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Intrinsic Stabilization Subsystem (ISS) — Brookbush Institute | Brentbrookbush.com

Intrinsic Stabilization Subsystem (ISS) Integration:

By Brent Brookbush DPT, PT, COMT, MS, PES, CES, CSCS, H/FS

The Intrinsic Stabilization Subsystem (ISS) is comprised of:

 

Function (Brief):

The ISS is comprised of muscles of the lumbar spine and pelvis that do not contribute, or contribute very little to motion. The function of this subsystem is to increase intra-abdominal pressure, tension in the thoracolumbar fascia (TLF), increase sacroiliac joint (SIJ) stiffness, and enhance segmental rigidity and alignment of the lumbar vertebrae. Although the ISS does not actively contribute to motion, the increased intra-abdominal pressure may decompress/traction the spine during motion. Further, an increase in intra-abdominal pressure, segmental rigidity and TLF stiffness may aid in eccentrically decelerating spinal flexion and lateral flexion. The ISS is “the stabilization subsystem”. Analogous to the rotator cuff of the shoulder, its optimal function may be essential for optimal performance of the other subsystems (muscles of the trunk). Inhibition of the ISS most often results in synergistic dominance of the Anterior Oblique Subsystem (AOS).

  • Concentric Function: None (potentially traction/decompression)
  • Isometric Function: Stabilization of the lumbar spine, SIJ and pelvis
  • Eccentric Function: Contributes to eccentric deceleration of lumbar flexion and lateral flexion.

Common Maladaptive Behavior

  • Under-active

Signs of POS Dysfunction

Practical Application

 

Anterior Thoracolumbar Fascia (TLF), Posterior Abdominal Fascia & More:

The fascial component of this myofascial synergy may be a spherical sheath that includes the anterior and middle layers of the TLF fascia, the posterior abdominal fascia, the investing fascia of the diaphragm and the fascia of the pelvic floor muscles.

The fascia of the internal obliques and transverse abdominis comprise the lateral wall of a loop that continues to form the posterior layer of abdominal fascia anteriorly, and the middle layer of the TLF posteriorly (1, 3-9). The middle layer of the TLF would also be continuous with the layer of TLF fascia that envelopes the multifidus (extensor retinaculum). There is not much published on the anterior layer of the TLF, likely because it is the thinnest of the 3 layers and may not contribute much to lumbar stability (3, 9). However, this layer may aid in communication between investing muscles. It envelops the psoas and quadratus lumborum (QL), may run continuous with the internal fascia of the transverse abdominisand potentially invests in the fascia of the pelvic floor and diaphragm. The continuity of this fascial linings seems likely, however more research is needed. Further, the relationships between this investing layer of fascia, the transverse fascia and the perotineum should also be the subject of further study.

As mentioned above, the ISS is a stabilization subsystem that does not function in isolation, but in synergy with subsystems better suited for movement. This notion is supported by research on fascial relationships and muscle recruitment.

Note, the fascial layers of the ISS are not generally addressed directly. The ISS, as a subsystem, is generally targeted with stabilization type exercise, specifically TVA Activation.

 

Muscular Layers of the Core - You can infer from this picture that the external oblique fascia runs continuous with the superficial abdominal fascia, and does not invest in the thoracolumbar fascia posteriorly. http://www.bandhayoga.com/shaktitest/images/Blog/b38_thoraco-lumbar_section.jpg

Consider how bilateral contraction of the TVA would increase tension on the thoracolumbar fascia, pulling laterally on both sides simultaneously increasing rigidity. Further, consider how an increase in pressure anterior to the lumbar vertebrae would resist and anterior translation and shear. – http://www.bandhayoga.com/online-courses/online-courses/shaktitest/images/Blog/b38_thoraco-lumbar_section.jpg

 

Transverse Abdominis

The function of the transverse abdominis (TVA) is relatively well researched, and will serve as a model for understanding similar behavior in other muscles of the ISS. The TVA contracts prior to planned activity, bilaterally regardless of the direction of limb motion, and activity increases with changes in posture including increased center of gravity height, increased respiration and speed of locomotion (20-24). The stabilizing function of the TVA appears to occur by two primary mechanisms. First, increased tension in the muscle results in increased tension of the thoracolumbar fascia (anterior and middle layers of the TLF) resulting in increased stiffness of the lumbar spine and force closure of the sacroiliac joint (25-31). TVA contraction (along with the other muscles of the ISS) also results in a decrease in intra-abdominal volume and an increase in intra-abdominal pressure, which increases rigidity of the spine, may decompress the spine and resist flexion torque (32-34). In studies by Morris et al. and Hodges et al. asymmetrical firing of the TVA was demonstrated with greater resistance and speed, and synergy was demonstrated between the TVAobliques and lumbar multifidus muscles (35-38). This implies that the TVA contracts bilaterally and relatively equally at lower intensities, and as larger forces are generated by the limbs (either due to increased weight or velocity), activity of the TVA on the more stressed side increases to match the additional load, and additional muscles (AOS) are recruited.

