Introduction

Considerable debate exists whether psoas major is important for spinal stability. It is in an ideal position to provide spinal stability, however its role remains unclear (McGill, 2002; Richardson et al 2004). Conversely, gluteus maximus is considered an important stability muscle of the lumbo-pelvic region. This is supported by motor control studies (Bullock – Saxton et al, 1994; Hungerford et al, 2003) and biomechanical analysis (Pool-Goudzwaard et al, 1998).

This last decade has brought a new insight into muscle function and the role of muscles in stability is now emerging. Some muscles appear to have a primary role, while others appear to have multiple roles. As well, there are different functional requirements for muscles (i.e. low load and high load). The purpose of this paper is to present evidence of a stability role for psoas major and introduce a new role for gluteus maximus in SIJ stability.

Lumbar Cylinder

A common model of lumbar stability shows the musculature forming a cylinder. The top of the cylinder is the diaphragm, the bottom is the pelvic floor and the wall is formed by the segmentally attaching abdominal and posterior spinal musculature, specifically transversus abdominus and the segmental fibers of lumbar multifidus (Morris et al, 1961; Bartlink, 1957; Richardson et al, 1999). There is growing evidence to show how these muscles coordinate to stabilize the spine. For example, transversus abdominis has been shown to co-contract with: the diaphragm (Hodges et al, 1997); the pelvic floor (Sapsford et al 2001); and the deep fibres of lumbar multifidus (Moseley et al, 2002). As well, in vivo porcine studies lend support that this mechanism contributes to the stability of the spine. A contraction of the diaphragm, transversus abdominis and an increase intra-abdominal pressure has been found to increase intervertebral stiffness (Hodges et al, 2003). Further, a transversus abdominus contraction was linked to an increase in the stiffness of the sacro-iliac joint (Richardson et al, 2002).

Muscle Function

Muscle function is much more complex than considering ‘origin to insertion’, or assessing activity profiles with electromyography (EMG). Information regarding muscle function may be obtained from four key sources. These are listed in table 1.

Table 1: Aspects of understanding muscle function for functional classification

Muscle Classification

A new classification system of muscle function has been presented (Comerford and Mottram, 2001). This divides muscles into local stabilizers, global stabilizers and global mobilizers. Of particular interest to this paper are the local stability muscles. Their characteristics are presented in table 2.

Table 2: Characteristics of local stability muscles (Review Comerford & Mottram, 2001)

Psoas Major

Psoas major has fibrous attachments to the anterior aspect of all lumbar transverse processes (posterior attachment) and to the antero-medial aspect of all the lumbar discs and adjoining bodies (anterior attachment) with the exception of the L5-S1 disc (Bogduk et al, 1992; Gibbons, 2004). The fiber length ranges from 3 to 8cm. The fascicles run infero-laterally to reach a central tendon where they descend over the pelvic brim and share a common insertion with iliacus to the lesser trochanter (Gibbons, 2004).

Psoas major also has significant fascial relations. The medial arcuate ligament is a continuation of the superior psoas major fascia that continues superiorly to the diaphragm. As psoas major descends, its infero-medial fascia becomes thick at its lower portion and is continuous with the pelvic floor fascia. This also forms a link with transversus abdominus and the internal oblique. As psoas major passes over the pelvic brim, it has a strong attachment to the pelvic brim. This may be an innominate ligament (Gibbons, 2005).

Biomechanical analysis of psoas major suggests it has minimal capability to produce movement in the lumbar spine. The dominant force is axial compression and this is always greater than shear (Bogduk et al 1992, Gibbons 2004, Rab et al 1977, Santaguida and McGill 1995). Compression from psoas major may create segmental stiffness (Janevic et al 1991) and can resist shear forces (McGill 2002). Due to their relative size, the posterior fasciculii have much smaller forces than the anterior fasciculii (Bogduk et al, 1992; Gibbons, 2004). Yoshio et al (2002) concluded that the primary role of psoas major was for lumbar stability and that psoas major contributed very little to hip flexion. The primary role for psoas major at the hip was for stability. This was achieved through maintaining the femoral head in the acetabulum.

Dangaria and Naesh (1998) assessed the CSA of psoas major in unilateral sciatica caused by disc herniation. There was significant reduction in the CSA of psoas major at the level and the site of disc herniation on the ipsilateral side. More recently, Barker et al (2004) found segmental atrophy in psoas major and lumbar multifidus in subjects with unilateral low back pain (LBP). These are similar patterns of atrophy previously observed in the deep segmental fibres of lumbar multifidus (Hides et al, 1994).

A specific exercise for psoas major was developed (Gibbons et al, 2001). Superficial EMG was recorded during the exercise from the multi-joint hip muscles that could contribute to the movement. Subjects either had a history of LBP and were pain free at the time or did not have a history of LBP. Psoas major was observed via ultrasound imaging at the pelvis brim and the neutral spine position was monitored with a pressure biofeedback. The subjects with a history of LBP exhibited higher amounts of EMG activity during the exercise and tended to loose the neutral spine position (Gibbons et al, 2005).

Gluteus Maximus

Gluteus maximus is composed of three distinct components: superficial sacral fibers, deep sacral fibers and deep ilium fibers. Superficial fibres from attach to the sacrum and run to the iliotibial band in 7-10 fascicular arrangements. Some of these fibres attach to the gluteal tuberosity. The deep ilium fibers run predominately to the gluteal tuberosity. Superiorly, the deep sacral fibers cross the sacro-iliac joint and attach to the posterior pelvic brim just lateral to the posterior superior iliac spine. Inferiorly, they are short and are orientated infero – laterally. These deep sacral fibers cross from the lateral sacrum to the posterior ischial spine, the ischial tuberosity and to the sacrotuberous ligament. These deep sacral fibers are continuous with the fascia of the deep hip intrinsics and the ischiococcygeus muscle. The deep sacral fibres of gluteus maximus (DSG) only cross the SIJ and it seems unlikely these fibres contribute to physiological range of motion (Gibbons and Mottram, 2004). Preliminary findings from a pilot study implicate a separate role for the DSG during vertical loading and during movements that may create sacral torsion. In another pilot study, subjects with a history of low back pain were compared to subjects with no history of back pain during an exercise designed to facilitate the DSG. The subjects with a history of low back pain had increased activity in rectus femoris, hamstrings and tensor fascia latae.

The mechanism for the DSG into the lumbar cylinder is unknown. One hypothesis may be that they co-contract with muscles of the lumbar cylinder to provide stability when the lumbar spine and pelvis are concurrently challenged. Their functional role may be in providing a stabilizing effect on the SIJ and a specific exercise for the DSG may have clinical significance. The anatomical relations to the posterior pelvic floor and to the lumbar multifidus provide interesting possibilities for co-contraction mechanisms in stability.

Conclusions

Based on the classification of local stabilizers in Table 2, there is evidence of a local stability role for psoas major. It does not produce significant range of movement, it shows a change in cross sectional area and also exhibits altered low threshold recruitment. The anticipatory timing of psoas major needs to be investigated in the posterior and anterior fasciculii. It has been hypothesized that the posterior fasciculii play a separate role in stability by controlling translation in the lumbar spine. Further research is needed to understand the role of psoas major. The gluteus maximus muscle appears to have three subdivisions, but it is unknown if they are individual muscles since a nerve supply was not located. The anatomy of the DSG suggest it has minimal movement capability. Preliminary data from pilot studies suggest an altered pattern of low threshold recruitment and an activation pattern to control translation. The DSG may be considered as a local stabilizer, however this is should be interpreted with caution. The DSG may provide a new avenue for mechanisms of stability at the SIJ and warrants further investigation.

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