  To be useful tools in real scenarios, humanoid robots must realize tasks
dynamically. This means that they must be capable of applying substantial
forces, rapidly swinging their limbs, and also mitigating impacts that may
occur during the motion. Towards creating capable humanoids, this letter
presents the leg of the robot TELLO and demonstrates how it embodies two new
fundamental design concepts for dynamic legged robots. The limbs follows the
principles of: (i) Cooperative Actuation (CA), by combining motors in
differential configurations to increase the force capability of the limb. We
demonstrate that the CA configuration requires half the motor torque to perform
a jump in comparison to conventional serial design configurations. And (ii)
proximal actuation, by placing heavy motors near the body to reduce the inertia
of the limb. To quantify the effect of motor placement on the robot's dynamics,
we introduce a novel metric entitle Centroidal Inertia Isotropy (CII). We show
that the design of state-of-the-art dynamic legged robots empirically increase
the CII to improve agility and facilitate model-based control. We hope this
metric will enable a quantifiable way to design these machines in the future.
