Humanoid robot arms, legs and actuators, explained

A humanoid robot is a pile of actuators with a body wrapped around it. Everything else exists to move those joints, so the two most useful numbers on any spec sheet are how many joints the machine has and where they sit.

Both counts come from the makers' spec pages. A Unitree G1 has 23 joints and sells for $13,500, about $587 a joint. A 1X NEO has 75, and 44 of those are inside its two hands. Those two numbers predict most of what each machine can do.

What an actuator actually is

One robot joint is one part you can buy. Inside it: an electric motor, a gearbox trading the motor's speed for torque, an encoder reporting the joint's angle, and a driver board running the control loop hundreds of times a second. That bundle is an actuator, and a walking humanoid carries between twenty and seventy-five of them.

You can price one, because Unitree sells joint motors in the same store it sells robots.

Joint motor	Price
Brushless digital servo	from $119
Unitree IM6014	from $319
GO-M8010-6	$369
Unitree B1	$3,000

Prices from shop.unitree.com.

The $369 unit's page describes a permanent magnet synchronous motor with a built-in field-oriented-control algorithm, a temperature sensor and an absolute-value encoder (shop.unitree.com). You are buying a motor that already knows where it is and how hot it is running. Multiply $369 by 23 and you reach $8,487 before a frame, a battery or a line of software. Unitree does not build the G1 from these exact parts, but the arithmetic shows the shape of the bill.

Makers count their own output the same way. Figure reports its factory's progress in actuators rather than robots, and in more than ten distinct part numbers.

Figure says its BotQ factory has now manufactured 9,000 actuators across more than 10 distinct SKUs (@Figure_robot, 2026-04-29)

The gearbox is where the money goes

A motor small enough to sit inside a knee spins fast and pushes weakly. A knee needs the opposite, so the gearbox does the conversion, usually a strain wave design sold under the trade name harmonic drive.

Three parts do the work, per Harmonic Drive's technology page: a wave generator (an elliptical hub), a flexspline (a thin steel cup with external teeth, squashed into that oval), and a circular spline (a rigid toothed ring with two more teeth than the cup). Roll the oval around inside the cup and the cup's teeth walk around the ring two teeth per revolution, so the output turns very slowly and very strongly. Ratios reach 320:1 in the volume where a single-stage planetary gearbox manages about 10:1 (strain wave gearing), and backlash goes to zero because, in the maker's words, "the gear teeth are always fully engaged in a region along the major axis."

Backlash is the free play you feel wiggling a socket wrench before the ratchet catches. A fraction of a degree of it at a shoulder becomes centimetres of wobble at the fingertips, and removing it takes precision machining on every flexspline, dozens of times per robot.

These gears also wind up like a torsional spring under light load, which is why better joints add a second encoder on the output side, as Unitree's IM6014 does.

A humanoid robot leg is six joints

Every walking humanoid whose spec sheet I can load uses six joints per leg. Booster spells out which six, publishing ranges for hip pitch, roll and yaw, then knee, then ankle pitch and roll (booster.tech). Three at the hip to aim the thigh anywhere, one at the knee to change the leg's length, two at the ankle to keep the sole flat on floors that are rarely level. There is no spare: lose the ankle roll and the machine only walks on ground that is level side to side.

Leg joints are the strong ones. The G1's knee peaks at 90 N.m standard and 120 N.m on the EDU (unitree.com/g1); the Booster T1 quotes 130 N.m maximum peak torque. Ninety newton-metres is roughly the effort of holding a 9 kg weight at the end of a metre-long bar, continuously.

A humanoid robot arm is five, six or seven

Six is the threshold: at least six degrees of freedom are required to put a hand at an arbitrary pose, meaning a position and an orientation, in three dimensional space. Seven adds redundancy, the trick your own arm uses to reach around a box without moving the box.

• Unitree G1: 5 per arm. One short of arbitrary orientation. The EDU configuration sells you two optional wrist degrees of freedom, which is exactly the missing freedom.
• Booster T1: 4 per arm, "extensible". A locomotion research platform, and the count says so.
• 1X NEO: 7 per arm. The full redundant human arm, on the machine sold for kitchens and laundry.

A five-joint arm still reaches a mug. It may have to turn the whole robot to grip it from the right angle, which is slower and needs more clear floor.

Where the joints live

Counts from each maker's spec page: Unitree, Booster, 1X, Engineered Arts.

Machine	Total	Legs	Arms and hands	Head, neck, torso
Unitree G1 (base)	23	12	10	1 waist
Booster T1 (base)	23	12	8	2 head, 1 waist
1X NEO	75	12	58 (14 arms, 44 hands)	3 neck, 2 spine
Ameca (Engineered Arts)	61	0	34 (whole upper body)	27 head and face

The leg block is constant: twelve joints, every walker here. All the variation sits above the waist, where NEO spends 44 joints on hands and Ameca spends 27 on a face and has no legs at all. A robot with 59 percent of its joints in its hands intends to manipulate objects. Whether it manages is a much harder question.

The arm that costs more than the whole robot

Unitree also sells a standalone arm, the Z1, at $15,999 (shop.unitree.com). Six axes, 740 mm reach, 2 kg payload on the AIR version, repeatability near 0.1 mm (unitree.com/z1). So: one six-joint arm for $15,999, or a 23-joint walking humanoid for $13,500. About $2,667 per joint against about $587.

The G1's arm carries about the same 2 kg, and its spec page claims no repeatability at all. The Z1 promises 0.1 mm, every time, for years. Precision is the expensive part; motion is comparatively cheap, and when a humanoid's arm looks affordable beside industrial equipment, repeatability is the missing line item.

Gears or tendons, and why it matters in a kitchen

A high reduction ratio multiplies the motor's torque on the way out. It multiplies the effort needed to move that joint from the other end too. Fine behind a safety fence, a problem in a house with a toddler in it.

1X took the other fork. NEO's spec page lists its actuation as "Low inertia tendon drives" and files it under Safety, beside "No pinchpoints" and a head injury criterion below 250. The figures given for those tendon drives: 95 percent backdrivability, 0.1 mm repetition, 22 dB of audible noise (1x.tech/neo). Backdrivable means you can push the limb and it moves, rather than fighting you.

Tesla's Optimus uses planetary roller screws instead of ball screws in its linear actuators, for shock resistance while walking (Optimus). Nobody at this end of the market picks hydraulics any more. Boston Dynamics retired the hydraulic Atlas on 16 April 2024 and announced an all-electric replacement the next day, promising a machine "stronger, with a broader range of motion than any of our previous generations" (Boston Dynamics). The company with the deepest hydraulic expertise on earth chose motors.

How to read this off a spec sheet

Five checks, in order:

1. Total joint count, then the breakdown. The headline number alone means little. Twelve in the legs is table stakes for anything that walks.
2. Arm degrees of freedom against six. Five means the body compensates. Seven means redundancy. Four means the arms were not the point.
3. Knee torque. Below roughly 90 N.m on an adult-sized biped, ask what it is rated to carry.
4. Encoders and repeatability. Dual encoders and a stated repeatability figure signal a joint built for precise work. Silence on repeatability is itself a spec.
5. Backdrivable or geared hard. For anything sharing a room with people, that is the safety question under the marketing.

Run those five and most spec sheets sort themselves. For the definitions underneath, start with what is a humanoid robot; for parts costs from the bottom up, open-source humanoid builds; for finished prices, every real price, dated.