In 1996, Hodges et al. published a pivotal study that demonstrated the TVA fired prior to the initiation of arm swing in asymptomatic individuals, but fired after the initiation of arm swing in individuals with low back pain (39). Hodges et al. would follow-up this study with research demonstrating that the same pattern also occurred during leg movement, various speeds of arm movement, and demonstrated the same altered recruitment pattern could be acutely created with injection induced low back pain (40-43). Further, they would show that low back pain results in not only decreased recruitment of intrinsic muscles (TVAobliques and lumbar multifidus), but increased reliance on “global muscles” (44). More recent research has demonstrated that altered recruitment patterns increase the likelihood of low back pain/injury in the following year, that unresolved altered recruitment patterns result in worse outcomes a year post physical therapy, changes in TVA recruitment patterns correlate strongly with a disability index, and that even adductor/groin injury/pain may result in altered  TVA recruitment strategies (45-48).

The practical application of TVA research is the inclusion of exercise that challenges or cues the “abdominal drawing in maneuver” (ADIM) (during Quadrupeds may be best).

Research has investigated the ADIM as assessment, as exercise, and as a cue during functional movement patterns in those with low back pain. Low back pain has been shown to reduce the ability to perform the ADIM (49), and conversely the ADIM has been shown to be a reliable assessment of decreased TVA function (50). Cuing the ADIM has been shown to preferentially recruit the TVA, to the exclusion of more superficial muscles (rectus abdominis and external obliques) (51), and has been shown to be effective for increasing TVA contraction during more functional tasks (52). Specific training of deep muscles results in immediate change to recruitment strategies, neutral spine has been demonstrated to result in greater TVA thickness, and somewhat strangely, adding ankle dorsiflexion to ADIM exercises also increases TVA activity (53-56). Quadrupeds (TVA Activation) with cuing of the ADIM resulted in similar TVA activity in asymptomatic individuals and those with low back pain, which may suggest this exercise is ideal for normalizing recruitment patterns (57). Leg raises resulted in greater TVA activity; however, this exercise also sharply increased erector spinae activity (a muscle prone to over-activity) (58). Although interventions should be based on assessment, research has demonstrated that generally,  stabilization exercise results in better outcomes than stretching, and a “segmental stabilization (TVA Activation)” approach results in better outcomes than global muscle training for low back pain (59, 60). Correlations have been made between TVA thickness, lumbar stability, and balance, which may have implications for injury prevention and sports performance (61).
The ADIM as a cue during functional activities, and quadrupeds with ADIM for core exercise are the interventions recommended by the Brookbush Institute for those exhibiting signs of TVA under-activity.  In fact, these recommendations are recommended for integration of the ISS as a whole, as the remainder of this article will cover how muscles investing in the fascial sheath noted. above are recruited in conjunction with the TVA and benefit from similar exercise.

Internal Obliques

The internal obliques can perform the same actions as the external obliques, but behave like the transverse abdominis. Studies have demonstrated these muscles are active during rotation (62), have the potential to contribute to side bending (63), and may contribute to flexion via transmission of force through the semilunar lines (64). However, internal obliques activity also increases with increases in respiration and gait speed (similar to the transverse abdominis) (23), are recruited bilaterally when lifting asymmetric loads (unlike the external obliques) (66), and activity increases when the spine is compressed, perhaps contributing to the “unloading/decompression” force generated when intra-abdominal pressure increases (34, 67). The internal obliques invest in the thoracolumbar fascia, enabling them to contribute to lateral force on the spinous process (primarily below L3), compression (force closure) of the sacroiliac joint (SIJ), and as mentioned above, increased intra-abdominal pressure via the abdominal tourniquet (25, 26, 29) Note: the external obliques do not invest in the TLF (68).
In two studies by Ng et al., increased external obliques activity and decreased internal obliques activity was noted during static and dynamic axial rotation in those exhibiting symptoms of low back pain (69, 70). Further, decreased internal obliques activity has been noted in those with “sway back” posture, and those exhibiting SIJ pain (71, 72). Although not always mentioned, the internal obliques are commonly studied in conjunction with the transverse abdominis. For example, Hodges et al. demonstrated that the internal obliques and the transverse abdominis fired before lower extremity activity in asymptomatic individuals, but both muscles demonstrated latent firing patterns in those with low back pain (21).  Further, in the study by Chon et al., adding dorsiflexion to exercises using the abdominal drawing-in maneuver (ADIM) increased activity of the internal obliques as well as the TVA  (56). In summary, despite having the potential to contribute to the same actions as the external obliques, the internal obliques behave like the TVA, including responding similarly to the ADIM.

