Knee extension moment arm variations relate to mechanical function in walking and running

The patellofemoral joint plays a crucial mechanical role during walking
and running. It increases the knee extensor mechanism’s moment arm and
reduces required knee extension muscle forces to generate the extension
moment that supports body weight, prevents knee buckling and propels the
centre-of-mass. However, the mechanical implications of moment arm
variation caused by patellofemoral and tibiofemoral motion remain unclear.
We used a data-driven musculoskeletal model with a 12-degree-of freedom
knee to simulate the knee extension moment arm during walking and running.
Using a geometric method to calculate the moment arm, we found smaller
moment arms during running than walking during the swing phase. Overall,
knee flexion causes differences between running and walking moment arms as
increased flexion causes a posterior shift in the tibiofemoral rotation
axis and patella articulation with the distal femur. Moment arms were also
affected by knee motion direction and best predicted by separating by
direction instead of across the entire gait cycle. Furthermore, we found
high inter-subject variation in the moment arm that was largely explained
by out-of-plane motion. Our results are consistent with the concept that
shorter moment arms increase the effective mechanical advantage of the
knee and may contribute to increased running velocity.