Very good question. I would interpret it based upon its function. However that will also lead to potential issues such as defining its function. It has attachments upon the lumbar spine as well the ribs so in a sense it is diverse in function. Traditionally, muscles are categorized based upon anatomical location and architectural features to determine line of pull. However, this has evolved into muscular activation to determine their function.
Fuel the Movement,
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.
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