The pelvic floor is often overlooked and misunderstood during movement assessment. The pelvic floor is integral to both breath and movement. This is good news, as it allows us to use movement to assess the pelvic floor. This is important because of the sensitivity of the pelvic structure. More invasive approaches like direct palpation of the pelvic floor compromises safety of both practitioner and client. Starting with movement assessment builds safety in the nervous system. In addition, movement assessment also allows pelvic floor assessment to be accessible to practitioners where direct contact with the pelvic floor is out of their scope of practice.
There are a few principles of movement assessment to consider with pelvic floor assessment. These include functional assessment, the biomechanics involved, and how to keep the container safe. Differentiating how the nervous system is responding from structural response becomes the foundation of functional assessment. Next, we need a clear understanding of the biomechanics involved. Because the pelvic floor is fundamental to the breathing apparatus, the pelvic floor is virtually participating in every possible movement. And perhaps most important, how we keep the container safe. The pelvic floor is a sensitive, limbically driven, aspect of our structure, and we must use caution so that we do not overstimulate the nervous system.
I’d like to talk with you about an anecdotal experience I recently had with a client.
I had a runner come to me who was experiencing knee pain. If I had stopped in my assessment at simply looking at the function of the knee, I would have missed the primary driver of the compensation pattern. Because I linked the relationship of the Anterior Kinetic Chain, and the core cylinder, I was able to correlate an internal oblique issue to the medial knee. It’s not uncommon for ligaments to compensate for the burden when muscular function is impaired. My ability to move beyond the obvious to a deeper level of inquiry – which is what we learn in Dynamic Neuromuscular Assessment™ Seminars – allowed me to get to the more profound root issue for my client.
Anatomy and kinesiology are two disciplines that give clarity to the interdependence of the structure and movement of the body.
Anatomy is the language used to describe the parts. These parts then fit together into systems that synergistically make up the whole organism.
Kinesiology is the language of movement. Through kinesiology, anatomy is given a context. If the language of movement is a symphony, the role of anatomy is to describe the source of each note of music.
Anatomy charts provide the fundamental foundation for understanding the names of bones, joints, ligaments, tendons, muscles, fascia, and so forth – the structure. Kinesiology then defines how each aspect of structure works together to create movement.
One of my teachers, early on in my career, imparted the importance of the breath, movement, and structure as being interdependent.
Movement is a translator to how the structure organizes, movement can’t lie. When the body is experiencing pain, the brain reorganizes movement so that we move around our pain instead of through it. This avoidance is a compensation to keep us in a perceived safe zone. As practitioners, our ability to see deviation in movement is paramount to assisting our clients. Often the walking gait is the lens through which we look during assessment. The 5 Primary Kinetic Chains provide a map of the gait.
The use of color in The 5 Primary Kinetic Chains illustrations imparts upon the teacher/student or practitioner/client, how the body organizes during movement. The kinetic chain charts further define how the body organizes in the optimal manner during gait. Why the gait? The gait is universal to human movement. From birth, our nervous system is prewired for developmental movement with the intention to get us upright and biped. If you have interest in a more in-depth conversation on the walking gait, see my blog on the Master Template. The synergistic organization, or sequential muscular activation, gives context to efficient movement and helps us to identify potential dysfunctional relationships that may not be obvious at first impression to the client or practitioner.
When the synergistic organization of our movement becomes less than optimal, or compensated, the result are over and underworked players. Synergistic dominance is the relationship between these over and under worked players. As a practitioner it is useful to have reference tools – like The 5 Primary Kinetic Chains Poster Set or Desktop Edition to help us dig deeper into the function and dysfunction presented by our clients.
Please leave a comment below about a powerful experience you had either as a client or practitioner where you or they went beyond the obvious to the profound!
While manual muscle testing can be a powerful tool, movement is infinitely variable. Because manual muscle testing attempts to put movement into a confined box with fixed positions, it has limitations. I have a problem with classical conventions that say “this movement is xyz muscle.” Muscles do not work in isolation. Muscles work synergistically with bones, joints, ligaments, tendons, muscles, fascia, and skin. We can isolate movement; we cannot isolate structure.
The idea that if you put the body in a specific position you are testing xyz muscle is limiting. Some 15+ years ago I was seeing a Physical Therapist for a shoulder injury. My colleague brought out a two volume encyclopedia of manual muscle tests. Each muscle had three divisions. Each division had three positions: open, closed, and middle. That equates to nine possibilities to assess a particular muscle in the structure.
