Probably a dumb question, because I know nothing about robotics, but:

> The "Zero RPM" Problem

> When a robot bends its knees to stand, the motor must constantly fight gravity. There is no skeletal structure to lock against. To an electric motor, holding a static load—known as stall torque—is the most punishing state possible.

Why not just add some kind of brake that can fully or partially lock the joint?

Brakes are being added to actuators but they're more useful for static holds / locking than dynamic balancing. Standing even in humans is a dynamic balancing activity.

Complexity, weight, failure modes, wear, maintenance, support burden of legacy parts.

Also the brake components are never in the same plane as the drive components, so now you have additional forces to engineer for.

You need to keep balancing fast enough. I don't think a break would give you this agility.