Iliopsoas

What it is

Not one muscle but two, joined near the end. Psoas major descends from the lumbar spine; iliacus fills the inner bowl of the pelvis. Their fibres converge into a single tendon that passes under the inguinal ligament and attaches to the lesser trochanter, a small bump on the inner side of the upper femur.

Because the two share that tendon and act together, anatomists name the combined structure iliopsoas and generally treat it as one functional muscle.

The thing that makes it unique: psoas major is the only muscle that links the spine directly to the leg. Every other connection between your upper and lower body runs through the pelvis. This one skips it.

What it does

It is the strongest flexor of the hip — the muscle that lifts your thigh towards your chest, or, when the leg is fixed, pulls your trunk towards your thigh. That second version is what happens in a sit-up.

Because psoas major attaches along the lumbar vertebrae, it also acts on the lower back. What exactly it does there is not fully agreed: it has been described as a stabiliser of the lumbar spine, as a contributor to the lumbar curve, and as both depending on posture and load. The honest position is that its spinal role is real, mechanically complicated, and less settled than the confident diagrams suggest.

What loads it

Lifting the knee against resistance, and holding the leg up. Walking loads it modestly; running, stairs and hill climbing much more. Sit-ups and leg raises load it heavily, which is worth knowing if you thought you were training your abdominals.

What is commonly got wrong about it

"Tight hip flexors" as a universal diagnosis. Sitting is widely blamed for shortening this muscle and causing back pain, in a chain of reasoning that sounds mechanically obvious. The supporting evidence is much weaker than the confidence with which it is stated, and the relationship between hip flexor length, posture and back pain is genuinely unsettled in the research.

Calling it "the muscle of the soul" or a store of emotional trauma. This is a modern claim, popular in some movement and wellness circles, presented as though it were anatomical. It is not an anatomical finding.

That sit-ups train the abdominals. They train the abdominals and the hip flexors, and past the first part of the movement, mostly the hip flexors — the trunk has finished curling and what is happening is the pelvis and torso being pulled towards the thighs.

Pronunciation, harmlessly. The p is silent: so-as.