Muscles.

Latissimus Dorsi.

This superficial back muscle affects the pelvis due to it's attachement to the thoracolumbar fascia, which also attaches to the lumbar spine and sacrum. Below it is the large fan shape muscle on the back.

Multifidus

This series of small but vital muscles help stabilise the spine and pelvis during movement, they also pump up the thoracolumbar fascia and can frequently become weak or completely inactive.

Erector Spinae

These three long muscles, the iliocostalis, longissimus and spinalis run down the sides of the spine to the pelvis. They also attach to the thoracolumbar fascia.

Quadratus Lumborum

Attaches to the ribs and the pelvis. A small, deep muscle that controls lateral movement.

Iliopsoas

Attaches the lumbar spine to the pelvis and also runs down to the femur to become a strong hip flexor. A very troublesome muscle for many people.

Abdominals

Transversus Abdominis

Lower abdominal muscle that when working correctly cues the multifidus to work. Often overlooked and therefore weak.

External and Internal Obliques

These muscles run down the side of the trunk and attach to the ilia. Their main job is to control rotation but if strong enough can help stabilise the pelvis. An imbalance can be found from side to side or between the internal and external obliques.

Rectus abdominus

This is the slab of muscle that runs down the body and if toned gives you the "six pack". As this muscle is used for curl ups it is often the most dominant abdominal. Attaches to the pubis symphis and the ribs.

Tensor Fascia Latae and Iliotibial Band (TFL and IT band)

A strong muscle that attaches the pelvis to the femur. An abductor of the hip this muscle frequently tests stronger than the other abductors and can cause Iliotibial Band pain and tightness. 80% of the Gluteus Maximus attaches to the TFL and IT band so it has a strong influence on the glutes.

Piriformis.

The only hip rotator that attaches to both the sacrum and the ilia, a very troublesome muscle that can tighten if the pelvis has a lack of support from the correct muscles. The piriformis is often a cause of sciatica from it's relationship with the sciatic nerve. On the diagram below the piriformis is the top muscle and the other muscles below it are the hip rotators.

Gluteus Maximus

The largest muscle in the body but very often weak in our modern world. The gluteus maximus attaches to the sacrum and coccyx and 80%of it attaches to the IT band. Very important for stabilising the pelvis.

Gluteus Medius

Another commonly weak muscle this is situated mostly beneath the glute max. It attaches to the iliac crest and the hip and is supposed to stabilise the pelvis while you perform one legged actions such as walking.

Hamstrings

Tight hamstrings are the most common finding in low back pain patients. The medial hamstrings are the semitendinous and the semimembranous and work to rotate the femur in wards (medially rotate). The lateral hamstring is the biceps femoris which extends and rotates the hip laterally.