Fig. 10. Diagrams of force and limb position at the midpoint of stance for a level (A) and a 12° incline (B) run. Filled circles indicate the locations of the centers of rotation of the ankle, knee and hip joints. The resultant ground reaction force GRF is closely aligned with the hip during level running (A), resulting in a small out-moment arm (R) and low joint muscle moment (M_m). During incline running (B), the GRF is oriented more forward of the hip, increasing R at the hip and decreasing it at the knee. The increase in R at the hip allows for higher force and work outputs at the hip during incline running. The decrease in R at the knee decreases the external moment and limits the external work that can be done at this joint.