Yudu - sportEX dynamics - 1999-2010

PELVIC INSTABILITY By Joanne Elphinston BPhty MA MCSP

GLUTEUS MAXIMUS Gluteus maximus (Gmax) is a large muscle with wide pelvic attachments. It con- tributes to powerful hip extension for explosive activities like power cleans and sprinting, as well as strong hip control as needed for squatting, skiing (1), uphill cycling (2) or a deep volley in tennis.

From a mechanical point of view, Gmax performs several key roles in controlling the relationship between femur, pelvis and trunk. Through its bony attachments along the ilium’s posterior superior sur- face, dorsal surface of sacrum and greater trochanter, Gmax influences vertical trunk orientation by rotating the pelvis over the femoral head in weight bearing. When acting as an extensor of the hip, Gmax can work concentrically (muscle fibres contracting to shorten) to propel the body forward over the fixed foot as in gait, or eccentrically (muscle fibres contracting but during lengthening) to control hip flexion as in a lunge or squat.

Presentations of anterior and medial knee pain of non-traumatic origin are often associated with deficits in gluteal activa- tion. Due to the inferolateral orientation of its fibres, Gmax has both lateral rota- tion and abduction influences on the femur. This controls the alignment of the knee with respect to the pelvis and the ankle, and therefore influences the

Latissimus dorsi

Thoraco- lumbar fascia

Ilium Sacroiliac

Iliotibial band

Femur Hamstrings

Gluteus maximus

Gluteus medius

Figure1: Anatomy of the pelvis

amount of stress on the patellofemoral joint, medial collateral ligament and medial meniscus. For example, in a step up, Gmax will ideally exert an external rotation and abduction force to keep the knee in optimal alignment with hip and ankle, while extending the hip to press the body upwards. If the gluteal group is weak or underactive, you will observe the knee moving medially, and the pelvis tip- ping laterally. Under these conditions, the structures of

the knee are put under

abnormal strain due to poor alignment and this may lead to painful presentations (3).

Gmax also plays a significant role in sta- bilising the sacroiliac joints (4). Fibres of Gmax attach into the thoracolumbar fas- cia (see fig.3a), a strong non-contractile sheet that is tensioned by the actions of muscles connecting into it to stabilise the trunk and pelvis (5).

Piriformis muscle

Figure 2: Gmax and the hamstrings

Via this thoracolumbar fascia, Gmax forms a partnership with the latissimus dorsi on the opposite side to form the posterior oblique myofascial sling. Like modified rub- ber bands, myofascial slings store and release elastic energy, providing stability when they are under tension, and increas- ing movement efficiency when releasing. By forming a diagonal connection across the pelvis and lumbar spine, the posterior oblique sling increases sacroiliac joint (SIJ) compression to help support the body weight in single leg stance (see fig.3a).

Weakness in Gmax will predispose the sacroiliac joint to injury (6) by decreasing the effectiveness of the posterior oblique sling. Due to the high loads passing through the SIJ during gait, the body will need to compensate in order to protect itself. If Gmax is weak or underactive on one side, the body may try to gain tension across the fascia by increasing the activa- tion of the opposing latissimus dorsi. As with any compensatory strategy,

this

Sacrotuberous ligament

Sacroiliac joint com- pression due to

Gmax and latissimus dorsi

Figure 3a: Gmax connecting to latissimus dorsi via the thoracolumbar fascia creating a myofascial sling

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Figure 3b: Piriformis and the sacrotuberous ligament

Gluteus maximus

Gluteus medius

affects function in other parts of the body, and in this case, body orientation is altered over the base of support, activa- tion of the deep trunk musculature is compromised, and shoulder biomechanics can be affected due to the latissimus dorsi insertion onto the scapula and humerus. During a weight bearing movement such as a lunge, step up or knee raise, this effect can be noted if one shoulder lowers slightly with respect to the other.

In addition to its diagonal musculo-fascial affiliations, Gmax is also the central link in a vertical chain comprising the paraspinals, gluteus maximus and ham- strings. Each of these muscle groups has