In piriformis syndrom, in Magee it says it is an overuse syndrome due to too much ER of the hip and excessive pronation of the foot which leads to abnormal femoral IR. I don’t understand how excessive motion of ER of hip and pronation of the foot leads to abnormal femoral IR. Please explain. Thanks.

Recall that the piriformis muscle is a lateral/extenal rotator (ER) of the hip. Excessive ER of the hip (as is observed with people who cross their legs while sitting most of the time), is implicated in the piriformis overuse syndrome. Incidentally, one of the long-term compensations for a chronic ER hip is excessive foot pronation, which when appraised upwards along the kinetic chain, also causes internal rotation (IR) of the tibia as well as IR of the femur.

The kinetic chain of the lower extremity usually follows this pattern:

Foot pronation > (leads to) IR Tibia > IR Femur > ER Coxa (hip).

Foot supination > ER Tibia > ER Femur > IR Coxa (hip).

You will notice that in both patterns, the long term adaptions at the hip is reverse that of the Tibia and Femur.