Action and Contribution of the Iliopsoas and Rectus Femoris as Hip Flexor Agonists Examined with Anatomical Analysis.

Objectives: To evaluate the difference in action between the iliopsoas and rectus femoris muscles in hip flexion by estimating the relative contribution to the maximal hip flexion torque and relative rotation speed.

Materials: We examined 22 lower limbs of 10 male and 12 female formaldehyde-fixed adult Japanese cadavers.

Methods: Using morphometric data from cadaver dissections, we calculated the moment arm length and physiological cross-sectional area for each muscle. We considered moment arm length and physiological cross-sectional area as indices of the maximal torque and compared them among the muscles at various hip joint angles. To evaluate the relative rotation speed, we calculated the increase of the hip joint angle for a 1% reduction of the muscle fiber length in each muscle.

Results: The rectus femoris contributed approximately 2/3 to the flexion torque in mild flexion up to 60°, whereas the iliopsoas contribution increased sharply beyond 60°. The relative iliopsoas rotation speed was 2.5- to 3-times higher than that of the rectus femoris in mild flexion up to 60° under the specific condition that each muscle had the same muscle contraction speed.

Conclusions: We found that the iliopsoas served as a rapid flexor, while the rectus femoris was a powerful flexor.