Anatomy Slings and Their Relationship to Low Back Pain

Introduction

The following wiki page will aim to investigate the body’s anatomical slings and the ways in which a dysfunction could potentially result in lower back pain. We will be discussing the stability of the lumbo-pelvic complex and the systems (both local and global) which govern it. We will also attempt to present the literature available in this underreported, but essential, area of dynamic stability whilst providing a detailed breakdown which will include exercises to target the individual myofascial slings. The authors hope that the information on this page helps the reader better understand stability and that it may influence a more comprehensive approach to instability and lower back pain.

''Stability in the Lumbo-Pelvic Region

As human beings our bodies are subject to constantly changing demands placed upon us by the external environment. Therefore, the ability to adapt our bodies in order to cope with these varying stresses is vital in order to protect internal structures. The lumbo-pelvic complex plays a key role in distributing load and maintaining stability during movement and changes in external demands. The primary function of this unit is to allow the transfer of forces safely in order to allow complex movement, without injury, and whilst facilitating efficient respiratory function (Lee 2011) It helps aid the prevention of injury to vital structures such as the spinal cord, as well as the bony and soft tissue structures of the area.

When investigated in an in-vitro environment, it has been estimated that the human spine can withhold loads of approximately 90N before buckling. However, research suggests that in functioning human beings this load can reach up to 1500N. This indicates a heavy dependence upon other structures in order to provide the stability required to cope with the forces that the spine is subjected to in reality. The relationship between the sacrum, pelvis and lumbar spine, alongside their surrounding structures, is fundamental to stability. The contribution of the design and structure of pelvic anatomy to stability is known as sacroiliac joint form closure. As alluded to before, the bony structure of the spine and pelvis alone is insufficient to deal with the forces the body is exposed to. Therefore, other structures such as ligaments, muscles, and fascia are required to distribute forces across the region. This is known as force closure of the sacroiliac joint (Hoffman and Gabel 2013).

A model for explaining stability of the spine was devised by Panjabi in 1992, and comprises of three components. The spinal column and its structural anatomy as described using the sacroiliac form closure theory is the first component, and is seen as a passive stabiliser. Secondly, the neural control unit is perceived as fundamental in order to mediate responses to movement and adapt spinal stability as required. The final component, as proposed by Panjabi is the muscular system, is an active stabiliser which consists of global and local muscle units (Panjabi 1992).

In spite of this theory, there is still discrepancy within the literature as to the contributors to spinal stability. A core stabilisation approach is commonly used by clinicians in order to treat pain in the spinal region. As purported by Norris (2008), this perspective focuses more on intrinsic mono or bi-articular muscles as primary stabilisers of the spine, and pays little attention to global systems.
A significantly broader approach is becoming increasingly common within the literature. This perspective considers the force closure stability mechanism as a complex system of selective co-contraction, between the deep and superficial muscles of the lumbo-pelvic region. It is believed that the term ‘control’ holds greater aptitude than ‘stability’, as it pertains to the constantly changing contribution of intrinsic and extrinsic muscles, and the ongoing mediation by the central nervous system (CNS) (Reeves et al. 2007). It is believed that in order to achieve functional control, the CNS has the ability to dampen some systems whilst exciting others; this may in fact result in reduced ‘stability’ in sacrifice for greater mobility (Lee 2011, Reeves et al. 2007).

The CNS has multiple strategies at its disposal, and utilises these depending upon the quantity of stabilisation required, the predictability of movement, and the risk to the bodily structures (Lee 2011). Work carried out by Richardson et al. (2002) upon subjects who carried out sacroiliac stability exercises in a static position, found a lower sacroiliac joint (SIJ) laxity when intrinsic muscles were contracted, compared to more superficial muscles. This demonstrates that different roles are played by the varying muscular groups in order to achieve optimal control. Another example of this is the recruitment of the intrinsic muscles when anticipating movement. The slow-twitch, type one fibres in the multifidus, for example, will be stimulated by the CNS to contract when a change in posture is predicted (Currie et al. 2012).
Within this harmony of stability control a different approach can be taken when viewing the more extrinsic and global musculature of the lumbo-pelvic region, and this will be explored in the further in the subsequent sections.

''Myofascial Slings

''Anterior Oblique Sling

''Posterior Oblique Sling

Evolution has seen human beings develop from quadrupedal into bipedal creatures. This adaptation has allowed us to carry out tasks at a more advanced level than we would have done previously, however, has also brought about changing demands upon the body. This has meant that the body has had to adapt in order to cope with different stresses. With the transformation of humans into upright-functioning beings, the demand upon the posterior structures of the body has changed dramatically, and these have had to adapt accordingly. For example, the gluteus maximus has evolved from a relatively small muscle (as observed in chimpanzees) to being the largest muscle in the body (Vleeming 2012). It has become part of a system which is specialised and integral in supporting functional control in movements such as human gait – the posterior oblique muscle sling (POS).

