The lower back, or lumbar region, supports the upper body’s weight and enables a wide range of movements. It provides stability and mobility for bending, twisting, and maintaining upright posture. Understanding these muscles helps appreciate their daily functions and overall body mechanics.
The Major Muscles of the Lower Back
The erector spinae group comprises the most prominent lower back muscles, running vertically along both sides of the spine. This group extends the spine, allowing straightening from a bent position, and contributes to lateral flexion (bending the trunk to the side).
The erector spinae group consists of three main muscles: the iliocostalis, longissimus, and spinalis. The iliocostalis is the most lateral, positioned closest to the ribs. Its fibers extend from the sacrum and iliac crest up to the ribs and cervical vertebrae.
The longissimus muscle lies in the middle of the erector spinae group, being the longest and largest. It spans from the sacrum and lumbar vertebrae up to the skull, extending the entire vertebral column. The spinalis is the most medial, closest to the spine’s bony processes. While smaller, it contributes to extending the upper thoracic and cervical spine. When these three muscles contract together, they produce strong spinal extension, enabling movements like arching the back.
Essential Deep Stabilizers
Beneath the larger superficial muscles, deeper muscles provide stability to individual spinal segments. Their deep location allows for precise control over small movements and spinal alignment.
The multifidus muscles are a key part of this deep stabilizing system, filling the grooves along the sides of the vertebral spinous processes from the sacrum to the neck. These small, segmented muscles span multiple vertebral levels, contributing to extension, lateral flexion, and rotation of the spine. They are continuously active in upright postures, providing ongoing support to the spinal joints.
The quadratus lumborum, often called the QL, is located in the posterior abdominal wall. This irregularly shaped muscle extends from the iliac crest and iliolumbar ligament to the twelfth rib and the transverse processes of the upper four lumbar vertebrae. The quadratus lumborum contributes to laterally flexing the trunk, pulling the rib cage down, and assisting in lumbar spine extension. It also helps stabilize the twelfth rib during breathing.
Important Connecting Muscles
Some muscles, not solely in the lower back, significantly influence its function due to their anatomical connections. These muscles connect the spine and lower limbs, impacting posture and movement. Their actions directly affect the lumbar spine’s position and curvature.
The psoas major is a long, thick muscle originating from the transverse processes and vertebral bodies of the lumbar spine, extending through the pelvis. It inserts onto the lesser trochanter of the femur, a bony prominence on the thigh bone. This muscle is a primary hip flexor, lifting the leg towards the torso or bringing the torso towards the leg.
The iliacus muscle, located within the pelvis, works with the psoas major. It originates from the inner surface of the hip bone and joins the psoas major to form the iliopsoas complex. This complex is the strongest hip flexor, and its connection to the lumbar spine means tightness or imbalance can directly affect lower back posture and contribute to discomfort.