Dr. Ida P. Rolf described the role of the psoas (brightly colored red muscle in the right image) in walking:
Let us be clear about this: the legs do not originate movement in the walk of a balanced body; the legs support and follow. Movement is initiated in the trunk and transmitted to the legs through the medium of the psoas.
(Rolf, 1977: Rolfing, the Integration of Human Structures, pg. 118).
The abdominal muscles and psoas have a great role in back pain experienced by most endurance athletes. As cyclists everything we do is in a sagital plane
We severely neglect the muscles responsible for abduction/adduction (motion that pulls a part or structure away/towards the midline of the body). The movements we produce on the bicycle are very eccentric when compared to how our musculature has evolved over thousands of years encompassing various functional movements. In other words we weren't designed to stabilize our torso with abducted shoulders (scaption) while hunched over with inordinately engaged pectoralis minor muscles, neglected transverse abdominals, and never giving our psoas a chance to fully stretch and contract, causing shortening. Now all this on it's own would be easier to mitigate if the majority of us didn't have a desk job, we spend all day combating an unnatural position at the desk and what do some us want to do to (unfortunately figuratively) decompress after a long day in the office? Get in the same position except rotated anteriorly a few more degrees and ride our bikes.
A dysfunctional psoas can have all kinds of effects on how you sit and operate on your bike. Your bike fitter might find a leg length imbalance or muscularly dominant leg, but the psoas is very strong and it can rotate your spine to the right or left and/or pull it forward and twist the pelvis into various distortions. So the leg length discrepancy might not be a real difference in femur or tibula/fibula length but a soft tissue "tweak". Is your RIGHT leg dominant? Don't be surprised to find yourself with a LEFT QL (Quadratus Lumborum) muscle spasm, as the overpowered QL spams in trying to overcome the pulling of the dysfunctional psoas.
The psoas muscles and the abdominal muscles are opposing pairs (agonist and antagonist) as well as synergists (mutual helpers). Closely coordinated interaction between the two is healthy; poor coordination between the two creates problems.
The psoas muscles lie behind the abdominal contents, from the lumbar spine to the inner thighs near the hip joints (lesser trochanters); the abdominal muscles lie in front of the abdominal contents, from the lower borders of the ribs (with the rectus muscles as high as the nipples) to the pubic bone.
Take a look at how you sit at your desk today, do you prefer to cross one leg compared to the other? Keep track of which one it is that you cross more often and know that it indicates a structural problem and perhaps to a much lesser extent a problem with ergonomics.
Stay tuned for more...