Figure 4. The internal moment arm curves of the quadriceps muscle group.

The internal moment arm curve for the single-joint quadriceps muscles slightly increases and peaks from 0-30 degrees of knee flexion, then gradually decreases from 30-100 degrees.⁶⁹ Similarly, the internal moment arm curve of rectus femoris peaks between 20-30 degrees of knee flexion but has a slightly steeper moment arm curve.⁷⁰ Deg, degrees; mm, millimeters; red shading, range of knee flexion associated with peak internal knee extension moment.