You are correct it has diverse functions. The QL stabilizes the lower ribs during forced inspiration. The QL is one of the muscles that are referred to as muscles of forced inspiration or accessory muscles of inspiration.
FYI the muscles of forced inspiration are typically used in healthy persons to increase the rate and volume of inspired air.
Any one of the muscles of forced inspiration may be used to compensate for dysfunction in one or more of the primary muscles of inspiration.
A scenario where this is common is within populations that have conditions such as COPD, CRPD, asthma, cystic fibrosis, and individuals who have undergone lung and/or heart transplant. The compensation happens as a consequence of hyperinflation of the lungs. Because their lungs are often filled with excessive air at the end of expiration it becomes difficult to inhale. Hence, it is forced.
The quadratic lumborum or QL, is one of the tightest an weakest muscles at the lumbopelvic junction
Per the research.
The origin and insertion and function has already been defined.
However, there are a few key other important points to learn about the muscle.
It is one of four “stabilizers” at the lumbopelvic junction.
Accompanied by the transverse abdominis(TVA), external obliques, and
Multifidi, all four muscles play a static and dynamic role in stabilizing.
Per the research by Paul hodges, PT,
He discovered through his RCT’S (randomized controlled trials),
That people with LBP, they were unable to properly contract their TvA
And their firing pattern for the multifidi, was improper.
Meaning when you ask a client who is prone to lift their alternate legs,
They will typically fire their gluteus, the hamstrings then multifidi.
The proper recruitment is multidisciplinary first, the flute max, then hamstrings.
Palate just laterally to the spinous process and ask the client providing your tactile cues to contract multifidi then lift their leg and you should see glute max fire next then hamstrings
I hope this helps and it should provide more in depth understanding
Then what has been previously outlined
The lateral muscles of the lumbar spine include the quadratus lumborum and psoas. The quadratus lumborum originates from the iliac crest and inserts at the twelfth rib and transverse process of the lower four lumbar vertebrae. The quadratus lumborum produces lateral bending of the lumbar spine with unilateral contraction and stabilizes the trunk with bilateral contraction. The psoas major muscle originates from the anterior surfaces of the transverse pricesses of all the lumbar vertebrae and inserts at the lesser trochanter of the femur. The psoas major flexes the trunk and the hip.
The spine and trunk muscles exist in pairs, one on each side of the body.In general, bilateral contraction results in movement in saggittal plane. The anterior muscles flex the spine, while the posterior muscles extend the spine. Unilateral contractions results in lateral bend or axial rotation.
Thank You for interesting questions,Miroslava.