The most important goal of character animation is to efficiently control the motions of a character. Until now, many techniques have been proposed for human gait animation, and some techniques have been created to control the emotions in gaits such as ``tired walking'' and ``brisk walking'' by using parameter interpolation or motion data mapping. Since it is very difficult to automate the control over the emotion of a motion, the emotions of a character model have been generated by creative animators. This paper proposes a human running model based on a one-leg planar hopper with a self-balancing mechanism. The proposed technique exploits genetic programming to find an optimal movement. We extend the energy minimization technique to generate various motions in accordance with emotional specifications, for instance, ``brisk running.''