch reflex system,
elasticity imparts potential energy to the muscles, since muscles
including the ability of muscle to sensitively register changes in
store energy when they are lengthened; the resulting rebound
length, which is a big part of their function––and a big part of
effect helps to maintain the integrity of the system.
what the Alexander Technique is all about. One option is to
Loss of length in muscle means that the structure cannot
address this by stretching and strengthening muscles, but this
support itself properly, with the result that various guy wires
will not work, because the system is designed to work as a
take on too much load; it also means loss of spring-like support
whole, with muscles functioning antagonistically in the context
and potential energy in muscle. We experience this in the
of postural support. No amount of strengthening, exercising,
heaviness and stiffness in our legs as we age, in contrast to the
relaxing, or balancing muscles can restore this condition when
spring-like legs of children.
the system is so maladjusted and sensory input so distorted.
Continued on page 22.
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The Organization of Movement, continued from page 20.
At the same time, muscles must maintain tone to stabilize
the tensegrity structure, which is made possible by the stretch
reflex system. This system can only work efficiently when
length is restored to muscles, which stimulates their reflex
activity. The antagonistic action of muscles, then, is the
condition under which the stretch reflex system operates to best
advantage. In the next installment in this four part series, we
will look at the role of the head and trunk in organizing the
working of this system as a whole.
Endnotes
1. See Sir Charles Sherrington, The Integrative Action of the
Nervous System (New Haven, CT: Yale University Press,
1961), 299–302.
2. Speaking of extensor activity in the decerebrate animal,
Sherrington writes: “These muscles counteract a force (gravity)
that continually threatens to upset the natural posture. The force
acts continuously and the muscles exhibit continued action,
tonus” (Integrative Action, 302). He later continues: “Two
separable systems of motor innervation appear thus controlling
two sets of musculature: one system exhibits those transient
phases of heightened reaction which constitute reflex
movements; the other maintains that steady tonic response
which supplies the muscular contractions necessary to
attitude” (Ibid.).
3. Sherrington, Integrative Action, 337: “[I]n the decerebrate
dog the tonic extensor rigidity of the leg appears reflexly
maintained by afferent neurones reaching the cord from the
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deep structures of the leg itself. Similarly, if the knee-jerk be
accepted as evidence in the spinal animal of a spinal tonus in
the extensor muscle, this tonus seems maintained by afferent
fibres from the extensor muscle itself, since the knee-jerk is
extinguished by severance of those fibres.”
4. Ibid., 86–100.
5. See Tristan D. M. Roberts, Understanding Balance: The
Mechanics of Posture and Locomotion (London: Chapman &
Hall, 1995); see also Tristan D. M. Roberts, “Reflexes, Habits
and Skills.” Direction–A Journal on the Alexander Technique,
no. 10, 23–28.
Drawings by Helen Leshinsky.
Dr. Theodore (Ted) Dimon received M.A. and Ed.D degrees in
Education from Harvard University and Alexander Technique
teacher certification from Walter Carrington. Dimon is the author
of five books: Anatomy of the Moving Body; The Body in Motion;
Your Body, Your Voice; The Elements of Skill; and The
Undivided Self. He is the founder and director of The Dimon
Institute in New York City and an adjunct professor of Education
and Psychology at Teachers College, Columbia University. More
information about Dimon’s work and The Dimon Institute can be
found at: www.dimoninstitute.org.
© 2013 Theodore Dimon. All rights reserved.
Photograph of Ted Dimon by Marie-France Drouet.
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