[FrojoS]

Boston Dynamics robots like Big Dog use hydraulic actuators [1] which act somewhat like a spring and are hence very robust towards perturbations. They also have more power (= force*velocity). ASIMO in contrast uses an electric motor with a high ratio gear drive [2] that makes the robot very stiff and slow while allowing for high torques. This setup doesn't allow for backdrive and hence can't handle impacts well. I don't know about the new ASIMO legs but in the video you can still hear the screaming of the gears.

[1] At least the orignal Raibert Hoppers at MIT also used pneumatic actuators http://www.ai.mit.edu/projects/leglab/robots/3D_hopper/3D_ho... [2] e.g. Harmonic Drives http://en.wikipedia.org/wiki/Harmonic_drive