Diaphragm

Hodges et al. (73) describes the relationship between breathing, diaphragm activity, and postural change as “time-locked” and potentially linked via reflex. Two studies by Hodges et al. demonstrated that stimulus of the diaphragm resulted in increased stiffness of the spine (74, 75), and various studies have noted a change in diaphragm activity during lifting, changes in position and posture, movement of the limbs, and changes in gait speed (73, 76-79, 81-82). A study by Hodges et al., may give inference of how the diaphragm accomplishes respiration and stabilization concurrently. In this study repetitive upper body motion (arm swings) resulted in an increase in average diaphragm activity (EMG) (78). Further, a study by Kolar et al. demonstrated lower average activity and a higher relative position in those exhibiting low back pain (80). Based on these findings we may be able to assume that postural stability with respiration is accomplished by higher average activity and a lower relative position, decreasing intra-abdominal space and increasing intra-abdominal pressure. This pattern is disrupted in those exhibiting signs of pain and dysfunction.

Lower activity/higher relative position may be part of the compensatory pattern associated with low back pain. The studies by Kolar et al. and Hodges et al. (78, 80) mentioned above are supported with additional studies on diaphragm activity and impairment (81, 83-86). Hodges et. al, demonstrated less diaphragmatic motion and activity in low back pain patients (86). Further, Vastatek et al. demonstrated that those with sacroiliac joint (SIJ) pain exhibited altered diaphragm activity during isometric lifting of the limbs; activity returned to normal when the SIJ was stabilized via manual compression of the pelvis (83). Several studies have noted increases in respiratory demand, and/or diaphragm fatigue result in a loss of lumbar stability (81, 85, 86). The studies by Jansen et al. demonstrate that low back patients are quicker to diaphragm fatigue, and that diaphragm fatigue resulted in a more rigid stabilization pattern during lifting (81, 85). Last, a study by McGill et al. demonstrated that compression and shear forces on the spine increase when ventilatory demand increases (87), which may imply that research is needed to determine the correlation between diaphragm activity and injury. Considering these studies together, the compensatory pattern adopted by the diaphragm in those exhibiting pain and dysfunction may be lower average activity, a higher relative position, with smaller excursion during respiration, which results in a more rigid (and likely weaker) stabilization strategy resulting in quicker fatigue, which in turn results in larger shear and compressive forces on the spine during motion and postural change.  To date, no prospective studies are available to determine whether altered diaphragm activity and motion may increase the risk of injury; however, studies have demonstrated that altered core control and an increase risk of injury (88, 89).

Hypothesized compensation pattern of the diaphragm:

  1. Lower average activity
  2. Higher relative position
  3. Decrease in excursion with respiration
  4. More rigid stabilization strategy (less change in response to changing stimulus)
  5. Quicker fatigue
  6. Larger shear and compressive forces on the lumbar spine

The study mentioned above by Vastatek et al. (83) implies that stabilization exercise may aid in both low back pain and diaphragm function.  Further, a randomized control trial by Mehling et al. demonstrated that “controlled-breath therapy” was effective for the treatment of low back patients (90).  Although core stabilization exercise are recommended by the Brookbush Institute (BI), “controlled-breathe therapy” admittedly deserves more attention.  Based on the research and compensatory patter above, it may be worth testing breathing from a higher activity/lower position. Not just deep breathing in-and-out, but breathing in deeper and trying to maintain normal tidal volume from a “deeper place”.  This could be added to tasks that challenges stability (e.g. standing chops).

The increase in activity with changes in posture, and the decrease in activity with increased length exhibited in those with signs of dysfunction is similar to the behavior of the TVA and internal obliques.

Pelvic Floor 

Several studies indicate that the pelvic floor muscles (PFM) behave like, and in conjunction with, the diaphragm and TVA. A study by Hodges et al. that mirrors their pivotal 1996 study on  TVA activity (39), demonstrated the PFM is active prior to deltoid activation during arm swing, and activity is not direction specific (91). Sapsford et al. demonstrated that the voluntary activation of core muscles during various common exercises also recruited PFM (92). Madill and colleagues showed the reverse relationship – voluntary contractions of the PFM resulted in an initial increase in lower vaginal pressure, along with an increase in PFM, rectus abdominusinternal obliques, and TVA activity, with later increases in pressure (last 30% maximum pressure) being associated with primarily increases in rectus abdominusinternal obliques, and TVA activity (93).
One study was located that demonstrated individuals with chronic pelvic pain also had a higher incidence of signs that are commonly associated with Sarcroiliac Joint Dysfunction (LSD) (94). A relationship between the pelvic floor and the sacroiliac joint seems very plausible, if for no other reason than the potential of a change in PFM length/tension with sacral motion. A study examining whether PFM activity continues to mimic the diaphragm and TVA in those experiencing low back pain, should also be explored as a continuation of the work by Hodges et al. mentioned above (39) .
Relevant to exercise, a study Critchley demonstrated that cuing PFM contraction during the quadruped exercise increased TVA contraction strength (95). This may suggest that although the PFM fires with the TVA reflexively, the addition of a voluntary PFM contraction may stimulate further recruitment of the TVA. Based on the function and behavior of the PFM the Brookbush Institute considers these muscles part of the Intrinsic Stabilization Subsystem (ISS) , behaving like the TVA and diaphragm. Based on the function of the PFM and the results of the study by Critchley and Culligan (95, 96) it is unlikely that specific exercise is necessary relative to LPHCD, but cuing a PFM contraction while doing ISS exercise may be beneficial.
Obturator Fascia – http://healingartsce.com/images/mm_deep-front-line-Pelvic-floor.jpg

