  A long-standing argument in model-based control of locomotion is about the
level of complexity that a model should have to define a behavior such as
running. Even though goldilocks model based on biomechanical evidence is often
sought, it is unclear what complexity level qualifies to be such a model. This
dilemma deepens further for bipedal robotic running with point feet, since
these robots are underactuated, while tracking center-of-mass (COM)
trajectories defined by the spring-loaded inverted pendulum (SLIP) model of
running allocates all control inputs, leaving angular coordinates of the
robot's trunk uncontrolled. Existing work in the literature approach this
problem either by trading off COM trajectories against upright trunk posture
during stance or by adopting more detailed models that include effects of trunk
angular dynamics. In this paper, we present a new approach based on modifying
foot placement targets of the SLIP model. Theoretical analysis and numerical
results show that the proposed approach outperforms these traditional
strategies.
