Simple is the operative word. I work for a robotics lab specializing in bipedal locomotion and we're developing a robot that exploits the periodic, superpositioned waveform nature of passive dynamics to produce a stable gait at varying speeds while only actuating at the hip. The mechanics behind the project involve tuning the sinusoid that governs the hip joint pitch in conjunction with the up/down motion at the knee. This tuning is done using a fairly neat tendon network since we only introduce mechanical power at the hip joint. The resultant behavior is clearly a non-linear controls problem, but at it's heart it's still trig. Just not simple trig.

The robot is modeled after an ostrich. You can read more about it on our organization's website if you want: http://www.ihmc.us/groups/fastrunner/