Obturator Fascia – http://healingartsce.com/online-courses/online-courses/images/mm_deep-front-line-Pelvic-floor.jpg

Multifidus

The middle layer of the thoracolumbar fascia (TLF) is continuous with the layer of TLF fascia that envelopes the multifidus (extensor retinaculum). The lumbar multifidus forms five distinct bands that are innervated segmentally by the nerve root corresponding to the vertebrae they attach to (97).  The primary forces they contribute to are extension and compression with have some potential to contribute to posterior shear; however, fiber arrangement suggests a larger role in frontal plane stabilization (98-101). In a resting position the multifidus are in a relatively shortened position, reaching optimal length/tension as the spine flexes (101). Although these muscles are often considered important to proprioception studies have demonstrated that these muscles have very low muscle spindle density (102-103).

The multifidus seem to function primarily as stabilizers. Although these muscles contract ipsilaterally during weighted trunk rotation, they contract bilaterally during unweighted rotation (104).  They are also active with spinal flexors during flexion and activity increases with increases in load (105).  Further, during asymmetric lifting tasks these muscles contract bilaterally (106). During isometric activities (holding a load in one hand) the multifidus contract first and fatigue faster when compared to the iliocostalis (107). Some distinction may need to be made between deep and superficial fibers. Several studies have shown that the multifidus are only active during active postures (upright sitting, walking, swinging both arms forward) (107 – 109); however, a study by Moseley et al. demonstrated the deep fibers behaved more like the transverse abdmonis and were active during static standing and during unilateral and bilateral arm swings in both directions (110). In summary, like other stabilizing muscles of the trunk, the multifidus contract bilaterally regardless of direction of motion, the deep fibers are active during low intensity tasks (static postures), activity increases during active postures and loading, and these muscles may fire before larger movers of the spine.

The case for mis-categorizing the multifidus as under-active relative to Lumbo Pelvic Hip Complex Dysfunction (LPHCD) starts with a group of studies showing atrophy, histochemical and connective tissue changes in those exhibiting chronic low back pain.  Several studies have shown that cross-sectional area of these muscles decreases in those with chronic low back pain. The atrophy is often segment specific and is likely to occur unilaterally in those with sided pain (111-123). Although preferential atrophy of type II fibers has been observed in some studies, it is not clear whether this is correlated with low back pain or a byproduct of surgery and/or age related changes (122, 124-127). Connective tissue changes are worthy of further investigation, as a study by Lehto et al, found fibrosis correlated with impaired ability to recover (128). Last, although not clinically relevant for human movement professionals, core-targetoid (moth eaten) changes to type I fibers may have relevance to diagnosis and prognosis if lab tests can be made easily implantable (129). It is important that we do not confuse atrophy with under-activity, relative to our categorization for use during exercise selection. Although it may be more difficult to hypothesize a process that results in atrophy from over-activity; research and clinical findings on activity relative to dysfunction must be considered in addition to findings of atrophy.

The activity of the multifidus is a bit complex. There are a couple of studies that imply dennervation is a cause of multifidus dysfunction (125, 130); however, this does not appear to be a consistent finding (124, 129). It seems more likely that denervation is specific to surgical intervention and retrolisthesis (125, 130). In those individuals exhibiting chronic low back pain the multifidus are less active especially in lumbar extended positions (131, 132), are weaker and fatigue faster (133, 134), are more active during static standing and stabilization (135-137), peak higher during flexion,  spontaneously discharge around hypermobile segments (132, 135, 137), and remain active for longer after a lift (138). Changes in motor behavior also include a “decoupling” of the bilateral contraction normally seen during lifting tasks (106, 139). Trigger point development may also be common (140-141). To summarize, optimal function of the multifidus is replaced with continuous low force output, unilateral and asymmetrical recruitment in response to stress, spasm or high-activity with any high intensity task or task requiring significant stability, and increased activity for a longer period post recruitment. This change in behavior would seem to replace an appropriately low intensity stabilization function, with bracing and over-activity. Even the loss of force output, quicker time to fatigue and inactivity in extended positions can be explained by adaptive shortening and altered length/tension, resulting in less efficient force output and active insufficiency in extended positions.

