Nachemson showed that the psoas major was active during upright standing, forward bending, and lifting. These observations prompted the inference that the
psoas major may function as a lumbar spine stabilizer.
Others have since proposed and found evidence for various roles that the psoas major may play with respect to
lumbar spine stability and movement. These roles include
psoas major being a flexor of the lumbar spine on the
pelvis, a lateral flexor of the lumbar spine, a stabilizer
of the lumbar spine,stabilizer of the hip,
power source for bipedal walking and running,and
controller of the lumbar lordosis when supporting difficult lumbar loads.