In the assessment process of DNA™, we take the process a step further. Instead of merely taking a “snap shot” of movement, we assess the complete “movie” of movement.
Movement has three components of players. One side of the tissues are in concentric action. This is the work production phase. The other side are the tissues that are lengthening. This is the eccentric phase of movement. This is often referred as the deceleration phase of movement. The third aspect of movement are the stabilizers that provide support for the movement. This is analogous to isometric movement though it is dynamic in application.
These three aspects of movement form a dynamic triangle that can respond to a changing environment. For example, the core subsystem in the Lateral Kinetic Chain creates a dynamic triangle that supports the pelvis/lumbar. The lateral hip stabilizers, adductors, and the opposite quadratus lumborum work together synergistically to maintain an upright axis of the spine when load bearing in the stance phase of gait.
Dynamic Neuromuscular AssessmentTM seminars take the skills you already possess and puts them into a context that will make those skills more effective.
When you employ what Joseph calls The Five Essential Skills with the corrective strategies you already use, the results of your work will have a quantum effect with your clients.
Essential Skill #1 – Hybrid Movement Assessment:
Learn to seamlessly transition between direct muscle testing strategies and indirect muscle testing – Manual Muscle Testing redefined
We will change the paradigm of testing muscles by assessing the response to movement. Can the nervous system and structure appropriately respond to the movement environment?
The Intrinsic Kinetic Chain has many players that cannot be evaluated with direct muscle testing strategies. Hybrid Movement Assessment strategies are essential for evaluating the players in respiration – a key element in assessing clients.
Essential Skill #2 – Completing the Feed-back Loop:
Afferent sensory neurons relay information about how movement is responding to the movement environment: this is feed-back motor control
Efferent motor neurons are the motor instructions to the periphery: this is feed-forward motor control
Using both feed-back and feed-forward movement completes the proprioceptive feed-back loop
Compensation is learned through the feed-back loop. Adaptation is need and response, two sides of the movement equation. Motor learning requires the integration of both feed-back and feed-forward communication to and from the brain. Feed-forward motor instructions allow the motor control center of the brain to capitalize on compensation patterns. DNA’s movement assessment strategies uncover hidden compensation.
Essential Skill #3 – The Functional Compass:
The functional compass provides a map for movement potential
Movement happens through non-linear spirals
Joints act in compression and distraction
Joint assessment using the functional compass evaluates the spectrum of movement potential
Shock Absorption of the Deep Longitudinal Kinetic Chain is interdependent with the ability of the joint capsule to translate compression to distraction over its range of motion. When the joint loses its ability to respond appropriately, compensation will show up as a symptom in ligaments, tendons, muscle and fascia. DNA’s joint by joint assessment strategies give laser focus attention on the root cause rather than the symptom.
This is analogous to non-painful dysfunctional movement
Movement functions can appear to be available with direct testing strategies
Hybrid Movement Assessment uncovers hidden layers of compensation
Movement functions can appear to be available with direct testing strategies. Challenging those movements with Hybrid Movement Assessment will uncover hidden layers of compensation: “just because you can doesn’t mean you should.”
Often, our clients are reinforcing compensation through their daily movement. DNA’s assessment strategies identify these patterns so they may be appropriately addressed.
Essential Skill #5 – Eccentric Movement Assessment:
Direct Concentric muscle testing is a mere snapshot of movement
Eccentric Movement Assessment challenges movement over a range rather than a snapshot
Eccentric Movement Assessment incorporates the SAID principle into assessment strategies
The rules of the SAID principle states that adaptation is specific to demand. If the motor program is not cued into the corrective strategy, it may not respond to the correction. Eccentric Movement Assessment cues the motor control center to a larger context of information. This brings up compensatory patterns that would not be revealed in standard concentric testing strategies.
DNA’s assessment strategies are unique as they incorporate both sides of the movement equation. Concentric activation must be balanced with Eccentric stabilization. This skill set can be explored through the core subsystems of The 5 Primary Kinetic Chains.
