(Late) Muscle of the week:
Psoas major and minor
These two muscles are part of the iliopsoas muscle group. The psoas major acts as what is more commonly known as the "hip flexor" muscle and brings the knee towards the trunk when the body is fixed (e.g. high knee drill), or brings the body towards the leg when the leg is fixed (e.g. doing a sit up). The psoas minor acts as a weak flexor of the lumbar spine.
In the case that the psoas major is extremely tight, it can anteriorly tilt the pelvis forward (and possibly into heightened lordosis, or increased inward curvature of the lumbar spine) and pull on the lumbar spine, causing lower back pain. Sitting for long periods of time may contribute to a tight psoas major, so make sure you stretch those hip flexors if you have a desk job or just like to binge watch Netflix shows for 6 hours straight!