This sling system consists of the latissimus dorsi (LD), the gluteus maximus (GM), and the inter-connecting thoracolumbar fascia (TLF) (Lee 2011). The POS, otherwise known as the back functional line, crosses approximately at the level of the sacro-lumbar junction. The lower portion of the sling, consisting of the distal GM fibres, passes underneath the iliotibial tract to attach to the posterolateral edge of the femur, thus this system becomes linked with the lateral sling (Myers 2013). Within recent decades’ clinicians have begun to identify that stability is a complex phenomenon, and “a system of assisting movement whilst stabilising” exists (Vleeming 1995). The POS is fundamental to this method of functioning.

The role of the POS is most distinguished during the single support (stance) phase of gait. Prior to heel strike, the ipsilateral hamstring muscle contracts in order to prepare the limb for weight-bearing. During this, the proximal hamstring also carries the role of stabilising the ipsilateral pelvis against the activity of the quadriceps, in order to prevent excessive anterior rotation of the ilium. However, once heel strike occurs, hamstring activity diminishes and its role of limiting ilium movement is largely undertaken by the GM. At this point the muscle is in a lengthened position. Simultaneously, counter-rotation of the trunk also begins to takes place. During this process, the arm contra-lateral to the stance leg is ante-flexed, undergoing an eccentric contraction of the LD in order to control the forward momentum of the limb, whilst taking the LD also into a lengthened position (Vleeming 2007; Chek 2011). The propulsive phase of gait then follows, with both the GM and contralateral LD concentrically contracting from a lengthened to shortened position, resulting in extension of the arm with the opposite propelling leg. When these two simultaneously occurring mechanisms are coupled, a contraction of GM alongside its contralateral LD is observed (Chek 2011). As discussed previously, this causes an increase in tension within the TLF, eliciting stabilisation of the SIJ and lumbar spine (Lee). This theory also adds credence to the remark that weakening of the GM aspect of the POS often results in hamstring dysfunction, due to the compensatory activity in order to stabilise the ilium (Sahrmann 2012).

As well as the SIJ compressional stability that this system produces through the TLF, some authors also believe that the mechanism acts a ‘smart spring’, using phasic contractions to release and store energy during gait. Vleeming (2007) theorises that kinetic energy is built up in the GM and LD as they lengthen prior to and during heel strike, respectively. This energy is then released as these muscles shorten immediately following the lengthening phase, causing this kinetic energy to be released. A similar response is observed when a finger rapidly returns to a neutral position, when it is released following passive full extension. There is debate surrounding whether this kinetic energy is stored within the muscles or the TLF (Dorman 1992). Regardless of this, it is widely believed that this mechanism reduces energy expenditure of surrounding muscles of locomotion, thus reducing the metabolic cost of gait (Vleeming 2007; Chek 2011).

Traditional exercise training to stabilise the SIJ focuses on ‘core’ units, often with the aim of isolating muscles in order to strengthen them. As discussed in the section “Stability in the Lumbo-Sacral Region” the demands upon human beings suggest that strengthening techniques should be incorporated into dynamic movements. Thus, in order to train the POS, the GM and LD should not be viewed in isolation, rather utilised in synergy with each other to promote efficient gait as described previously. A good example of an exercise that can be used for treating POS dysfunction is the reverse lunge. A therapist should utilise this once a patient can achieve pain-free hip motion and satisfactory static stability (Nickelston 2013). A demonstration of this exercise is shown in the video below:

As alluded to previously, it is vital that the muscle slings of the body function harmoniously in order to facilitate efficient movement and prevent injury. This is especially pertinent between the anterior oblique sling (AOS) and the POS. These systems can be viewed similarly to a muscle pair, with an antagonist and an agonist: whilst one is contracting, the other may work to control the movement being produced. An example of this in the AOS and POS is during the swing of a tennis racket. The movement and power is produced by the AOS which causes a rotation and forward movement of the pelvis, trunk and arm. However, the POS is also crucial during this action in order decelerate the movement when appropriate, using eccentric control. This helps to aid an individual in maintaining their balance during a highly dynamic movement such as this, whilst stabilising the lumbo-pelvic hip complex.

Deep Longitudinal Sling

''Lateral Sling

''How Do These Link to Low Back Pain?

''Conclusion