Muscles produce work in the body. They come in two distinct types, smooth and striated. Smooth muscles are governed by the autonomic nervous system. Their function is automatic. Smooth muscles perform the regulatory functions. The tissues that make up organs, the GI tract, and arteries utilize smooth muscles to perform their unique functions. Conversely, striated muscles are governed by the rules of conscious motor control. Striated muscles are often referred to as skeletal muscles. Their job is to act on the skeleton for posture and movement.
Skeletal muscles have a spectrum of roles. Highlights include: work production, multiple joint stabilization, and position sense. Muscles need to be available to do their job in the movement equation. If they can’t participate appropriately, the brain will come up with a coping strategy. This is a survival-based mechanism, and this is what we call compensation. Compensation has many flavors, and despite a bad rap, it is the intelligence of the body doing its best to keep you safe.
Muscles come in many configurations. Generally, the large powerhouse muscles are more superficial, while the intrinsic stabilizers are deeper. Some muscles are specific in fibril orientation and function while others are available for multiple roles. For example, the large powerhouse muscles of the posterior chain, the latissimus dorsi and gluteus maximus, have multiple fibril orientations that look like a fan. This gives these muscles mechanical advantage over the range-of-motion spectrum.
For simplicity, let’s categorize muscles into two sets: short and long-lever. Short-lever muscles are the dependable hardworking muscles. They have mechanical advantage on the joint. The brain likes to use them as the go-to muscle during work production. Long-lever muscles cross multiple joints and have multiple attachments. Long-lever muscles are best suited for stabilization during work production. Their role is key when movement deviates and unknown variables occur in the environment.
Compensation patterns have a common trait among short and long-lever muscles: short-lever muscles are the heroes. They come to rescue when the long-lever muscles are not responding appropriately in the movement environment.
– cross one joint
– mechanical advantage
– commonly up-regulated
– cross multiple joints
– stabilizer during work production
– commonly down-regulated
Short-Lever ~ Long-Lever
popliteus ~ bíceps femoris
bíceps femoris short head ~ biceps femoris long head
iliacus ~ psoas
multifidus ~ erector spinea
subclavius ~ pectoralis major
brachialis ~ biceps brachii
These examples are samples of utilizing short-lever ~ long-lever muscle relationships to assess movement compensation patterns. The kinetic chain charts in The 5 Primary Kinetic Chains provide a map for investigating synergistic dominance, regional interdependence, and functional opposite musculoskeletal relationships. Muscles are in constant response to joint position in the movement environment. Can the muscles in conjunction with motor control instructions respond appropriately to the environment?
My upcoming Dynamic Neuromuscular Assessment™ workshops (learn more here) will provide an integrated strategy for movement assessment in a changing environment. Some of the key skill-sets we will employ:
utilizing a hybrid that combines direct assessment with indicator testing to uncover functional dysfunctional movement
utilizing feed-forward motor control to assess structure that cannot be directly tested
completing the proprioceptive feedback loop to assess both motor instructions and structural response
investigating long series kinetic chains because muscles do not work in isolation, they work in synergistic sequences during movement
investigating dynamic stability as a two-part equation: concentric action balanced by eccentric action — eccentric movement evaluation uncovers hidden layers of compensation
There seems to be lot of “dysfunctional psoas causing back pain” articles. I’d like to offer another viewpoint.
Say a person does have an inhibition in their psoas. What effects would that have on posture?
The short answer is: a general facilitation along the anterior kinetic chain. The body doesn’t like to be in a position it cannot stabilize. If it is weak in an action such as flexion, the body will move more into flexion, which gives the illusion of being in a safe position.
This position then affects the ability of the hamstring to act on the ischial tuberosity. How do you think the lumbar is going to respond when it does not have the reciprocal muscles balancing extension?
The next question that we should be asking is why is the psoas inhibited in the first place? Is that the causation or a symptom of something else?
Lots of questions, and each person has their unique answer.
Looking deeper into causation instead of chasing symptoms is a good practice.
Don’t just treat what you find, look deeper. Peel away the layers.
Ask for next level factors. It could be structural. It could be physiological. It could be emotional/cognitive. There are environmental factors as well as habitual influences that could be in play. How we work and how we move are all considerations as well.
We are complex human beings, not just muscles and a nervous system.
The psoas is involved in posture, stability, and breath. Read more about the “Mighty Psoas” here.
The limbic system is the master regulatory system of the body. Information is gathered from both the outer environment and the inner environment. That information, or inputs, are then collated and prioritized so that the process of homeostasis occurs on structural, physiological, and emotional levels.
