Brushless motors have an excellent power to weight ratio, and with the advent of quadcopters and electric flight have become relatively cheap.   The idea was to leverage this for DogBot.  The first design was a linear actuator, with a brushless motor driving a threaded rod.

The main problem with this is that it is too slow and wouldn’t be back-drivable or compliant.  This means you would need separate sensors to measure the force applied by the actuator. Linear actuators are also quite large

DogBot needs a fast response time that is suitable for reacting to sensor feedback - it has to be sufficient to enable movement for balancing on rough terrain etc.