Hypothesized compensation pattern of the Multifidus:

  1. Low activity at all times (not quite during static posture)
  2. Weaker and fatique faster
  3. Bilateral activation replaced by directions specfic unilateral activation
  4. Increased activity in response to load and instability
  5. Increased activity for longer after lifting
  6. Trigger point development

Exercise has been shown to be effective for increasing multifidus cross sectional area, although it would seem that stabilization exercises are generally more effective than loaded extension (142-143). Research by Hides et al. has shown that recovery of the multifidus is not automatic post low back pain; however, stabilization exercises are effective for both short-term reduction of symptoms and reduction of low back pain recurrence long-term (144-146). Last, it may be prudent to not “wait for injury”, as a study by Lee et al. demonstrated that imbalance between spine flexors and extensors was a risk factor for low back injury in a 5-year prospective study (147).  Based on the compensation pattern adopted, and the stabilization exercise being more effective than specific multifidus strengthening, the Brookbush Institute recommends treating these muscles as short/over-active (release and lengthening) in those exhibiting dysfunction. However, careful attention during core integration exercise is also recommended; specifically using cuing to enhance bilateral activation during Instrinsic Stabilization Subsystem Integration.

Rotatores, Intertransversarii, and Interspinales

A comprehensive analysis of core muscle function should not exclude any muscles crossing the lumbar spine. Deep to the multifidus are several small sets of muscles fibers known as the rotatores (from spinous process to transverse process spanning 1 or 2 segments), intertransversarii (running vertically between transverse processes), and interspinales (running vertically between spinous process). These fibers are multipennate (especially the rotatores) highly aerobic, with fibers organized in parallel, an arrangement that may be advantageous for “fine-tuning” of vertebral movements (148). However, McGill (12) noted that these deeper muscles are so small and have such small moment arms that is unlikely they contribute to motion.

There is evidence to suggest that the rotatores, intertransversarii, and interspinales serve a sensory function. Two studies demonstrate these muscles have a much higher muscle spindle concentration (4.5 to 7.3 times higher) than the multifidus  or erector spinae (149-150). Another study by Waters and Morris demonstrated that these muscles are active in conjunction with the multifidus; however, the graphs depicting EMG activity in the published paper seem to illustrate the onset of deep muscle activity prior to the multifidus (151). Note: the difference in onset timing of the multifidus and deep rotators was not evaluated for statistical significance. Assuming the onset timing was significant, it is plausible that the deep rotators initially respond to stretch with an increase in activity, followed by a reflexive increase in activity of the larger multifidus. Supporting this hypothesis are two studies that show the supraspinous, interspinous and iliolumbar ligaments are abundantly innervated by Pacinian receptors and Ruffini endings (152, 153). The supraspinaous and interspinous ligaments are continuations of the fascial sheath of the interspinous and intertransversarii muscle fibers.  Although additional studies show that these receptors may only be stimulated at end range as limit detectors, the finding reinforces the role of these fibers as primarily sensory (154, 155).

The rotatores, intertransversarii, and interspinales are often omitted from analysis of core muscles, were not included in the original description of subsystems (which also excluded all the muscles of the ISS), and these muscles are scarcely mentioned in research and texts on the intrinsic stabilizers. The BI asserts these muscles may be viewed as a deeper set of segmental stabilizers/extensors serving a role as proprioceptors or sensory organs.  More research should be designed to determine whether a feedback loop exists that results from an initial lengthening of these deep muscles and reflexive increases in tone of the multifidus, and perhaps a cascade that stimulates the ISS.

 

Image of multifidus and erector spinae

Modified image from Gray’s Anatomy – https://en.wikipedia.org/wiki/Multifidus_muscle

 

Quadratus Lumborum

There is not much research on the quadratus lumborum (QL), likely due its location deep to other paraspinal muscles. Studies have demonstrated a moment arm too small to contribute to large amounts of force, and motion of the lumbar spine results in little change to QL length (156, 157). Lying lateral flexion and the side plank result in significant recruitment of the QL (158, 159); however, the muscle’s size and short moment arm suggests its acting as a “stabilizer”, and not a prime mover (159). An interesting finding by McGill et al. demonstrated an increase in EMG activity with increased load during bilateral carrying (159), and two studies have shown that the QL is still active during the “flexion-relaxation phenomenon” (158, 160). The bilateral carrying results in comprehensive forces, which may imply the increase in QL activity is similar to that of the TVA and internal obliques in response to compression of the lumbar spine. Further, activity during the flexion-relaxation phenomenon is another trait of intrinsic stabilizing muscles (ISS).

To our knowledge no studies exist comparing the EMG activity of the QL in symptomatic low back pain patients to asymptomatic controls; however, two MRI studies have shown a decrease in QL cross-sectional area in those with chronic low back pain (161-162). A study by Hua et al. demonstrated good reliability for locating myofascial trigger points when matched to tenderness upon palpation and replication to patient symptoms (163), and a case study by Graber reported targeted inhibtion/relaxation of the QL to resolved low back pain symptoms (164)). Activity similar to other stabilizers, development of trigger points, and atrophy in those with low back pain is similar to the behavior noted in the multifidus.  It is the recommendation of the BI that the QL and multifidus are addressed as short/over-active muscles, with release techniques, lengthening techniques when necessary, and ISS (stabilization) exercises.