The sensory apparatus of the body is under a constant input from the outer and inner environment. We are familiar with the five traditional senses: sight, sound, smell, taste,and touch. There are also an array of non-traditional senses. Our kinesthetic sense is one of these. Our balance sense, relationship to gravity, and proprioception are components of our kinesthetic sense.
There is an array of sensory inputs to which we are registering and responding. They can be categorized as tangible and non-tangible. The tangible senses would be our response to the five traditional senses and the kinesthetic senses. What about electromagnetic fields, barometric pressure, spectrums of light, wave lengths of sound, or cosmic energy? These are measurable influences even if we cannot consciously quantify them in our experience. Our subconscious mind is sure to have the capacity to respond to these inputs. Many people are sensitive to these influences. I get headaches when I travel next to big overhead wires and I can sense the weather changing in my joints a week before the event occurs. When I wear special colored glasses my brain function integration improves, and so forth. These are just a few examples of tangible sensory inputs. There is a vast array of non-tangible as well.
If we look at the image of the iceberg, about 25% is above water and 75% is below. The 25% is the tangible realm, that which we can quantifiably create context. The other 75% is where it gets really interesting. This is the mystery. This is the realm to which some people have access. This information is often regarded as psychic senses or the paranormal; for most of us, it is our intuition. These inputs are unknowingly affecting us 24/7.
It is in the non-tangible that limbic resonance resides. Empathy comes to mind as I think of a non-tangible. There is a connection to the earth, the plant and mineral kingdom, another living being, a fellow human being, and so forth. Resonance is vibration. The harmonics are able to mesh and respond to each other. How many times have we heard “I had chemistry with that person.” This is limbic resonance.
In the therapeutic context, it’s a matching of vibration. The intention of the practitioner is clear, connected, and confident while remaining neutral, receptive, and present. It is a wonderful example of mindfulness. When we have limbic resonance with our clients, vistas of new possibilities can present themselves. We are out of the technical application of our craft and engaged in the inquisitive creative side of our consciousness. This does not take away from technical capacity. Limbic resonance is complementary.
The homework cards allow the practitioner to give specific homework based on their clients’ presentation. They serve as a reminder for the client to stay on track between sessions. They also provide a template for greater client education and understanding by emphasizing both manual release and integration exercises that work in tandem for success in recovery.
The cards are easy to use. The kinetic chain illustration is on the front of the card and there are four entries on the back of the card.
These are the player/s not engaged. This is the part of the movement equation that needs to get back in the game of keeping the structure safe.
These are the player/s that are overworked by trying to do the job for the down-regulated player/s. Often, these up-regulated player/s create secondary down system effects. Good detective work discovers the primary relationship between the up and down regulated players so that the application of the release and integration is effective at restoring balance back to the structure.
This is the first step in repatterning. The release of the fixated segment or inappropriate tension allows for a new pattern to be learned. There are many appropriate interventions, as well there are ways of asking the body what it needs. This is up to the practitioner and their toolbox.
There is a window of opportunity for the nervous system to learn a new pattern, and to get the player/s that have been disengaged back in the game. The manual release acts as a hack. By temporarily removing the option for compensation, the nervous system must learn a new coping strategy. Activating the down-regulated player/s give the structure the support it needs to recover balanced action.
The order of cuing the motor control center is important so that effective change and reinforcement of the pattern becomes a learned behavior. If the compensated player is not temporarily taken out of the movement equation, then subsequent movement work often will reinforce a maladaptive pattern. The idea is to displace a maladaptive pattern with a more bio-mechanically efficient pattern. Displacing maladaptive compensation with appropriate movement integration keeps the container of coping mechanisms safe.
To summarize, the homework cards are the place where you:
Identify the underworked player/s ~
Identify the overworked player/s ~
Temporarily remove the overworked player from the movement equation ~
Integrate the underworked player back into the movement equation ~
Please note this particular series of blogs will describe each of the four muscles and their relationship to the five principal actions described in the charts of The 5 Primary Kinetic Chain Poster Set I’ve developed. This is Part Two of four. You can find Part One on the Piriformis here.
The sacrum, or sacred bone, is unique in the body. Mystics regard the sacrum as the focal point for kundalini, the spiraling energy that rises from the root through the crown. This triangular shaped bone provides the base of support for the spinal column.
