The psoas major muscle: A three dimensional anatomical and mechanical study with respect to the spine

The psoas major muscle is a large muscle affecting both the lumbar spine and hip joints. There is, however, limited detail regarding its morphology and its exact function with respect to the lumbar spine. Cadaver dissections (n = 7) were carried out to clarify discrepancies regarding its association with the iliopsoas muscle complex and to collect data on muscle and tendon lengths. Since electromyographic study of the muscle is restricted due to its location within the body, a computer model was developed to determine the biomechanical potential of this muscle. This geometrical model attempts to quantify the potential contribution of psoas major muscle in generating forces and moments during static spinal postures and at all lumbar levels. The force producing potential (muscle cross-sectional areas (CSA) and moment arms) of the psoas major was determined from Magnetic Resonance Imaging (MRI) scans (N = 15) of the muscle at all lumbar levels. The scans were collected in three lordosis conditions of neutral, flexed and extended lumbar conditions. There was no statistical difference in the muscle CSA or location of the muscle centroid (n = 5) in these three conditions. The model predicted that the principle mechanical potential of the psoas major is ipsilateral side flexion for neutral and end-range flexion and extension conditions and at all lumbar levels except the T12/L1 level. Similarly, the model predicted that the psoas major can exert large compressive forces, particularly in the symmetrical activation condition where the forces are additive.