Psoas

The psoas is a hip flexor; however, it is likely not a primary hip flexor until the end of hip flexion range of motion (165, 166). This muscle is most active during activities requiring maximal effort hip flexion, and only active up to 25% of maximum voluntary isometric contraction (MVIC) during activities like push-ups (166). Further, in a study by Anderson et al., the psoas was shown to be active during upright sitting and frontal plane stabilization of the lumbar spine when carrying a contralateral load, but quiet during static standing (167). The duel function of the psoas as a lumbar spine stabilizer and hip flexor is further highlighted by the duel nerve innervation from the femoral nerve and direct branches from the anterior rami of nerve roots L1 – L3 (168 – 170). More consideration should be given to the psoas belonging to a group of lumbar stabilizers and perhaps less to its function as a hip flexor.
The loss of hip extension, increased lordosis and anterior pelvic tilt commonly noted in those exhibiting signs of LPHCD would suggest this muscle is short, and if over-active could contribute to these impairments (171 – 177). Further, this muscle may contribute to spinal compression, to a lesser degree anterior shear (mostly at L5, S1), and flexion of the lower lumbar spine (increased lordosis) (178-179), all of which have been implicated as contributing factors (or irritants) to a herniated nucleus pulposus (HNP) and/or nerve root impingement. This muscles contribution to hip external rotation would also be worthy of exploration given the decrease in hip internal rotation associated with LPHCD (180 – 185); at this time no studies could be located.  Several studies have shown atrophy of the psoas on the side of low back pain, and potentially segmentally specific atrophy corresponding to the level of HNP (186-190). There is some evidence that strength may be further affected by fatty infiltrate in the psoas post injury, but conflicting research exists (191-192). The activity, compensation, atrophy and morphologic change in response to low back pain of the psoas is very similar to findings for the multifidus and QL.
Clinically, the BI notes the presence of trigger points and increased tissue density in the iliacus more often than the psoas. Findings based on palpation may be less reliable, but are presented here to inspire a potential direction for research.  It is the assertion of the BI that the psoasQL and multifidus function and compensate similarly, and should be addressed similarly with release techniques (when necessary), lengthening techniques (when necessary), and ISS (stabilization) exercises.

Summary of Research Findings: 

  • The fascial component of this myofascial synergy may be a spherical sheath that includes the anterior and middle layers of the TLF fascia, the posterior abdominal fascia, the investing fascia of the diaphragm and the fascia of the pelvic floor muscles.
  • The muscles investing in this fascial sheath include the transverse abdominis (TVA), internal obliques, pelvic floor (levator ani and coccygeus), diaphragm, multifidus, rotatores, interspinales & intertransversarii, and potentially the quadratus lumborum and psoas.
  • The ISS is a stabilization subsystem that does not function in isolation, but in synergy with subsystems better suited for movement.
  • The TVA and internal obliques enhance stability by increasing tension in the TLF, compressing the (force closure) SIJ, and decreasing intra-abdominal volume which may increase intra-abdominal pressure.
  • Lumbo Pelvic Hip Complex Dsyfunction (LPHCD) results in a decrease in TVA and internal obliques activity.
  • Although the internal obliques can function like the external obliques, they behave like the TVA.
  • The diaphragm may adopt a higher average activity level and lower position in response to stability demands (likely contributing to an increase in intra-abdominal pressure)
  • Pain and dysfunction results in the diaphragm adopting a lower average activity level, higher relative position, more rigid stabilization strategy and faster time to fatigue.
  • The pelvic floor muscles (PFM) are recruited with the TVA, internal obliques and diaphragm.
  • Contracting the PFM during ISS exercise may increase TVA activity.
  • The multifidus function like other stabilizing muscles of the spine (including the TVA, pelvic floor and diaphragm)
  • The compensatory pattern adopted by the multifidus in those exhibiting dysfunction suggests a muscle that is short/over-active.
  • The rotatores, intertransversarii, and interspinales may act as a sensory organ, initiating a reflexive increase in activity of the muscles of the ISS when lengthened.
  • Recovery from multifidus atrophy is best accomplished via stabilization exercise, not loaded extension.
  • The quadratus lumborum and psoas are recruited and compensate (short/over-active) similar to the multifidus, and show the same atrophy and morphological change in those with chronic low back pain.
  • The ADIM and quadruped (stability exercise) are effective for enhancing recruitment and hypertrophy of ISS muscles.