The sacrum articulates with the pelvis through the sacral iliac joint, SIJ. The kinetic energy of ground force reaction moves from the feet engaging the earth, up through the legs, into the pelvis. The energy crosses through the pelvis into the sacrum and up through the axis of the spine. The manner by which this energy moves into and through the axis of the spine defines our ability to respond to ground force reaction.
There are four important muscles that act directly on the sacrum.
These four high level muscles often are not engaged with their task of stabilizing the sacrum through a spectrum of movement. When we look at the function of these four muscles, and the various movement they are involved in, there is a trend we see in most people’s presentation that are seeking therapeutic intervention.
The anterior surface muscles are often up-regulated. These muscles are over worked and do not respond appropriately. One of the flavors of synergistic dominance is when one group of fibers becomes up-regulated, those dominant fibers then down-regulate the function of that muscle over its spectrum of movement.
The posterior surface muscles are often down-regulated and are not available to respond appropriately to movement.
The relationship of how these four muscles work together in coordination changes over the spectrum of movement. The 5 Primary Kinetic Chains have unique Principal Actions that inform the sequence of movement. This series of essays will describe each of the four muscles and their relationship to the five principal actions.
The iliacus is a large primary muscle of the pelvis that attaches to the bowl of the pelvis, the iliac fossa. This muscle has a large surface area as it fans across the inner bowl of the pelvis. The multiple direction of these fibers give them advantage over a range of functions.
As the fibers of the iliacus come off the pelvic bowl, they knit together multiple structures of the pelvis. Fibers attach to the anterior sacral ligaments, the sacrum, the psoas, and the lessor trochantor of the femur.
Looking at the web of connective tissue between the iliacus, the psoas, the anterior sacral ligaments, and the direct attachment on the body of the sacrum, it becomes clear that the iliacus has a profound effect on the sacrum.
The fibers of the iliacus are joined by the fibers of the psoas. Together they create a common tendon attachment on the lesser trochanter. This makes the iliacus and the psoas an important synergistic pair, yet they have some different roles in movement.
The psoas is a multi-segmented muscle. The psoas crosses multiple joints of the lumbar spine. Muscles that cross multiple joints have an important role as a stabilizer during the work production phase of movement. The shorter fibered muscles that cross one joint are the hard working prime mover. The important distinction here is that the psoas is acting on the lumbar spine while the iliacus is acting on the pelvis. When these two muscles are not playing well as individuals, or as a synergistic pair, the result is a destabilized lumbar-pelvis.
The iliacus is considered one of the more common up-regulated muscles. The bracing, or shortening action of an up-regulated iliacus, affects the sacroiliac joint, SIJ.
As the iliacus acts on the ilium, the relationship of a neutral SIJ becomes altered. The movement of the sacrum is self-limiting by the SIJ, while the ilium has more freedom to move around the sacrum creating a mobile/stable relationship. Hip rotation, hip hiking, and hip flare are relationships to sagittal, coronal, and transverse plane movement. The iliacus is involved in these movements even if it isn’t the driver.
Concentric Actions of The Iliacus:
Sagittal ~ hip flexion, ilium rotation, & sacral flexion
An up-regulated iliacus is the action of the exhalation phase thereby affecting the inhalation phase of the breath. This is a relationship of reciprocal inhibition.
The iliacus attaches on the bowl of the pelvis creating an extrinsic boundary. An up-regulated iliacus partners with the pelvic floor. During the inhalation phase of the breath, the pelvic floor’s action is eccentric lengthening. An up-regulated pelvic floor loses this ability.
Deep Longitudinal ~ Shock Absorption
An up-regulated iliacus interferes with the kinetic wave of shock absorption. The up-regulated iliacus is a bracing strategy for the SIJ. Compression in the SIJ functionally acts as an abutment to the kinetic wave of ground force reaction.
The body’s appropriate response to the kinetic wave of shock absorption is to counter with the push reflex. Imagine stepping off the curb. The hip must descend so that the foot can meet the ground. This is an eccentric action of the quadrates lumborum, the QL. An up-regulated iliacus down-regulates the push reflex.
Lateral ~ Axial Stability
The adductor magnus, a lateral kinetic chain subsystem muscle, needs to play well with the iliacus. The adductor magnus is a synergist with the adductor longus. The iliacus is synergist with the adductor longus.
