Figure 3. The default musculoskeletal model used in our experiments.
a) The leg model comprises six muscles, the iliacus (
), the rectus femoris (
), the vastus intermedius (
), the gluteus maximus (
), the long biceps (
), and the short biceps (
); 
and 
represent the height of the end-effector and the ground respectively, and 
represents the height of the hip. 
and 
show the centers of mass of the pelvis, femur and tibia, respectively. 
and 
are the lengths of the femur and the tibia, respectively; the centers of mass of these bodies are located in the geometrical center of the body. b) The 3-element muscle model used; it consists of a spring (
) and a damper (
) in parallel to the contractile element (
).