Two Muscles of the ISS can be separated into 2 categories:

Long/Under-active (Less or latent recruitment, relative increase in length, decrease in force production, quicker to fatique)

Short/Over-active (Higher average activity, prone to spikes, higher activity level for longer after lifting, loss of bilateral activation in response to load, may show marked atrophy in those with chronic low back pain)

Dr. Brent Brookbush instructs patient/client on how to perform a dynamic quadruped

Dynamic Quadruped

More on Function:

Stabilization Subsystem – The transverse abdominis (TVA)internal obliques, diaphragm and pelvic floor muscles increase tension in the fascial sheath of the ISS, and increase intra-abdonimal pressure which aids in creating a posterior to anterior force and a traction effect on the lumbar spine (24-34, 39, 69, 74-75). The multifidusrotatores, intertransversarii, and interspinalesmultifidusquadratus lumborum and psoas act on the lumbar spine and sacroiliac joint, aiding in as segmental stabilization by optimizing alignment and stiffness (98-101, 148, 151, 158-160, 167). Although these two groups of muscles adopt opposite compensatory patterns in response to dysfunction, when functioning optimally they share several similar traits. Muscles of the ISS have a propensity to fire prior to movement of the limbs, bilaterally in a non-direction specific manner, do not produce significant motion, and must work in synergy other subsystems during higher intensity tasks (1-2, 10-14, 17-24, 34, 39, 66, 67, 73, 76-79, 81, 82, 91, 104-106, 148, 151). This subsystem is analogous to the rotator cuff of the shoulder; optimal function of the ISS may be essential to optimal performance of all other trunk muscles.

Eccentric Decelerator of Flexion and compression – Although this subsystem does not contribute significantly to motion, it may aid in decelerating motion.  Research shows that increased stiffness in the thoracolumbar fascia (TLF) results in increased stiffness of the lumbar spine and force closure of the sacroiliac joints (25-31). Several studies have noted the potential of  the TLF to eccentrically decelerate lumbar flexion (3, 8, 9). Further, an increase in intra-abdominal pressure, increases rigidity of the spine, may decompress the spine, and may resist flexion and lateral flexion torques (32-34).

Compensations and models of postural dysfunctionThe ISS (as well as the POS) is commonly under-active. As demonstrated in the research described above, dysfunction results in the muscles listed as long/under-active having a propensity toward a decrease in activity and/or latent recruitment (21, 39- 44, 69 – 72, 78, 80, 81, 83 – 86, 94), and those muscles that are short/over-active having a propensity toward atrophy resulting in a decreased ability to produce force (111-123, 161-162, 168 – 170). This is the “stability subsystem”, and the function of all subsystems may be dependent on its optimal activity and recruitment. In the Brookbush Institute’s predictive models of Upper Body Dysfunction (UBD), Lumbo Pelvic Hip Complex Dysfunction (LPHCD), Sacroiliac Joint Dysfunction (SIJD), and Lower Extremity dysfunction (LED) this sub-system is noted as under-active. Commonly, the under-activity of the ISS results in synergistic dominance of the Anterior Oblique Subsystem (AOS), altered reciprocal inhibition of the Posterior Oblique Subsystem (POS), and synergistic dominance of the Deep Longitudinal Subsystem (DLS).

 

Practical Application

Based on available research on individual muscles, the Brookbush Institute’s (BI’s) predictive models of postural dysfunction, and observations in practice, this subsystem is categorized as “under-active“.  As subsystems are synergies, dynamic multi-segmental postural assessment is recommended for determining compensatory activity, and under-activity should be addressed with multi-joint exercise. The BI uses the research and theory of subsystems as a means of refining core and integrated exercise selection. Note, the BI’s Corrective-Template addresses individual structures prior to addressing subsystems to reduce relative flexibility, synergistic dominance and compensatory patterns.

Assessment: The Research has demonstrated that the muscles of the ISS are recruited similarly (51 – 57, 73, 76 – 79, 81 – 82, 107-109, 151, 158-160, 167), and likely reflexively in synergy.  It may be most appropriate to think of assessment and strength/stability exercise intervention for any muscles of the ISS, as assessment and exercise for all the muscles of the ISS. Most of the research on assessment and intervention is on the TVA (and internal obliques) (49 – 61). Further, studies investigating other muscles have mentioned similar exercises as those used in TVA studies (stabilization exercise) (59-60, 83, 95-96, 142-143); and several have demonstrated better results than muscle specific interventions (i.e. lumbar extension for multifidus strengthening) (95-96, 142-143).