During the transition phases of the gait, mid stance, late stance, propulsion, and shift, this synergistic pair action is eccentric lengthening. This lengthening is storing elastic energy that will be released in the swing phase of the gait.
The lateral kinetic chain is in contralateral relationship with the anterior spiral kinetic chain: stance/swing. This movement requires stability across the anterior surface of the sacrum. The iliacus and contralateral piriformis become functional synergists during the swing phase of the gait. Looking at these kinds of contralateral relationships is an important aspect in movement assessment.
Posterior Spiral ~ Generation of Stored Elastic Energy
The coiling of the thoracolumbar fascia acts on the sacrum and the SIJ. The hip is extending and externally rotating. The iliacus is actively engaged in eccentric lengthening, or is a functional opposite.
An up-regulated iliacus is going to down-regulate the coiling action of the posterior spiral kinetic chain. This is important when looking at the posterior surface muscles that act on the sacrum. Often, multifidus/sacrospinalis and gluteus maximus are down-regulated and need to get back into the equation for sacral stability.
Anterior Spiral ~ Translation of Stored Elastic energy
The iliacus is a powerful hip flexor. An up-regulated iliacus will look for recruits to assist in hip flexion during the swing phase of the gait. The common players the body looks to recruit are the psoas, tensor fasciae latea, rectus femoris, sartorius, and all the adductors.
The anterior spiral pairs with the contralateral lateral kinetic chain. At the moment when hip extension translates into hip flexion, the iliacus and the contralateral piriformis are in functional synergist relationship. This creates stability across the anterior sacrum during shock absorption.
The body starts to look for recruitments to assist an up-regulated and fatigued muscle/s. One common recruitment pattern are muscles in contralateral pairs. The pectoralis minor and the iliacus are common up-regulated muscles, they work together in the contralateral shoulder to hip relationship of the anterior spiral.
Manual Therapy Application:
One important aspect of any manual intervention is to ask the body directly if the modality is appropriate. This can be verified by doing a little bit of release. Go back to the relationship and take notice. Did the response change in a favorable way? If it did, then the release technique was appropriate. If it did not, then the nervous system needs something else to restore the coordination.
Here are a few strategies I regularly employ when working with an up-regulated iliacus.
Strain Counter Strain:
This is a one of my favorite go to techniques. It is gentle and effective. There is little risk to further irritation of an up-regulated iliacus. The lessor trochanter, the common tendon junction and the bowl of the pelvis are good entry points for this gentle technique.
This active bilateral release can have a dramatic positive effect in the SIJ. The belt puts the SIJ in compression while the bilateral activation of internal/external rotation resets the receptors. The therapist can approach the release in two ways. One is to use feedback pressure to activate the balance between internal and external rotation. The other is to use bilateral pressure on both piriformi to reset the muscle spindles.
Active Muscle Spindle:
This is a technique that resets the muscle spindles interpretation of muscle length. Support clients leg with thigh vertical, leg horizontal. Have the client hold their leg in place to accurately access the common tendon junction near the bowl of the pelvis. The placement of the practitioners fingers should be such that there is zero visceral impingement. Once appropriate contact is made, the clientslowly extends the leg and draws back to the starting position.
Pin and Stretch:
This flossing technique is a mixed bag. It can either be highly effective or over stimulates the nervous system. Ask the body if it is appropriate to the client’s presentation.
When assessing the players involved with sacral stability, ask if the identified players can cooperate with each other. Getting all the players back on the same team make for a happy sacrum.
Concentric activation ~ The muscle fibers are shortening; the muscle attachments are moving toward one another.
Eccentric activation ~ The muscle fibers are lengthening; the muscle attachments are moving away from one another.
Synergist ~ Muscles that work together during movement.
Functional Opposite ~ Muscles that work opposite to one another. One muscle is lengthening while the other is shortening.
Up-Regulated ~ An overstimulated muscle that is compensating for other muscle/s that are not participating. Often the muscle will become overworked and fatigued and unable to respond appropriately.
Down-Regulated ~ An under stimulated muscle. The function is impaired and unable to respond appropriately.
Reciprocal Inhibition ~ When a muscle/s is contracting, the opposite muscle/s must be lengthening. If the opposite muscle/s are unable to lengthen, being up-regulated for example, then that will functionally inhibit the muscle that is contracting.