Lengths of the external hip rotators in mobilized cadavers indicate the quadriceps coxa as a primary abductor and extensor of the flexed hip

- We modeled the piriformis, obturator internus, and gemellii muscles.
- Lengthening, moment arms, kinematics, and force-lengths all indicate one muscle.
- The Quadriceps Coxa, maximally stretched by flexion and adduction.
- Quadriceps Coxa, a primary extensor and abductor muscle from flexed positions.
- Quadriceps Coxa, of probable major importance in rising and propulsive motions.

BackgroundThe primary function of the external rotators of the hip is inadequately described. The descriptions for peak strength and stretch take no account of how these muscles change their length during normal movement. The latter relationship is known to greatly influence contraction forces and reflect moment arms. The aim of the present study was to indicate positions and directions for peak strength and stretch of piriformis and obturator internus (including the gemelli), collectively defined as the quadriceps coxa, by measuring their changes in length due to normal movements.MethodsRepeated measurements of muscle lengths and range of motions were acquired from dissected muscles on human cadaver hips. We measured at every 15° of flexion with and without adding end ab/adduction, rotations, and combinations thereof. Measurements were taken in three normal hips (1 female aged 59 years, 2 males aged 68 and 70 years) using a custom-engineered frame, electronic calipers, goniometer, and a string muscle model. Movement-lengthening relations were differentiated into movement-moment arm relations.FindingsThe piriformis and obturator internus were maximally lengthened (35 and 30 mm) by 105° flexion and 10° adduction and relaxed by extension and abduction. With significant moment arms for extension and abduction in the movement-lengthening range deduced as force-efficient, our findings indicate peak strength by extension and abduction at 60° to 90° flexed positions.InterpretationThis cadaver study indicates that the quadriceps coxa is a primary abductor and extensor from flexed positions, a strength function which may be of major importance in rising and propulsive motions.