Low back pain has been shown to reduce the ability to perform the ADIM (49), and conversely the ADIM has been shown to be a reliable assessment of decreased TVA function (50). That is, the inability to perform the ADIM or perform the ADIM without co-contraction of the rectus abdominis and external obliques (Anterior Oblique Subsystem (AOS)) may be a sign of ISS dysfunction. During multi-joint/functional movement patterns, abdominal distension may be viewed as a loss of the reflexive recruitment of the ISS and the ADIM. Further, based on the correlations between low back pain, osteokinematic dysfunction and altered recruitment patterns, signs of Lumbo Pelvic Hip Complex Dysfunction (LPHCD) may imply ISS dysfunction. This could include the following signs on the Overhead Squat Assessment (OHSA):

Core Exercise: Again, several studies have demonstrated the effectiveness of stabilization exercise for the muscles of the ISS (59-60, 83, 95-96, 142-143); and some research has demonstrated better results than muscle specific interventions (i.e. lumbar extension for multifidus strengthening) (95-96, 142-143). Cuing the ADIM has been shown to preferentially recruit the TVA, to the exclusion of more superficial muscles (rectus abdominis and external obliques) (51), has been shown to be effective for increasing TVA contraction during more functional tasks (52), and specific training of deep muscles results in immediate change to recruitment strategies (53). Neutral spine, adding ankle dorsiflexion to ADIM exercises, and cuing pelvic floor muscle contraction (PFM) may increase TVA activity (54-56, 95-96). Although, stabilization exercise will enhance diaphragm function, “controlled-breath therapy” during AIDM exercise may also be beneficial (83, 90). Quadrupeds (TVA Activation) with cuing of the ADIM resulted in similar TVA activity in asymptomatic individuals and those with low back pain, which may suggest this exercise is ideal for normalizing recruitment patterns (57). Leg raises resulted in greater TVA activity; however, this exercise also sharply increased erector spinae activity (a muscle prone to over-activity) (58). Correlations have been made between TVA thickness, lumbar stability, and balance, which may have implications for injury prevention and sports performance (61).

Cueing the ADIM during functional activities, and quadrupeds progressions (with the additional cues noted above) with ADIM for “core integration” techniques are the ISS specific interventions recommended by the Brookbush Institute.

Additional Cues:

  • Neutral Spine
  • Dorsiflexion
  • Pelvic Floor Contraction
  • Controlled Breathing

Integrated Exercise: As the ISS does not include muscles that cross the hip or shoulder girdle (the psoas being an exception), there are no Integrated Exercises specific to the ISS. However, the ADIM should be cued during all integrated exercises.

Instrument Assisted Soft Tissue Mobilization (IASTM) of the TLF: Fascia’s response to specific intervention is complex.  Responses may include strain hardening with repetitive or constant tension, enhanced shear and a reduction in cross-bridging between fascial layers, changes in both muscle tone and fibroblast mediated pre-tension (with sustained pressure), and/or altered transmission of force of multiple muscles acting to stabilize a joint (195-197). A study by Levangin et al. demonstrated that a chronic low back pain group showed approximately 20% less shear strain between layers of TLF tissues than asymptotic controls (194). These finding may suggest that techniques with the potential to improve shear between the posterior layer of the POS and the anterior and middle layers of the AOS and ISS may be worth attempting. IASTM is hypothesized to improve shearing, although research does not currently exist to support or refute this claim. Assessment, intervention, and careful re-assessment is recommended (as with all techniques) to determine efficacy on an individual basis. The BI has noted that IASTM of the TLF may not have a large impact on the OHSA, but may be generally effective for improving lumbar flexion, extension and rotation range of motion. Care should be taken in addressing these tissues directly, as there is evidence to suggest that the TLF could be a source of nociceptive input for those suffering from chronic low back pain (196-201).

Subsystems Summary:

Subsystem Common Behavior Core Integration 
Intrinsic Stabilization Subsystem (ISS)
Under-active Transverse Abdominis (TVA) Activation N/A
Anterior Oblique Subsystem (AOS) Over-active

Chop Progression

Avoid: planks, crunches, resisted hip flexion

Legs with Push
Posterior Oblique Subsystem (POS)
Under-active Chop Progression and Bridge Progression Legs with Pull
Deep Longitudinal Subsystem (DLS) Over-active Avoid: leg curls, lumbar extensions Avoid: straight-leg deadlifts, kettle bell windmills, Nordic curls

 

 

Signs of ISS Under-activity

Practical Application

ISS Core Integration (Quadruped Progression with ADIM)

  1. Quadruped
  2. Quadruped Single Arm Raise
  3. Quadruped Opposite Arm & Leg Extension
  4. Quadruped Single Leg Raise
  5. Quadruped with Glute Activation
  6. Gali-peds
  7. Hardest Quadruped Progression Ever
  8. Dynamic Quadruped
  9. Dynamic Quadruped 2
    • Alternate Progression: Quadruped with Ball Resisted Cervical Retraction

Intrinsic Stabilization Subsystem Activation (TVA Activation):

ISS (TVA) Activation and Gluteus Maximus Activation Progressions:

Gali-peds:

Hardest Quadruped Progression Ever:

Dynamic Quadruped (Quadruped Crawl)

Dynamic Quadruped 2 (Additional Quadruped Crawl Variations)

Quadrupeds with Deep Cervical Flexor Activation:

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    • Multifidus
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