The Posterior Longitudinal Sling (PLS) connects the erector spinae, multifidus, thoracolumbar fascia and the biceps femoris. This sling allows for movement in the sagittal plane and also plays a major role in local or core stability of the lumbar-pelvic region.
The muscles in this sling trigger a contraction to bring about nutation and bring the sacroiliac joint into its close packed (stable) position.
Nutation is a flexion moment of the sacrum in relation to the iliac or posterior rotation of the iliac in relation to the sacrum. This position can put strain and stress on the Sacro-Iliac ligaments.
Perhaps easier to see here when we flex forward. With the legs straight there is a balance met by the hamstrings pulling downward posteriorly almost like a belay rope to meet the pull anteriorly of the body and spinal flexion. The force of these muscles continue to work in response to different loads, for example bending down to tie a shoe lace vs. dead lifting.
This sling is in full time employment due to the main function of erector spinae in standing up and maintaining posture and also due to the stabilizing role of Multifidus, against segmental rotation.
Stay tuned this week on Instagram for exercises that really tap into this sling!